High temperature induced antibiotic sensitivity changes in Pseudomonas aeruginosa.

Pseudomonas aeruginosa, which was resistant to a wide variety of antibiotics, became sensitive to several of these antibiotics when grown and tested at 46 degrees C. Cell wall antibiotics such as penicillin G and ampicillin were only effective when added to cells growing at 46 degrees C prior to a temperature shift to 37 degrees C. Antibiotics which penetrate the cytoplasmic membrane to express their inhibiting action present a pattern different from those which are active against the outer cell wall. In order that these compounds be effective, the permeability of the cytoplasmic membrane must be further altered with agents such as EDTA which allow the penetration of actinomycin D. Inhibitors of protein synthesis, such as streptomycin and chloramphenicol, have increased access to their sites of action in cells grown at 46 degrees C. Cells grown at 46 degrees C have 40% less lipopolysaccharide (LPS) than cells grown at 37 degrees C and the LPS aggregates were of large molecular size in cells grown at 46 degrees C. Growth at 46 degrees C affects the permeability properties of the outer cell wall more than the permeability properties of the cytoplasmic membrane and this was due, in part, to the selective release of LPS of LPS-protein complexes at elevated growth temperatures.     

TRANSFORMATION OF NONPOLAR FILAMENTS OF THE BLUE-GREEN ALGA GLOEOTRICHIA ECHINULATA U. WISC. 1052 INTO DOUBLE HELICES12

Normally straight filaments of Gloeotrichia echinulata U. Wisc. 1052 transform into double helices when a critical culture density has been attained. In inorganic Zehnder-Gorham's medium No. 11, the alga initially is morphologically uniform and forms single, lightly curved, polar filaments. When a stage of close proximity of the filaments is reached, excessively long filaments are observed that are nonpolar. Some of these nonpolar filaments form helices. Two independent single helices may entwine to form a double helix. Double helices also form when both ends of an excessively long filament meet and start entwining until a loop is left at about the original middle of the filament. In another manner of double-helix formation, a straight filament makes a hairpin bend at about its middle; then 2 helices form starting at the bend and they entwine into a double helix, leaving a loop at the location of the original bend. Under proper culture conditions, the structures of double helices show an amazing regularity. Once double-helix formation has started, some strong force brings the process to completion. In a mature culture, helices disintegrate into apparently healthy, short pieces of filaments or single cells, while the straight or slightly curved polar filaments still persist. Helical transformation of nonpolar filaments does not appear to be a sign of nutritional or other stress but rather appears to serve a specific purpose. One might speculate that a genetic exchange between opposite cells takes place.     

A Temperature-Sensitive Mutation Affecting Synthesis of Outer Arm Dyneins in Tetrahymena thermophila

ABSTRACT. We have characterized a novel, temperature-sensitive mutation affecting motility in Tetrahymena thermophila . Mutants grew and divided normally at the restrictive temperature (38°C), but became nonmotile. Scanning electron microscopic analysis indicated that nonmotile mutants contained the normal number of cilia and that the cilia were of normal length. Transmission electron microscopic analysis indicated that axonemes isolated from nonmotile mutants lacked outer dynein arms, so the mutation was named oad I ( outer arm defficient ). Motile mutants shifted to 38° C under conditions that prevent cell growth and division (starvation) remained motile suggesting that once assembled into axonemes at the permissive temperature (28° C) the outer arm dyneins remain functional at 38° C. Starved, deciliated mutants regenerated a full complement of functional cilia at 38° C, indicating that the mechanism that incorporates the outer arm dynein into developing axonemes is not affected by the oad I mutation. Starved, nonmotile mutants regained motility when shifted back to 28° C, but not when incubated with cycloheximide. We interpret these results to rule out the hypothesis that the oad I mutation affects the site on the microtubules to which the outer arm dyneins bind. Axonemes isolated from mutants grown for one generation at 38° C had a mean of 6.0 outer arm dyneins, and axonemes isolated from mutants grown for two generations at 38° C had a mean of 3.2 outer arm dyneins. Taken together, these results indicate that the oad I mutation affects the synthesis of outer arm dyneins in Tetrahymena .     

