Localization of Mad2 to Kinetochores Depends on Microtubule Attachment, Not Tension

A single unattached kinetochore can delay anaphase onset in mitotic tissue culture cells (Rieder, C.L., A. Schultz, R. Cole, G. Sluder. 1994. J. Cell Biol. 127:1301–1310). Kinetochores in vertebrate cells contain multiple binding sites, and tension is generated at kinetochores after attachment to the plus ends of spindle microtubules. Checkpoint component Mad2 localizes selectively to unattached kinetochores (Chen, R.-H., J.C. Waters, E.D. Salmon, and A.W. Murray. 1996. Science. 274:242–246; Li, Y., and R. Benezra. Science. 274: 246–248) and disappears from kinetochores by late metaphase, when chromosomes are properly attached to the spindle. Here we show that Mad2 is lost from PtK₁ cell kinetochores as they accumulate microtubules and re-binds previously attached kinetochores after microtubules are depolymerized with nocodazole. We also show that when kinetochore microtubules in metaphase cells are stabilized with taxol, tension at kinetochores is lost. The phosphoepitope 3f3/2, which has been shown to become dephosphorylated in response to tension at the kinetochore (Nicklas, R.B., S.C. Ward, and G.J. Gorbsky. 1995. J. Cell Biol. 130:929–939), is phosphorylated on all 22 kinetochores after tension is reduced with taxol. In contrast, Mad2 only localized to an average of 2.6 out of the 22 kinetochores in taxol-treated PtK₁ cells. Therefore, loss of tension at kinetochores occupied by microtubules is insufficient to induce Mad2 to accumulate on kinetochores, whereas unattached kinetochores consistently bind Mad2. We also found that microinjecting antibodies against Mad2 caused cells arrested with taxol to exit mitosis after ~12 min, while uninjected cells remained in mitosis for at least 6 h, demonstrating that Mad2 is necessary for maintenance of the taxol-induced mitotic arrest. We conclude that kinetochore microtubule attachment stops the Mad2 interactions at kinetochores which are important for inhibiting anaphase onset.     

Reconstitution of glucosylceramide flip-flop across endoplasmic reticulum: implications for mechanism of glycosphingolipid biosynthesis

Most glycosphingolipids are synthesized by the sequential addition of monosaccharides to glucosylceramide (GlcCer) in the lumen of the Golgi apparatus. Because GlcCer is synthesized on the cytoplasmic face of Golgi membranes, it must be flipped to the non-cytoplasmic face by a lipid flippase in order to nucleate glycosphingolipid synthesis. Halter et al. (Halter, D., Neumann, S., van Dijk, S. M., Wolthoorn, J., de Mazière, A. M., Vieira, O. V., Mattjus, P., Klumperman, J., van Meer, G., and Sprong, H. (2007) Pre- and post-Golgi translocation of glucosylceramide in glycosphingolipid synthesis. J. Cell Biol. 179, 101–115) proposed that this essential flipping step is accomplished via a complex trafficking itinerary; GlcCer is moved from the cytoplasmic face of the Golgi to the endoplasmic reticulum (ER) by FAPP2, a cytoplasmic lipid transfer protein, flipped across the ER membrane, then delivered to the lumen of the Golgi complex by vesicular transport. We now report biochemical reconstitution studies to analyze GlcCer flipping at the ER. Using proteoliposomes reconstituted from Triton X-100-solubilized rat liver ER membrane proteins, we demonstrate rapid (t_½ < 20 s), ATP-independent flip-flop of N-(6-((7-nitro-2–1,3-benzoxadiazol-4-yl)amino)hexanoyl)-d-glucosyl-β1–1′-sphingosine, a fluorescent GlcCer analog. Further studies involving protein modification, biochemical fractionation, and analyses of flip-flop in proteoliposomes reconstituted with ER membrane proteins from yeast indicate that GlcCer translocation is facilitated by well characterized ER phospholipid flippases that remain to be identified at the molecular level. By reason of their abundance and membrane bending activity, we considered that the ER reticulons and the related Yop1 protein could function as phospholipid-GlcCer flippases. Direct tests showed that these proteins have no flippase activity.     

Dietary Factors Impact on the Association between CTSS Variants and Obesity Related Traits

Background/Aims

Cathepsin S, a protein coded by the CTSS gene, is implicated in adipose tissue biology–this protein enhances adipose tissue development. Our hypothesis is that common variants in CTSS play a role in body weight regulation and in the development of obesity and that these effects are influenced by dietary factors–increased by high protein, glycemic index and energy diets.

Methods

Four tag SNPs (rs7511673, rs11576175, rs10888390 and rs1136774) were selected to capture all common variation in the CTSS region. Association between these four SNPs and several adiposity measurements (BMI, waist circumference, waist for given BMI and being a weight gainer–experiencing the greatest degree of unexplained annual weight gain during follow-up or not) given, where applicable, both as baseline values and gain during the study period (6–8 years) were tested in 11,091 European individuals (linear or logistic regression models). We also examined the interaction between the CTSS variants and dietary factors–energy density, protein content (in grams or in % of total energy intake) and glycemic index–on these four adiposity phenotypes.

