Calcium-binding and temperature induced transitions in equine lysozyme: new insights from the pCa-temperature "phase diagrams"

The most universal approach to the studies of metal binding properties of single-site metal binding proteins, i.e., construction of a "phase diagram" in coordinates of free metal ion concentration-temperature, has been applied to equine lysozyme (EQL). EQL has one relatively strong calcium binding site and shows two thermal transitions, but only one of them is Ca(2+)-dependent. It has been found that the Ca(2+)-dependent behavior of the low temperature thermal transition (I) of EQL can be adequately described based upon the simplest four-states scheme of metal- and temperature-induced structural changes in a protein. All thermodynamic parameters of this scheme were determined experimentally and used for construction of the EQL phase diagram in the pCa-temperature space. Comparison of the phase diagram with that for alpha-lactalbumin (alpha-LA), a close homologue of lysozyme, allows visualization of the differences in thermodynamic behavior of the two proteins. The thermal stability of apo-EQL (transition I) closely resembles that for apo-alpha-LA (mid-temperature 25 degrees C), while the thermal stabilities of their Ca(2+)-bound forms are almost indistinguishable. The native state of EQL has three orders of magnitude lower affinity for Ca(2+) in comparison with alpha-LA while its thermally unfolded state (after the I transition) has about one order lower (K = 15M(-1)) affinity for calcium. Circular dichroism studies of the apo-lysozyme state after the first thermal transition show that it shares common features with the molten globule state of alpha-LA.     

Protein purification by affinity binding to unilamellar vesicles

A novel purification technique is proposed which employs affinity-ligand-modified liposomes to specifically purify bioactive macromolecules from solution. This process is demonstrated with avidin as the model biomolecule and biotin as the affinity ligand. Biotin is covalently bound to the surface of small unilamellar vesicles composed of dimyristoyl phosphatidylcholine (DMPC) and dimyristoyl phosphatidylethanolamine (DMPE). The number of accessible binding sites on the liposomes is determined by titration with avidin, and the kinetics of binding are evaluated by monitoring the concentration of free avidin in solution after the addition of biotinylated liposomes. The specificity of the process is determined by following the affinity binding of avidin to biotinylated liposomes in the presence of model impurities (i.e., lysozyme and cytochrome C). Liposome-bound avidin is separated from the impurities by ultrafiltration through a membrane which retains the liposomes.     

Left-sided substrate binding of lysozyme: evidence for the involvement of asparagine-46 in the initial binding of substrate to chicken lysozyme.

The "right-sided" and "left-sided" substrate binding modes at the lower saccharide binding subsites (D-F sites) of chicken lysozyme were investigated by utilizing mutant lysozymes secreted from yeast. We constructed the following mutant lysozymes; "left-sided" substitution of Asn46 to Asp, deletion of Thr47, and insertion of Gly between Thr47 and Asp48 and "right-sided" substitution of Asn37 to Gly. Analyses of their activities and substrate binding abilities showed that Asn46 and Thr47 are involved in the initial enzyme-substrate complex and Asn37 is involved in the transition state. These results support an earlier proposal that interactions between substrate and residues at the left side of lysozyme stabilize a catalytically inactive enzyme-substrate complex, while interactions between substrate and residues at the right side stabilize the catalytically active complex [Pincus, M. R., & Scheraga, H. A. (1979) Macromolecules 12, 633-644]. These results are also consistent with the proposed kinetic mechanism for lysozyme reaction that the rearrangement of an initial enzyme-substrate complex (beta-complex) to another complex (gamma-complex) is required for catalytic hydrolysis [Banerjee S. K., Holler, E., Hess, G. P., & Rupley, J. A. (1975) J. Biol. Chem. 250, 4355-4367].     

Surface-plasmon microscopic observation of site-selective recognition reactions

We demonstrate that surface-plasmon microscopy allows one to monitor the specific binding of streptavidin to biotinylated lipid molecules selectively enriched in one of the two coexisting phase domains of a phospholipid monolayer transferred in its phase transition region from the water-air interface to a solid support.     