A temperature-sensitive mutation affecting synthesis of outer arm dyneins in Tetrahymena thermophila.

We have characterized a novel, temperature-sensitive mutation affecting motility in Tetrahymena thermophila. Mutants grew and divided normally at the restrictive temperature (38 degrees C), but became nonmotile. Scanning electron microscopic analysis indicated that nonmotile mutants contained the normal number of cilia and that the cilia were of normal length. Transmission electron microscopic analysis indicated that axonemes isolated from nonmotile mutants lacked outer dynein arms, so the mutation was named oad 1 (outer arm deficient). Motile mutants shifted to 38 degrees C under conditions that prevent cell growth and division (starvation) remained motile suggesting that once assembled into axonemes at the permissive temperature (28 degrees C) the outer arm dyneins remain functional at 38 degrees C. Starved, deciliated mutants regenerated a full complement of functional cilia at 38 degrees C, indicating that the mechanism that incorporates the outer arm dynein into developing axonemes is not affected by the oad 1 mutation. Starved, nonmotile mutants regained motility when shifted back to 28 degrees C, but not when incubated with cycloheximide. We interpret these results to rule out the hypothesis that the oad 1 mutation affects the site on the microtubules to which the outer arm dyneins bind. Axonemes isolated from mutants grown for one generation at 38 degrees C had a mean of 6.0 outer arm dyneins, and axonemes isolated from mutants grown for two generations at 38 degrees C had a mean of 3.2 outer arm dyneins. Taken together, these results indicate that the oad 1 mutation affects the synthesis of outer arm dyneins in Tetrahymena.     

Regulation of Cell Division in Escherichia coli: Characterization of Temperature-Sensitive Division Mutants

A temperature-sensitive division mutant of Escherichia coli was isolated by using differential filtration to select for filaments at 42 C and normal cells at 30 C. Cells shifted from 30 to 42 C stop dividing almost immediately, suggesting the temperature-sensitive element is required for cell division late in the cell cycle. Cells returned to 30 from 42 C divide abruptly, suggesting accumulation of division potential at 42 C. Inhibitors of protein, deoxyribonucleic acid, and ribonucleic acid synthesis do not block division during the recovery period at 30 C. Cycloserine does not stop cell division, vancomycin shows some effect on cell division, whereas penicillin completely stops cell division during this period. The addition of high concentrations of NaCl to filaments at 42 C results in a burst of cell division. The final cell number is equivalent to the control which is grown at 30 C if sufficient salt is added (11 g/liter, final concentration). After the original burst, cell division ceases at the nonpermissive temperature even at increased osmolality. Chloramphenicol, puromycin, vancomycin, and penicillin prevent division during the recovery in the presence of NaCl. Kinetic data indicate division potential decays to a reversible inactive intermediate which rapidly decays to an irreversible inactive form. Conversion of division potential to the inactive form is correlated with a 100- to 1,000-fold derepression of the synthesis of division potential. The mutation appears to involve a stage in cross-wall synthesis which is required during the terminal stages of division.     

Characterization of gliding motility in Flexibacter polymorphus

Motility of the marine gliding bacterium Flexibacter polymorphus was studied by using microcinematographic techniques. Following adhesion to a glass surface, multicellular filaments and individual cells usually began to glide within a few seconds at a speed of approximately 12 micron per second (at 23 degrees C). Adhesion to the glass surface was evidently mediated by multitudes of extremely fine extracellular fibrils. Gliding velocity was independent of filament length but directly related to electron-transport activity and substratum temperature in the range 3-35 degrees C. The rate of gliding was inversely related to medium viscosity, suggesting that the locomotor apparatus functions at constant torque. Forward motion was occasionally interrupted by direction reversals, somersaults (observed primarily in single cells of short filaments), or spinning of filaments tethered by one pole. The frequency of direction reversal was found to be an inverse function of filament length. Translational motility was invariably accompanied by sinistral revolution about the longitudinal axis of a filament. The sense and pitch of revolution were constant among filaments of different length. Polystyrene microspheres or India ink particles adsorbed to gliding cells were actively displaced in either direction, their movement tracing either a regular zigzag or helical path along the filament surface. Because microspheres were also observed to move on nonmotile filaments, particle translocation was evidently not obligatorily linked to gliding locomotion. Multiple particles adsorbed to a single filament often moved independently. The data are consistent with a motility mechanism involving limited motion in numerous mechanically independent (yet functionally coordinated) domains on the cell surface.     