Results

We found several associations between CTSS polymorphisms and anthropometric traits including baseline BMI (rs11576175 (SNP N°2), p = 0.02, β = −0.2446), and waist change over time (rs7511673 (SNP N°1), p = 0.01, β = −0.0433 and rs10888390 (SNP N°3), p = 0.04, β = −0.0342). In interaction with the percentage of proteins contained in the diet, rs11576175 (SNP N°2) was also associated with the risk of being a weight gainer (p_interaction = 0.01, OR = 1.0526)–the risk of being a weight gainer increased with the percentage of proteins contained in the diet.

Conclusion

CTSS variants seem to be nominally associated to obesity related traits and this association may be modified by dietary protein intake.     

Intrinsic microtubule GTP-cap dynamics in semi-confined systems: kinetochore–microtubule interface

In order to quantify the intrinsic dynamics associated with the tip of a GTP-cap under semi-confined conditions, such as those within a neuronal cone and at a kinetochore–microtubule interface, we propose a novel quantitative concept of critical nano local GTP-tubulin concentration (CNLC). A simulation of a rate constant of GTP-tubulin hydrolysis, under varying conditions based on this concept, generates results in the range of 0-420 s⁻¹. These results are in agreement with published experimental data, validating our model. The major outcome of this model is the prediction of 11 random and distinct outbursts of GTP hydrolysis per single layer of a GTP-cap. GTP hydrolysis is accompanied by an energy release and the formation of discrete expanding zones, built by less-stable, skewed GDP-tubulin subunits. We suggest that the front of these expanding zones within the walls of the microtubule represent soliton-like movements of local deformation triggered by energy released from an outburst of hydrolysis. We propose that these solitons might be helpful in addressing a long-standing question relating to the mechanism underlying how GTP-tubulin hydrolysis controls dynamic instability. This result strongly supports the prediction that large conformational movements in tubulin subunits, termed dynamic transitions, occur as a result of the conversion of chemical energy that is triggered by GTP hydrolysis (Satarić et al., Electromagn Biol Med 24:255–264, 2005). Although simple, the concept of CNLC enables the formulation of a rationale to explain the intrinsic nature of the “push-and-pull” mechanism associated with a kinetochore–microtubule complex. In addition, the capacity of the microtubule wall to produce and mediate localized spatio-temporal excitations, i.e., soliton-like bursts of energy coupled with an abundance of microtubules in dendritic spines supports the hypothesis that microtubule dynamics may underlie neural information processing including neurocomputation (Hameroff, J Biol Phys 36:71–93, 2010; Hameroff, Cognit Sci 31:1035–1045, 2007; Hameroff and Watt, J Theor Biol 98:549–561, 1982).     

Diameter Oscillation of Axonemes in Sea-Urchin Sperm Flagella

The 9 + 2 configuration of axonemes is one of the most conserved structures of eukaryotic organelles. Evidence so far has confirmed that bending of cilia and flagella is the result of active sliding of microtubules induced by dynein arms. If the conformational change of dynein motors, which would be a key step of force generation, is occurring in a three-dimensional manner, we can easily expect that the microtubule sliding should contain some transverse component, i.e., a motion in a direction at a right angle to the longitudinal axis of axonemes. Using a modified technique of atomic force microscopy, we found such transverse motion is actually occurring in an oscillatory manner when the axonemes of sea-urchin sperm flagella were adhered onto glass substrates. The motion was adenosine triphosphate-dependent and the observed frequency of oscillation was similar to that of oscillatory sliding of microtubules that had been shown to reflect the physiological activity of dynein arms (S. Kamimura and R. Kamiya. 1989. Nature. 340:476–478; 1992. J. Cell Biol. 116:1443–1454). Maximal amplitude of the diameter oscillation was around 10 nm, which was within a range of morphological change observed with electron microscopy (F. D. Warner. 1978. J. Cell Biol. 77:R19–R26; N. C. Zanetti, D. R. Mitchell, and F. D. Warner. 1979. J. Cell Biol. 80:573–588).     