The effect of protein stability on protein-monoglyceride interactions

We have previously shown that proteins such as beta-lactoglobulin and lysozyme insert into monoglyceride monolayers and are able to induce an L(beta) to coagel phase transition in monoglyceride bilayers. These studies gave a first indication that protein stability could be an important factor for these interactions. This study therefore aims at further investigating the potential role of protein stability on protein-monoglyceride interactions. To this end we studied the interaction of stable and destabilized alpha-lactalbumin with monostearoylglycerol. Our results show that protein stability is important for the insertion of proteins into a monostearoylglycerol monolayer, such that the lower the stability of the protein the better the protein inserts. In marked contrast to beta-lactoglobulin and lysozyme we found that destabilized alpha-lactalbumin does not induce the L(beta) to coagel phase transition in monoglyceride bilayers. We propose that this is due to an increased surface coverage by the protein which could result from the unfolding of the protein upon binding to the interface.     

Hexafluoroacetone hydrate as a structure modifier in proteins: characterization of a molten globule state of hen egg-white lysozyme.

A molten globule-like state of hen egg-white lysozyme has been characterized in 25% aqueous hexafluoroacetone hydrate (HFA) by CD, fluorescence, NMR, and H/D exchange experiments. The far UV CD spectra of lysozyme in 25% HFA supports retention of native-like secondary structure while the loss of near UV CD bands are indicative of the overall collapse of the tertiary structure. The intermediate state in 25% HFA exhibits an enhanced affinity towards the hydrophobic dye, ANS, and a native-like tryptophan fluorescence quenching. 1-D NMR spectra indicates loss of native-like tertiary fold as evident from the absence of ring current-shifted 1H resonances. CD, fluorescence, and NMR suggest that the transition from the native state to a molten globule state in 25% HFA is a cooperative process. A second structural transition from this compact molten globule-like state to an "open" helical state is observed at higher concentrations of HFA (> or = 50%). This transition is characterized by a dramatic loss of ANS binding with a concomitant increase in far UV CD bands. The thermal unfolding of the molten globule state in 25% HFA is sharply cooperative, indicating a predominant role of side-chain-side-chain interactions in the stability of the partially folded state. H/D exchange experiments yield higher protection factors for many of the backbone amide protons from the four alpha-helices along with the C-terminal 3(10) helix, whereas little or no protection is observed for most of the amide protons from the triple-stranded antiparallel beta-sheet domain. This equilibrium molten globule-like state of lysozyme in 25% HFA is remarkably similar to the molten globule state observed for alpha-lactalbumin and also with the molten globule state transiently observed in the kinetic refolding experiments of hen lysozyme. These results suggest that HFA may prove generally useful as a structure modifier in proteins.     

The specificity of monoglyceride-protein interactions and mechanism of the protein induced L(beta) to coagel phase transition

This study aims at gaining insight into the specificity and molecular mechanism of monoglyceride-protein interactions. We used beta-lactoglobulin (beta-LG) and lysozyme as model proteins and both monostearoylglycerol and monopalmitoylglycerol as defined gel phase monoglycerides. The monoglycerides were used in different combinations with the two negatively charged amphiphiles dicetylphosphate and distearylphosphate. The interactions were characterized using the monolayer technique, isothermal titration calorimetry, (2)H-nuclear magnetic resonance (NMR) using deuterium labelled monoglycerides and freeze fracture electron microscopy (EM). Our results show that lysozyme inserts efficiently into all monolayers tested, including pure monoglyceride layers. The insertion of beta-LG depends on the lipid composition of the monolayer and is promoted when the acylchains of the negatively charged amphiphile are shorter than that of the monoglyceride. The binding parameters found for the interaction of beta-LG and lysozyme with monoglyceride bilayers were generally similar. Moreover, in all cases a large exothermic binding enthalpy was observed which was found to depend on the nature of the monoglycerides but not of the proteins. (2)H-NMR and freeze fracture EM showed that this large enthalpy results from a protein mediated catalysis of the monoglyceride L(beta) to coagel phase transition. The mechanism of this phase transition consists of two steps, an initial protein mediated vesicle aggregation step which is followed by stacking and probably fusion of the bilayers.     