Growth, Cell Division, and Fragmentation in a Species of Flexibacter

Flexibacter FS-1, a gram-negative gliding bacterium was grown in liquid culture as long (over 100-mum) filaments. The filaments possessed a triple-track wall which resembled that found in other gram-negative bacteria. Although phase-contrast microscopy indicated that the long filaments were nonseptate, electron microscopy revealed three or four septa along the length of each filament. The septa contained lysozyme-sensitive, electron-opaque material, presumed to be peptidoglycan, sandwiched between cell membranes. The outer triple track wall was not part of the septum. Mesosomes were seen in various areas of the cell and frequently were observed attached to septa in different stages of completion. Studies of the organism in slide culture revealed that individual filaments grew in an exponential fashion and divided in the middle despite the long length and multiseptate condition. When the temperature of a liquid culture growing exponentially with a generation time of 90 minutes was shifted from 30 to 35 C, the filaments fragmented into three or four shorter cells within 120 min. The short cells continued to grow exponentially at 35 C at approximately the same rate as at 30 C. When the culture was shifted back to 30 C, the cells immediately stopped dividing and began to elongate. After a period of 2 to 3 hr, cell division resumed. It is suggested that the shift-up in temperature induced the completion of the cross wall (centripetal growth of the triple-track wall) and cell separation at the sites of previously formed septa, whereas the shift-down in temperature caused a transient inhibition of cross-wall formation but not of growth. Fragmentation was inhibited by sodium azide but took place despite the inhibition of protein synthesis by chloramphenicol or the inhibition of deoxyribonucleic acid synthesis by mitomycin C.     

Flagellin inhibits Myoviridae phage φCTX infection of Pseudomonas aeruginosa strain GuA18: purification and mapping of binding site

PhiCTX is a double-stranded DNA phage of the Myoviridae family that converts Pseudomonas aeruginosa into a cytotoxin producer. A 42-kDa phiCTX-inhibiting protein was purified from the outer membrane fraction of P. aeruginosa strain GuA18 by octyl-beta-glucoside extraction, DEAE-chromatography, and mono-Q HPLC. This protein had an isoelectric point of 5.4 and bound specifically [125I]-labeled phiCTX. The N-terminal amino acid sequence of six out of seven Lys-C fragments was highly similar (87%) to that of the entire of type-a flagellin of P. aeruginosa strain PAK. At a concentration of 14 nM, purified flagellin protein caused a 50% decrease in the phage titer after a 20-min incubation at 37 degrees C (PhI50). The presence of ethanol was necessary to reconstitute the inhibitory activity. In contrast, no ethanol treatment was necessary for the inhibitory activity of the sheared flagellin filaments from P. aeruginosa strain GuA18, which consists of the 42-kDa flagellin subunits and the synthesized 17-mer phage-binding-peptide NGSNSDSERTALNGEAK, representing flagellin residues 100-116 of P. aeruginosa strain PAK. The PhI50 was 10 nM and 200 nM, respectively. Antisera against the flagellin filament protein as well as against the 17-mer peptide neutralized phage infection. These results indicated that the amino acid region 100-116 of the flagellin subunit of strain GuA18 is involved in phiCTX binding. This region might play a role in phage attachment.     