The highly processive kinesin-8, Kip3, switches microtubule protofilaments with a bias toward the left

Kinesin-1 motor proteins walk parallel to the protofilament axes of microtubules as they step from one tubulin dimer to the next. Is protofilament tracking an inherent property of processive kinesin motors, like kinesin-1, and what are the structural determinants underlying protofilament tracking? To address these questions, we investigated the tracking properties of the processive kinesin-8, Kip3. Using in vitro gliding motility assays, we found that Kip3 rotates microtubules counterclockwise around their longitudinal axes with periodicities of ∼1 μm. These rotations indicate that the motors switch protofilaments with a bias toward the left. Molecular modeling suggests 1), that the protofilament switching may be due to kinesin-8 having a longer neck linker than kinesin-1, and 2), that the leftward bias is due the asymmetric geometry of the motor neck linker complex.The founding member of the kinesin superfamily, the cargo-transporting kinesin-1, has been studied in great detail. Dimeric kinesin-1 constructs 1) are mechanical processive, taking ∼100 of 8-nm steps in a hand-over-hand fashion without detaching from the microtubule; and 2), walk parallel to the axis of microtubule protofilaments as they step from one tubulin dimer to the next. The latter was inferred from gliding motility assays, where microtubules propelled by motors bound to a planar substrate surface rotated around their longitudinal axis with periodicities corresponding to the helical course of the protofilaments in supertwisted microtubules (1,2). Interestingly, protofilament tracking of kinesin-1 is lost in nonprocessive, monomeric constructs (3). There, and also for other nonprocessive microtubule motors such as the kinesin-14 Ncd (4) or axonemal dynein (5), significantly shorter pitches of microtubule rotations in gliding motility assays were observed. As suggested previously (6) this may indicate that protofilament tracking is an inherent property of processive microtubule motors.To explore this idea further, we investigated the rotations of 14-protofilament microtubules (left-handed helical pitch of ∼8 μm (2)) in gliding motility assays using kinesin-8 that has been observed to perform ≈12 μm-long processive runs in vitro (7). Streptavidin-coated quantum dots (QDs), sparsely bound to the microtubules, served as reporters of microtubule rotations (Fig. 1 A). Information on the three-dimensional paths of the QDs—and thus on microtubule rotations—were obtained from 1), two-dimensional tracking of the QDs with nanometer precision in x and y (8), in combination with 2), z information derived from fluorescence-interference contrast (FLIC) (2) (Fig. 1, B–D). FLIC originates from destructive and constructive interference effects close to reflecting surfaces and gives rise to a modulation of the detected intensity of a fluorescent object depending on its height above the surface. Specifically, the microtubule-attached QDs appear dark when they are in close proximity to the surface (i.e., when being located between the microtubule and the surface) but brighten up significantly when being further away (i.e., when on the microtubule lattice pointing away from the surface). In our experiments, we observed counterclockwise rotations (looking from the trailing microtubule plus-end in the direction toward the leading minus-end) with an average pitch of 0.93 μm ± 0.20 μm (mean ± SD, N = 75; N is the number of complete rotations obtained from 15 gliding microtubules). Considering the geometry of the assay, the counterclockwise directionality of the rotations corresponds to the motors stepping with a perpetual bias (∼1 protofilament switch event per forward movement over 10 tubulin dimers) toward the left.Open in a separate windowFigure 1Monitoring Kip3-driven microtubule rotations in gliding motility assays. (A) Schematic of the experimental setup. Imaging is performed on top of a reflective silicon surface using fluorescence interference contrast (FLIC) microscopy (2). (B) Maximum projection of the fluorescence signal of a microtubule-attached quantum dot in the Kip3 gliding motility assay. (C) FLIC intensity (red) and lateral distance from the microtubule path (blue) of the quantum dot shown in panel B versus traveled distance along the microtubule path. The periodic FLIC signal is indicative of repeated up- and down-motion. (D) Schematic of the deduced Kip3 path (red) in comparison to the protofilament axis (green) on a 14-protofilament microtubule.The behavior observed in our experiments is in stark contrast to kinesin-1, for which counterclockwise rotations with an average pitch of 7.9 μm were previously observed using the same experimental technique (2). Consequently, the question arises: which structural determinants decide whether a kinesin acts as a strict protofilament tracker (and—if it does not—from where the directional bias of the off-axis stepping originates)? Assuming motility in a hand-over-hand fashion, it will matter which binding sites on the microtubule lattice are within reach of the forward swinging motor head. This reach is primarily set by the neck linkers, the structural elements that connect the two motor heads to the coiled-coil neck domain. More precisely, the reach is a function of the length of the neck linkers and their three-dimensional path dictated by the volumes that are occupied by the motor heads when bound to the microtubule. Based on primary sequence alignment between Kip3 with other members of the kinesin-8 family and prediction of the start of the coiled-coil dimerization domain with the program PCOILS, we assigned the neck linker region to the amino acids K436–H452 (i.e., 17 amino acids). Accounting for neck linker docking of the rear motor head (K436–Q447) (9), the corresponding length of the neck linkers between both heads, composed of five amino acids from the undocked part of the rear-head neck linker and 17 amino acids from the front-head neck linker, is estimated to be 85 Å (see the Supporting Material).We then modeled all configurations of Kip3 with both heads bound simultaneously to adjacent tubulin dimers (Fig. 2, A and B, and see the Supporting Material). The estimated three-dimensional distances between the positions where the neck linkers protrude from the motor heads, respecting the volumes of the heads (Fig. 2 C, gray column; see also the Supporting Material), are measures for the minimally required neck linker lengths for each two-head bound configuration. Comparison between the three-dimensional distances obtained from the model and the available neck linker length (85 Å) suggests that a forward-swinging Kip3 head can most readily reach the tubulin dimer in the front (53 Å needed) and can switch to the protofilament on the left (79 Å for left and 93 Å for front-left needed), but it has difficulties in stepping to the protofilament on the right, which would require a longer neck-linker than it actually exhibits (103 Å for front-right and 105 Å for right needed). The main reason why these long neck-linker distances are required (i.e., >100 Å) is that, to reach the tubulin dimer on the right (or front-right), the neck linker has to bend over the humpy back of the front head (see Fig. 2 B, right and front-right). On the contrary, to reach the tubulin dimer on the left (or front-left), this detour is avoided (see Fig. 2 B, left and front-left). The model-derived preference for left-stepping over right-stepping is in agreement with our experimental observations.Open in a separate windowFigure 2Virtual three-dimensional reconstruction of Kip3 stepping. (A) Tubulin dimer: composed of alpha-tubulin (α) and beta-tubulin (β) monomers, with the unstructured surface-exposed E-Hooks (e). Kip3 front head: Shown with undocked neck linker (U) and following coiled-coil (cc) dimerization domain. Kip3 rear head: Shown with docked (D) and undocked (U) neck linker parts. (B) Illustration of different Kip3 configurations bound with both heads to adjacent tubulin dimers (first heptad repeat of the coiled coil region is artificially unfolded to illustrate all binding configurations). (C) Estimated three-dimensional distances between the positions where the neck linkers protrude from the motor heads, respecting the volumes of the heads (nomenclature as in panel B). For comparison, the direct distances between the tubulin dimers are given.Modeling as described can be applied for kinesin-1, whose neck linkers are three-amino-acids shorter than the neck linkers of Kip3. Whereas the modeled minimally required neck linker lengths for each two-head-bound configuration are almost identical to the values for Kip3, we estimate an available neck linker length of 63 Å (see the Supporting Material), which explains the strict forward stepping of kinesin-1.In summary, we have shown what to our knowledge is the first example of a highly processive kinesin motor (run length of several μm) switching between protofilaments of microtubules. Our modeling suggests that protofilament switching may be due to kinesin-8 having a longer neck linker than kinesin-1 so that it is able to reach the extra distance required to change protofilaments. The leftward bias cannot be explained by the geometry of the microtubule lattice alone (Fig. 2 C, last column) but follows from the additional consideration of the asymmetric geometry of the motor neck linker complex. A leftward torque component, which may be present in the powerstrokes of the individual heads (3,4), may further promote the leftward bias but is not strictly necessary. While our results were under review, left-handed spiraling along microtubules of beads coated with a modified kinesin-1 (with extended neck linkers (10)) was reported (11); the handedness of the bead rotations is consistent with the handedness of our microtubule rotations and our model.Our results may also provide an alternative explanation for the short-pitch, counterclockwise rotations of microtubules gliding on surfaces coated by dimeric kinesin-5 (Eg5) motors (6). The authors of this report attributed the short pitch to the low processivity of Eg5, arguing that during processive episodes the motor follows the protofilament axis, but when detaching generates an off-axis force leading to microtubule rotation. Considering the structure of Eg5 (neck linker length of 18 amino acids (12)), protofilament switching may, however, also be possible during the processive episodes. For kinesin-2 (neck linker length of 17 amino acids, although reduced in length by ∼5 Å due to proline in cis-conformation at position 13 (12,13)), the propensity to switch protofilaments is controversially discussed and may depend on the stability of the neck domain (11,14).Previously, Kip3, has been found to depolymerize microtubules in a length-dependent manner (7). The underlying mechanism has been described by an antenna model, where Kip3 binds along the entire microtubule lattice and subsequently walks to the microtubule plus-end relying on its high processivity that is ∼20 times the run length of kinesin-1. During such long runs, motors in vivo are expected to frequently encounter obstacles, such as microtubule-associated proteins. In the case of kinesin-1, shown to follow the microtubule''s protofilament axis (1), obstacles cause motor stalling or accelerated detachment. It is exciting to speculate that Kip3 uses protofilament switching to bypass obstacles on the microtubule surface avoiding premature motor release or stalling that could reduce the efficiency of targeting and subsequent depolymerization of the microtubule plus-ends.     