Three-dimensional reconstruction of the flagellar filament of Caulobacter crescentus. A flagellin lacking the outer domain and its amino acid sequence lacking an internal segment

We obtained a three-dimensional reconstruction of the flagellar filament of Caulobacter crescentus CB15 from electron micrographs of negatively stained preparations. The C. crescentus filament appears, both in negative stain and in the frozen-hydrated state, significantly smoother and narrower than other filaments. Its helical symmetry, and unit cell size, however, are similar to that of other filaments. Although the molecular weight of the C. crescentus flagellin is about half that of other plain flagellins, there is only one monomer per unit cell as indicated by diffraction studies and by linear mass density measurements with the scanning transmission electron microscope. Alignment of the primary amino acid sequences of Salmonella typhimurium (serotype i) and C. crescentus (29,000 Mr) flagellins shows that whereas there is homology at the amino and carboxyterminal ends of the two sequences, the central segment of the S. typhimurium sequence has no homology to that of C. crescentus. A correlated comparison between the three-dimensional reconstructions of the two filaments and primary amino acid sequences of the two flagellins suggests that: (1) the C. crescentus subunit is missing the outer molecular domain but is, otherwise, similar to that of S. typhimurium; (2) the outer molecular domain in S. typhimurium corresponds, therefore, to a central stretch of the primary amino acid sequence; and (3) the outer molecular domain, missing in C. crescentus, is not obligatory for flagellar motility.     

Coalignment of microtubules, cytokeratin intermediate filaments, and collagen fibrils in a collagen-secreting cell system

The distribution of microtubules and intermediate filaments in the collagen-secreting scleroblasts of the goldfish scale was investigated by immunofluorescence and electron microscopy. Many of the microtubules and cytokeratin type intermediate filaments formed bundles that were aligned with the underlying, parallel collagen fibrils. The intermediate filament bundles were evenly spaced and located adjacent to the basal plasma membrane. The microtubules, on the other hand, were located further away from the membrane, although many were found very close to the intermediate filament bundles. No detectable change was observed in scleroblast microtubules when cells on scales were treated with colchicine or cooled (greater than or equal to 0 degrees C) for up to 1 h. Cells had to be cooled overnight before the microtubules were affected. The final number and length of the microtubules in the cell depended only on the final steady-state temperature and not the temperature history of the scale cell, and steady state was reached more slowly at colder temperatures. The microtubules but not the intermediate filaments rapidly (within 5 min) and reversibly depolymerized when cells were chilled to -2 approximately -4 degrees C. When chilled cells were warmed, the microtubules polymerized back, within 15 min at room temperature, to the same pattern of parallel coalignment with the underlying collagen. They appeared to repolymerize via two different pathways: (1) a radial growth outwards from the microtubule organizing center followed by a progressive realignment with the underlying collagen and (2) a gradual and simultaneous polymerization along cold-stable, antitubulin staining fibers. These fibers were also aligned with the collagen fibrils and may be related to the aligned intermediate filaments.     

Alternative flagellar filament types in the haloarchaeon Haloarcula marismortui

Many Archaea use rotation of helical flagellar filaments for swimming motility. We isolated and characterized the flagellar filaments of Haloarcula marismortui, an archaeal species previously considered to be nonmotile. Two Haloarcula marismortui phenotypes were discriminated--their filaments are composed predominantly of either FlaB or FlaA2 flagellin, and the corresponding genes are located on different replicons. FlaB and FlaA2 filaments differ in antigenicity and thermostability. FlaA2 filaments are distinctly thicker (20-22 nm) than FlaB filaments (16-18 nm). The observed filaments are nearly twice as thick as those of other characterized euryarchaeal filaments. The results suggest that the helicity of Haloarcula marismortui filaments is provided by a mechanism different from that in the related haloarchaeon Halobacterium salinarum, where 2 different flagellin molecules present in comparable quantities are required to form a helical filament.     