Posttranslational modification of distinct microtubule subpopulations during cell polarization and differentiation in the mouse preimplantation embryo

During the course of preimplantation development, the cells of the mouse embryo undergo both a major subcellular reorganization (at the time of compaction) and, subsequently, a process of differentiation as the phenotypes of trophectoderm and inner cell mass cell types diverge. We have used antibodies specific for tyrosinated (Kilmartin, J. V., B. Wright, and C. Milstein. 1982. J. Cell Biol. 93:576-582) and acetylated (Piperno, G., and M. T. Fuller. 1985. J. Cell Biol. 101:2085-2094) alpha-tubulin in immunofluorescence studies and found that subsets of microtubules can be distinguished within and between cells during the course of these events. Whereas all microtubules contained tyrosinated alpha-tubulin, acetylated alpha-tubulin was detected only in a subpopulation, located predominantly in the cell cortices. Striking differences developed between the distribution of the two populations during the course of development. Firstly, whereas the microtubule population as a whole tends to redistribute towards the apical domain of cells as they polarize during compaction (Houliston, E., S. J. Pickering, and B. Maro. 1987. J. Cell Biol. 104:1299-1308), the microtubules recognized by the antiacetylated alpha-tubulin antibody became enriched in the basal part of the cell cortex. After asymmetric division of polarized cells to generate two distinct cell types (termed inside and outside cells) we found that, despite the relative abundance of microtubules in outside cells, acetylated microtubules accumulated preferentially in inside cells. Treatment with nocodazole demonstrated that within each cell type acetylated microtubules were the more stable ones; however, the difference in composition of the microtubule network between cell types was not accompanied by a greater stability of the microtubule network in inside cells.     

Thermostabilization of ovalbumin by alkaline treatment: Examination of the possible roles of d-serine residues

It was revealed from the crystal structure analysis of S-ovalbumin (S-OVA) formed by alkaline treatment that Ser164, Ser236, and Ser320 take the d-amino acid residue configuration (Yamasaki et al., J Biol Chem 2003; 278:35524–35530). To address the implications of a d-configuration for these Ser residues in S-OVA formation, three mutant OVAs (S164A, S236A, and S320A) were generated to compare their thermostabilities before and after alkaline treatment. Following alkaline treatment, S236A showed a marked increase in melting temperature similar to the wild type (ΔT_m, +9°C) which corresponded to the formation of S-OVA, whereas the increment in T_m for both S164A and S320A was only 4.5°C. Furthermore, the T_m value of the double mutant S164/320A remained unchanged after alkaline treatment, supporting the relevance of Ser164 and Ser320 for thermostabilization of OVA. As Arg142 was predicted to interact with D-Ser164 upon S-OVA formation, it was substituted to Ala to generate R142A. The resulting increment in T_m of mutant R142A after alkaline treatment was 5.8°C. The double mutant R142/S320A was therefore prepared to eliminate the participation of Ser320 in thermostabilization, and its T_m value was compared before and after alkaline treatment. As expected, the increase in T_m for the double mutant was only 1.2°C. Taken together, the data suggest that d-configuration of Ser164 caused by alkaline treatment favors interaction with Arg142 through conformational changes of the side chain. These results strongly supported the participation of the configurational inversion of both Ser164 and Ser320 residues in the formation of S-OVA.     

No‐tillage and fertilization management on crop yields and nitrate leaching in North China Plain

A field experiment was performed from 2003 to 2008 to evaluate the effects of tillage system and nitrogen management regimes on crop yields and nitrate leaching from the fluvo-aquic soil with a winter wheat (Triticum aestivum L.)–maize (Zea mays L.) double-cropping system. The tillage systems consisted of conventional tillage (CT) and no-tillage (NT). Three nitrogen management regimes were included: 270 kg N ha⁻¹ of urea for wheat and 225 kg N ha⁻¹ of urea for maize (U), 180 kg N ha⁻¹ of urea and 90 kg N ha⁻¹ of straw for wheat and 180 kg N of urea and 45 kg N ha⁻¹ of straw for maize (S), 180 kg N ha⁻¹ of urea and 90 kg N ha⁻¹ of manure for wheat and 180 kg N ha⁻¹ of urea and 45 kg N ha⁻¹ of manure for maize (M). An array of tension-free pan lysimeters (50 cm × 75 cm) were installed (1.2 m deep) to measure water flow and -N movement. No significant effect of the N management regime on yields of winter wheat and maize grain was found in the 5-year rotation. Tillage systems had significant influences on -N leaching from the second year and thereafter interacted with N management regimes on -N loads during all maize seasons. The average yield-scaled -N leaching losses were in order of CTS < NTS< CTU < NTU −1 for winter wheat system and from 0.99 (CTS) to 6.27 (NTM) kg N Mg⁻¹ for summer maize system for 5 rotation years. The results showed that CTS decreased the yield-scaled -N leaching losses while sustaining crop grain yields. Considering the lower costs, NTS could be a potential alternative to decrease yield-scaled -N leaching losses and improve soil fertility while maintaining crop yield for the winter wheat–maize double-cropping systems in the North China Plain.     

Structure-Function Relationships in Ribonuclease S: the Work of Frederic M. Richards

The Preparation of Subtilisin-modified Ribonuclease and the Separation of the Peptide and Protein Components(Richards, F. M., and Vithayathil, P. (1959) J. Biol. Chem. 234, 1459–1465)The Three-dimensional Structure of Ribonuclease-S. Interpretation of an Electron Density Map at a Nominal Resolution of 2 Å(Wyckoff, H. W., Tsernoglou, D., Hanson, A. W., Knox, J. R., Lee, B., and Richards, F. M. (1970) J. Biol. Chem. 245, 305–328)Frederic Middlebrook Richards (1925–2009) was born in New York City. He attended the Massachusetts Institute of Technology and, after a brief stint in the military, received his B.S. in 1948. Richards then enrolled in graduate school at Harvard Medical School, where he worked with Barbara Low and received his Ph.D. in 1952. After graduating he remained at Harvard for another year as a research fellow with Edwin Joseph Cohn, who was featured in a previous Journal of Biological Chemistry (JBC) Classic (1). Richards then moved to the Carlsberg Laboratory in Denmark where, with Kaj Linderstrøm-Lang and others, he began working on ribonuclease.Open in a separate windowFrederic M. RichardsAfter a short stint as a postdoctoral fellow at Cambridge University, Richards joined the faculty of the Department of Biochemistry at Yale University in 1955 as an assistant professor. He rose rapidly through the ranks, becoming professor in 1963. That year, Richards was also appointed chairman of the Department of Molecular Biology and Biophysics at Yale, which entailed a move from the Medical School to the Yale College campus. Following a sabbatical at Oxford University in 1967–1968, for which Richards and his wife Sally sailed their own boat with a small crew across the Atlantic Ocean, Yale merged the Medical School Department of Biochemistry and the Yale College Department of Molecular Biology and Biophysics to form a new university-wide Department of Molecular Biophysics & Biochemistry (MB&B) with Richards as its founding chair (1969–1973). Richards remained at Yale for his entire research career, eventually becoming Sterling Professor of Molecular Biophysics and Biochemistry.Much of Richard''s early research centered on bovine pancreatic ribonuclease (RNase). During his time at the Carlsberg laboratory, he showed that cleavage of RNase by the protease subtilisin produces a modified RNase (RNase S) that is still active (2). After starting his own lab at Yale, Richards was able to separate RNase S into a 20-residue S-peptide and a 102-residue S-protein, both of which lacked enzymatic activity. However, when the peptide and protein were recombined, the activity was recovered. Richards published an initial paper on this finding in 1958 (3). He followed this up with a more extensive article in the JBC, which is reprinted here as the first JBC Classic. In this paper, Richards and co-workers purified and characterized RNase S, separated it into S-peptide and S-protein, showed that almost all enzymatic activity is recovered when the two components are recombined, and also reported that the only observed change in covalent structure during the conversion of RNase A to RNase S is the hydrolysis of the peptide bond between residues 20 and 21.The demonstration that two separate, inactive fragments of the enzyme RNase A could be reconstituted to form an active enzyme provided the first experimental evidence that the ability of a protein to form a three-dimensional structure is an intrinsic property of its amino acid sequence. This work also foreshadowed the extensive RNase A refolding studies performed by Nobel laureate Christian Anfinsen, as discussed in a previous JBC Classic (4).In the 1960s Richards teamed up with Harold Wyckoff to solve the three-dimensional structure of RNase S. Initially, in 1967, they produced a 3.5 Å electron density map (5), which they used to determine the approximate conformation of the peptide chain. Three years later, they collected data to 2 Å, as reported in the second JBC Classic reprinted here. Using these data, Richards, Wyckoff, and colleagues produced an electron density map, which they used to determine the complete three-dimensional structure of RNase S. This structure tied with three others for the third protein structure ever solved to atomic resolution. Richards also showed that RNase S was enzymatically active in crystal form, putting to rest the widely held view at that time that protein crystal structures were irrelevant to the conformation and behavior of enzymes in solution.Richards received many honors and awards for his scientific achievements, including the Pfizer-Paul Lewis Award in Enzyme Chemistry (1965), election as Fellow of the American Academy of Arts and Sciences (1968), election to the National Academy of Sciences (1971), the Kai Linderstrøm-Lang Prize in Protein Chemistry (1978), the American Society for Biochemistry and Molecular Biology Merck Award (1988), the Stein and Moore Award of the Protein Society (1988), and the State of Connecticut Medal of Science (1995). He was also president of ASBMB (1979) and the Biophysical Society (1972–1973).     

Dances with leukocytes: how tetraspanin-enriched microdomains assemble to form endothelial adhesive platforms

Rather than just providing an unstructured adhesive surface for leukocytes, cytokine-activated endothelial cells assemble preexisting tetraspanin-enriched microdomains to form endothelial adhesive platforms (EAPs) and endothelial docking structures. In this issue of the Journal of Cell Biology, Barreiro et al. (Barreiro, O., M. Zamai, M. Yáñez-Mó, E. Tejera, P. López-Romero, P.N. Monk, E. Gratton, V.R. Caiolfa, and F. Sánchez-Madrid. 2008. J. Cell Biol. 183:527–542) show how the immunoglobulin superfamily adhesion molecules intercellular adhesion molecule (ICAM)–1 and vascular cell adhesion molecule (VCAM)–1 form nanoclusters with the tetraspanins CD9 and CD151 in a physiologically relevant system. Furthermore, convincing biochemical data suggest that these structures are distinct from lipid rafts.     