Filament formation in Clostridium acidiurici under conditions of elevated temperatures

Terry, David R. (Brigham Young University, Provo, Utah), Abdul Gaffar, and Richard D. Sagers. Filament formation in Clostridium acidiurici under conditions of elevated temperatures. J. Bacteriol. 91:1625-1634. 1966.-Vegetative cells of Clostridium acidiurici, when grown at temperatures up to 42 C, are straight rods varying from 2.5 to 4 mu in length. When grown at 43 C, the cells show a definite tendency to elongate, and, when grown at 44 C, filaments are formed, often exceeding 500 mu in length. Only an occasional cross wall is apparent in the heat-induced long forms, but as the temperature is lowered they readily form cross walls and fragment into short, single cells. Chromatin material is distributed in evenly spaced clusters throughout the length of the filaments. The filaments grown at 44 C are gram-negative, whereas cells grown at 37 C are gram-positive. However, filament formation and gram-negativity apparently are not due to magnesium deficiency, since the gram-negative filaments are formed in concentrations of magnesium ranging from 10(-6) to 10(-2)m. The rapid transition from filaments to single cells upon lowering the temperature from 44 to 37 C suggests that the temperature-related repression of the cross wall-forming system is a phenotypic response rather than the selection of specific mutants which produce the observed phenomena.     

Structural Basis for Stabilization of the Hypervariable D3 Domain of Salmonella Flagellin upon Filament Formation

The hypervariable D3 domain of Salmonella flagellin, composed of residues 190-283, is situated at the outer surface of flagellar filaments. A flagellin mutant deprived of the complete D3 domain (ΔD3_FliC) exhibited a significantly decreased thermal stability (T_m 41.9 °C) as compared to intact flagellin (T_m 47.3 °C). However, the stability of filaments formed from ΔD3_FliC subunits was virtually identical with that of native flagellar filaments. While D3 comprises the most stable part of monomeric flagellin playing an important role in the stabilization of the other two (D1 and D2) domains, the situation is reversed in the polymeric state. Upon filament formation, ordering of the disordered terminal regions of flagellin in the core part of the filament results in the stabilization of the radially arranged D1 and D2 domains, and there is a substantial increase of stability even in the distant outermost D3 domain, which is connected to D2 via a pair of short antiparallel β-strands. Our experiments revealed that crosslinking the ends of the isolated D3 domain through a disulfide bridge gives rise to a stabilization effect reminiscent of that observed upon polymerization. It appears that the short interdomain linker between domains D2 and D3 serves as a stabilization center that facilitates propagation of the conformational signal from the filament core to the outer part of filament. Because D3 is a largely independent part of flagellin, its replacement by heterologous proteins or domains might offer a promising approach for creation of various fusion proteins possessing polymerization ability.     

Three-dimensional reconstruction of a simple Z-band in fish muscle

The three-dimensional structure of the Z-band in fish white muscle has been investigated by electron microscopy. This Z-band is described as simple, since in longitudinal sections it has the appearance of a single zigzag pattern connecting the ends of actin filaments of opposite polarity from adjacent sarcomeres. The reconstruction shows two pairs of links, the Z-links, between one actin filament and the facing four actin filaments in the adjacent sarcomere. The members of each pair have nearly diametrically opposed origins. In relation to one actin filament, one pair of links appears to bind along the final 10 nm of the actin filament (proximal site) and the other pair binds along a region extending from 5 to 20 nm from the filament end (distal site). Between one pair and the other, there is a rotation of approximately 80 degrees round the filament axis. A Z-link with a proximal site at the end of one actin filament attaches at a distal site on the oppositely oriented actin filaments of the facing sarcomere and vice versa. The length of each Z-link is consistent with the length of an alpha-actinin molecule. An additional set of links located 10-15 nm from the center of the Z-band occurs between actin filaments of the same polarity. These polar links connect the actin filaments along the same direction on each side of the Z-band. The three-dimensional structure appears to have twofold screw symmetry about the central plane of the Z-band. Only approximate twofold rotational symmetry is observed in directions parallel to the actin filaments. Previous models of the Z-band in which four identical and rotationally symmetrical links emanate from the end of one actin filament and span across to the ends of four actin filaments in the adjacent sarcomere are therefore incorrect.     