Cytoplasmic Dynein and Dynactin Are Required for the Transport of Microtubules into the Axon

Previous work from our laboratory suggested that microtubules are released from the neuronal centrosome and then transported into the axon (Ahmad, F.J., and P.W. Baas. 1995. J. Cell Sci. 108: 2761–2769). In these studies, cultured sympathetic neurons were treated with nocodazole to depolymerize most of their microtubule polymer, rinsed free of the drug for a few minutes to permit a burst of microtubule assembly from the centrosome, and then exposed to nanomolar levels of vinblastine to suppress further microtubule assembly from occurring. Over time, the microtubules appeared first near the centrosome, then dispersed throughout the cytoplasm, and finally concentrated beneath the periphery of the cell body and within developing axons. In the present study, we microinjected fluorescent tubulin into the neurons at the time of the vinblastine treatment. Fluorescent tubulin was not detected in the microtubules over the time frame of the experiment, confirming that the redistribution of microtubules observed with the experimental regime reflects microtubule transport rather than microtubule assembly. To determine whether cytoplasmic dynein is the motor protein that drives this transport, we experimentally increased the levels of the dynamitin subunit of dynactin within the neurons. Dynactin, a complex of proteins that mediates the interaction of cytoplasmic dynein and its cargo, dissociates under these conditions, resulting in a cessation of all functions of the motor tested to date (Echeverri, C.J., B.M. Paschal, K.T. Vaughan, and R.B. Vallee. 1996. J. Cell Biol. 132: 617–633). In the presence of excess dynamitin, the microtubules did not show the outward progression but instead remained near the centrosome or dispersed throughout the cytoplasm. On the basis of these results, we conclude that cytoplasmic dynein and dynactin are essential for the transport of microtubules from the centrosome into the axon.     

Enzymatic suppression of the membrane conductance associated with the glutamine transporter SNAT3 expressed in Xenopus oocytes by carbonic anhydrase II

The transport activity of the glutamine/neutral amino acid transporter SNAT3 (former SN1, SLC38A3), expressed in oocytes of the frog Xenopus laevis is associated with a non-stoichiometrical membrane conductance selective for Na⁺ and/or H⁺ (Schneider, H.P., S. Bröer, A. Bröer, and J.W. Deitmer. 2007. J. Biol. Chem. 282:3788–3798). When we expressed SNAT3 in frog oocytes, the glutamine-induced membrane conductance was suppressed, when carbonic anhydrase isoform II (CAII) had been injected into the oocytes. Transport of substrate, however, was not affected by CAII. The reduction of the membrane conductance by CAII was dependent on the presence of CO₂/HCO₃ ⁻, and could be reversed by blocking the catalytic activity of CAII by ethoxyzolamide (10 μM). Coexpression of wild-type CAII or a N-terminal CAII mutant with SNAT3 also reduced the SNAT3- associated membrane conductance. The catalytically inactive CAII mutant V143Y coexpressed in oocytes did not affect SNAT3-associated membrane conductance. Our results reveal a new type of interaction between CAII and a transporter-associated cation conductance, and support the hypothesis that the transport of substrate and the non-stoichiometrical ion conductance are independent of each other. This study also emphasizes the importance of carbonic anhydrase activity and the presence of CO₂-bicarbonate buffers for membrane transport processes.     

A Dynein Light Chain Is Essential for the Retrograde Particle Movement of Intraflagellar Transport (IFT)

Several enzymes, including cytoplasmic and flagellar outer arm dynein, share an M_r 8,000 light chain termed LC8. The function of this chain is unknown, but it is highly conserved between a wide variety of organisms. We have identified deletion alleles of the gene (fla14) encoding this protein in Chlamydomonas reinhardtii. These mutants have short, immotile flagella with deficiencies in radial spokes, in the inner and outer arms, and in the beak-like projections in the B tubule of the outer doublet microtubules. Most dramatically, the space between the doublet microtubules and the flagellar membrane contains an unusually high number of rafts, the particles translocated by intraflagellar transport (IFT) (Kozminski, K.G., P.L. Beech, and J.L. Rosenbaum. 1995. J. Cell Biol. 131:1517–1527). IFT is a rapid bidirectional movement of rafts under the flagellar membrane along axonemal microtubules. Anterograde IFT is dependent on a kinesin whereas the motor for retrograde IFT is unknown. Anterograde IFT is normal in the LC8 mutants but retrograde IFT is absent; this undoubtedly accounts for the accumulation of rafts in the flagellum. This is the first mutation shown to specifically affect retrograde IFT; the fact that LC8 loss affects retrograde IFT strongly suggests that cytoplasmic dynein is the motor that drives this process. Concomitant with the accumulation of rafts, LC8 mutants accumulate proteins that are components of the 15-16S IFT complexes (Cole, D.G., D.R. Deiner, A.L. Himelblau, P.L. Beech, J.C. Fuster, and J.L. Rosenbaum. 1998. J. Cell Biol. 141:993–1008), confirming that these complexes are subunits of the rafts. Polystyrene microbeads are still translocated on the surface of the flagella of LC8 mutants, indicating that the motor for flagellar surface motility is different than the motor for retrograde IFT.     

Polymerization of tubulin in vivo: direct evidence for assembly onto microtubule ends and from centrosomes

Microtubule assembly in vivo was studied by hapten-mediated immunocytochemistry. Tubulin was derivatized with dichlorotriazinylaminofluorescein (DTAF) and microinjected into living, interphase mammalian cells. Sites of incorporation were determined at the level of individual microtubules by double-label immunofluorescence. The haptenized tubulin was localized by an anti-fluorescein antibody and a second antibody conjugated with fluorescein. Total microtubules were identified by anti-tubulin and a secondary antibody conjugated with rhodamine. Contrary to recent studies (Salmon, E. D., et al., 1984, J. Cell Biol., 99:2165-2174; Saxton, W. M., et al., 1984, J. Cell Biol., 99:2175-2186) which suggest that tubulin incorporates all along the length of microtubules in vivo, we found that microtubule assembly in interphase cells was in vivo, as in vitro, an end-mediated process. Microtubules that radiated out toward the cell periphery incorporated the DTAF-tubulin solely at their distal, that is, their plus ends. We also found that a proportion of the microtubules connected to the centrosomes incorporated the DTAF-tubulin along their entire length, which suggests that the centrosome can nucleate the formation of new microtubules.     

Identification of a Microtubule-associated Motor Protein Essential for Dendritic Differentiation

The quintessential feature of the dendritic microtubule array is its nonuniform pattern of polarity orientation. During the development of the dendrite, a population of plus end–distal microtubules first appears, and these microtubules are subsequently joined by a population of oppositely oriented microtubules. Studies from our laboratory indicate that the latter microtubules are intercalated within the microtubule array by their specific transport from the cell body of the neuron during a critical stage in development (Sharp, D.J., W. Yu, and P.W. Baas. 1995. J. Cell Biol. 130:93– 104). In addition, we have established that the mitotic motor protein termed CHO1/MKLP1 has the appropriate properties to transport microtubules in this manner (Sharp, D.J., R. Kuriyama, and P.W. Baas. 1996. J. Neurosci. 16:4370–4375). In the present study we have sought to determine whether CHO1/MKLP1 continues to be expressed in terminally postmitotic neurons and whether it is required for the establishment of the dendritic microtubule array. In situ hybridization analyses reveal that CHO1/MKLP1 is expressed in postmitotic cultured rat sympathetic and hippocampal neurons. Immunofluorescence analyses indicate that the motor is absent from axons but is enriched in developing dendrites, where it appears as discrete patches associated with the microtubule array. Treatment of the neurons with antisense oligonucleotides to CHO1/MKLP1 suppresses dendritic differentiation, presumably by inhibiting the establishment of their nonuniform microtubule polarity pattern. We conclude that CHO1/MKLP1 transports microtubules from the cell body into the developing dendrite with their minus ends leading, thereby establishing the nonuniform microtubule polarity pattern of the dendrite.     

Desmoplakin: an unexpected regulator of microtubule organization in the epidermis

Despite their importance in cell shape and polarity generation, the organization of microtubules in differentiated cells and tissues remains relatively unexplored in mammals. We generated transgenic mice in which the epidermis expresses a fluorescently labeled microtubule-binding protein and show that in epidermis and in cultured keratinocytes, microtubules stereotypically reorganize as they differentiate. In basal cells, microtubules form a cytoplasmic network emanating from an apical centrosome. In suprabasal cells, microtubules concentrate at cell-cell junctions. The centrosome retains its ability to nucleate microtubules in differentiated cells, but no longer anchors them. During epidermal differentiation, ninein, which is a centrosomal protein required for microtubule anchoring (Dammermann, A., and A. Merdes. 2002. J. Cell Biol. 159:255-266; Delgehyr, N., J. Sillibourne, and M. Bornens. 2005. J. Cell Sci. 118:1565-1575; Mogensen, M.M., A. Malik, M. Piel, V. Bouckson-Castaing, and M. Bornens. 2000. J. Cell Sci. 113:3013-3023), is lost from the centrosome and is recruited to desmosomes by desmoplakin (DP). Loss of DP prevents accumulation of cortical microtubules in vivo and in vitro. Our work uncovers a differentiation-specific rearrangement of the microtubule cytoskeleton in epidermis, and defines an essential role for DP in the process.