Factors that influence the permeability assay of the outer membrane of Pseudomonas aeruginosa

Brief exposure of Pseudomonas aeruginosa to a temperature of 10 degrees C or lower caused a significant leakage of the periplasmic beta-lactamase into the medium. The extent of leakage increased as the incubation temperature was lowered to 4 degrees C and reached a maximum at 0 degrees C. Cells grown in the presence of beta-lactamase inducers were unsuitable for the permeability assay. It was found that the diffusion rates of beta-lactams through the outer membrane of P. aeruginosa were much lower than those previously reported, as assayed under refined conditions. The diffusion rates of beta-lactams in one of the mutants tested were an order of magnitude lower than those of the other strains, despite the fact that the outer membrane protein profile of the strain appeared to be indistinguishable from those of the others. These results suggest that beta-lactam antibiotics diffuse through the outer membrane of P. aeruginosa, at least partly, through a non-porin pathway.     

Regulation of Cell Division in a Temperature-Sensitive Division Mutant of Escherichia coli

Escherichia coli fil ts forms multinucleate filaments when suspensions of about 10(7) organisms per ml are shifted from 37 to 43 C in rich medium. Occasional septation continues, chiefly at the poles, and immediately becomes more frequent when the filaments are returned to 37 C. The addition of chloramphenicol (200 mug/ml) at either temperature initially stimulates the formation of polar septa. When very dilute suspensions of the strain (<10(6) organisms per ml) are shifted to the restrictive temperature, the inhibition of septation is more complete and only seldom reversible. Conversely, cell division is little affected when suspensions of >10(8) organisms per ml, or microcolonies of several hundred organisms on agar, are incubated at 43 C; evidence is presented that this is a consequence of a slight reduction in the mutant's growth rate. In certain media, septation is blocked irreversibly by even brief exposure to 43 C, after which cell elongation without division proceeds at 37 C for some hours. Several findings, when considered together, suggest that the cytoplasmic membrane is normal at the restrictive temperature, and that the block in septation is caused by a defect in the cell wall: it is largely overcome by NaCl, but not by sucrose; in some circumstances the filaments become swollen and develop localized bulges in the wall, yet the membrane remains intact and retains its selective permeability; lastly, the strain is insensitive to deoxycholate at both temperatures. The mutation has been mapped between arg B and thr, at a locus which appears to be distinct from others known primarily to influence cell division.     

Gene cloning, overproduction, and characterization of thermolabile alkaline phosphatase from a psychrotrophic bacterium

The gene encoding alkaline phosphatase from the psychrotrophic bacterium Shewanella sp. SIB1 was cloned, sequenced, and overexpressed in Escherichia coli. The recombinant protein was purified and its enzymatic properties were compared with those of E. coli alkaline phosphatase (APase), which shows an amino acid sequence identity of 37%. The optimum temperature of SIB1 APase was 50 degrees C, lower than that of E. coli APase by 30 degrees C. The specific activity of SIB1 APase at 50 degrees C was 3.1 fold higher than that of E. coli APase at 80 degrees C. SIB1 APase lost activity with a half-life of 3.9 min at 70 degrees C, whereas E. coli APase lost activity with a half-life of >6 h even at 80 degrees C. Thus SIB1 APase is well adapted to low temperatures. Comparison of the amino acid sequences of SIB1 and E. coli APases suggests that decreases in electrostatic interactions and number of disulfide bonds are responsible for the cold-adaptation of SIB1 APase.     

An electron microscopic study of the interaction in vitro of vimentin intermediate filaments with vesicles prepared from Ehrlich ascites tumor cell lipids

The interaction of intermediate filaments prepared from pure, delipidated vimentin with vesicles obtained from Ehrlich ascites tumor (EAT) cell lipids was studied employing sucrose density gradient centrifugation in combination with electron microscopy. In negative stain electron microscopy, preformed vimentin filaments were seen in lateral association with lipid vesicles; end-on contacts of filaments with liposomes were rarely detected. When the reaction of filaments with vesicles was carried out at 0 degree C, sucrose density gradient equilibrium centrifugation of the reaction products led to the banding of relatively light filament-vesicle meshworks in clear separation from free filaments and free vesicles. With certain vimentin and lipid preparations, occasionally partial breakdown of the filaments during centrifugation and banding of vesicle-free fragments in denser regions of the sucrose gradients was observed. However, when the reaction mixtures were incubated at 37 degrees C prior to sucrose gradient analysis, all filaments were released from vesicles and totally fragmented during centrifugation. Electron microscopy showed unraveling of the filament fragments into subfilament strands. Employing lipid vesicles labeled with [3H]cholesterol, a low but significant amount of radioactivity was found to be associated with the fragments in a non-vesicular form. Filament reconstitution experiments performed in the presence of EAT cell lipids revealed an inhibitory effect of vesicles on filament assembly, particularly at lower temperatures. The mechanical labilization of the filament structure by lipid vesicles might play a role in the redistribution of intermediate filaments in the course of certain cellular processes involving turnover and fragmentation of intracellular membrane systems.     

Aurora-B phosphorylates the myosin II heavy chain to promote cytokinesis

Cytokinesis, the final step of mitosis, is mediated by an actomyosin contractile ring, the formation of which is temporally and spatially regulated following anaphase onset. Aurora-B is a member of the chromosomal passenger complex, which regulates various processes during mitosis; it is not understood, however, how Aurora-B is involved in cytokinesis. Here, we show that Aurora-B and myosin-IIB form a complex in vivo during telophase. Aurora-B phosphorylates the myosin-IIB rod domain at threonine 1847 (T¹⁸⁴⁷), abrogating the ability of myosin-IIB monomers to form filaments. Furthermore, phosphorylation of myosin-IIB filaments by Aurora-B also promotes filament disassembly. We show that myosin-IIB possessing a phosphomimetic mutation at T¹⁸⁴⁷ was unable to rescue cytokinesis failure caused by myosin-IIB depletion. Cells expressing a phosphoresistant mutation at T¹⁸⁴⁷ had significantly longer intercellular bridges, implying that Aurora-B-mediated phosphorylation of myosin-IIB is important for abscission. We propose that myosin-IIB is a substrate of Aurora-B and reveal a new mechanism of myosin-IIB regulation by Aurora-B in the late stages of mitosis.     

Actin filament content and organization in unstimulated platelets

The extent of actin polymerization in unstimulated, discoid platelets was measured by DNase I inhibition assay in Triton X-100 lysates of platelets washed at 37 degrees C by gel filtration, or in Triton X-100 lysates of platelets washed at ambient temperatures by centrifugation in the presence of prostacyclin. About 40% of the actin in the discoid platelets obtained by either method existed as filaments. These filaments could be visualized by electron microscopy of thin sections. Similar results were obtained when the actin filament content of discoid platelets was measured by sedimentation of filaments from Triton X-100 lysates at high g forces (145,000 g for 45 min). However, few of these filaments sedimented at the lower g forces often used to isolate networks of actin filaments from cell extracts. These results indicate that actin filaments in discoid cells are not highly crosslinked. Platelets isolated by centrifugation in the absence of prostacyclin were not discoid, but were instead irregular with one or more pseudopodia. These platelets also contained approximately 40-50% of their actin in a filamentous form; many of these filaments sedimented at low g forces, however, indicating that they were organized into networks. The discoid shape of these centrifuged platelets could be restored by incubating them for 1-3 h at 37 degrees C, which resulted in the reversal of filament organization. High g forces were then required for the sedimentation of the actin. Approximately 80-90% of the actin in platelets washed at 4 degrees C was filamentous; this high actin filament content could be attributed to actin polymerization during the preparation of the platelets at low temperatures. These studies show that platelet activation involves mechanisms for the structural reorganization of existing filaments, in addition to those previously described for mediating actin polymerization.