The specificity of monoglyceride–protein interactions and mechanism of the protein induced Lβ to coagel phase transition

This study aims at gaining insight into the specificity and molecular mechanism of monoglyceride–protein interactions. We used β-lactoglobulin (β-LG) and lysozyme as model proteins and both monostearoylglycerol and monopalmitoylglycerol as defined gel phase monoglycerides. The monoglycerides were used in different combinations with the two negatively charged amphiphiles dicetylphosphate and distearylphosphate. The interactions were characterized using the monolayer technique, isothermal titration calorimetry, ²H-nuclear magnetic resonance (NMR) using deuterium labelled monoglycerides and freeze fracture electron microscopy (EM). Our results show that lysozyme inserts efficiently into all monolayers tested, including pure monoglyceride layers. The insertion of β-LG depends on the lipid composition of the monolayer and is promoted when the acylchains of the negatively charged amphiphile are shorter than that of the monoglyceride. The binding parameters found for the interaction of β-LG and lysozyme with monoglyceride bilayers were generally similar. Moreover, in all cases a large exothermic binding enthalpy was observed which was found to depend on the nature of the monoglycerides but not of the proteins. ²H-NMR and freeze fracture EM showed that this large enthalpy results from a protein mediated catalysis of the monoglyceride L_β to coagel phase transition. The mechanism of this phase transition consists of two steps, an initial protein mediated vesicle aggregation step which is followed by stacking and probably fusion of the bilayers.     

Functionalization of poly(oligo(ethylene glycol) methacrylate) films on gold and Si/SiO2 for immobilization of proteins and cells: SPR and QCM studies

Thin films of a biocompatible and nonbiofouling poly(oligo(ethylene glycol) methacrylate) ( pOEGMA) with various thicknesses were formed on gold and Si/SiO 2 substrates by a combination of the formation of self-assembled monolayers (SAMs) terminating in bromoester-an initiator of atom transfer radical polymerization (ATRP)-and surface-initiated ATRP. After the formation of the pOEGMA films, terminal hydroxyl groups of side chains divergent from the methacrylate backbones were activated with N, N'-disuccinimidyl carbonate (DSC), and the DSC-activated pOEGMA films were reacted with (+)-biotinyl-3,6,9-trioxaundecanediamine (Biotin-NH 2) to form biotinylated pOEGMA films. By surface plasmon resonance experiments with the target protein (streptavidin) and model proteins (fibrinogen and lysozyme), we verified that the resulting films showed the enhanced signal-to-noise ratio ( approximately 10-fold enhancement) for the biospecific binding of streptavidin compared with the biotinylated substrate prepared from carboxylic acid-terminated SAMs. Quartz crystal microbalance measurements were also carried out to obtain the surface coverage of streptavidin and fibrinogen adsorbed onto the biotinylated pOEGMA films with various thicknesses and to investigate the effect of film thicknesses on the biospecific binding of streptavidin. Both the binding capacity of streptavidin and the signal-to-noise ratio of streptavidin/fibrinogen were found to be saturated at the 20 nm thick pOEGMA film. In addition, to demonstrate a wide applicability of the pOEGMA films, we constructed micropatterns of streptavidin and cells by microcontact-printing biotin-NH 2 and poly- l-lysine onto the DSC-activated pOEGMA films, respectively.     

Characterization of the transition state of lysozyme unfolding. II. Effects of the intrachain crosslinking and the inhibitor binding on the transition state

The reversible refolding of a lysozyme derivative containing an extra crosslink between Glu 35 and Trp 108 was observed at pH 3.7 in solutions of 4.5M LiBr and 4.SM 1-PrOH. The rate constants of unfolding and folding for crosslinked lysozyme were compared with those for intact lysozyme. In LiBr solutions, the crosslinking greatly increases k_f but only slightly decreases k_uf. This means that the crosslinking restricts possible conformations of the U state to some conformations on the folding pathway, whereas the possible conformations of the transition state are little restricted. Although the crosslinking produces an apparently different effect on the rate constants in a PrOH solution, this could be explained by assuming that the crosslinking counteracts the effect of PrOH on the transition state. These observations suggest that the Glu 35 and Trp 108 of intact lysozyme already come into contact with each other in the course of refolding to the transition state. Kinetics of the unfolding and folding reactions of intact lysozyme were measured in the presence of 8mAf (NAG)₃. The apparent folding rate (k) was nearly equal to k_f, while the apparent unfolding rate (k) decreased sixfold. This means that the inhibitor binding stabilizes the native conformation but makes no contribution to the stabilization of the transition state. Specific interactions between (NAG)₃ and the cleft of lysozyme become available only at the last stage of folding.     

Mechanisms controlling the temperature-dependent binding of proteins to poly(N-isopropylacrylamide) microgels

Poly(N-isopropylacrylamide) (PNIPA) microgels may offer several advantages over PNIPA-modified surfaces when used as sorbents in temperature-sensitive chromatography. Yet, a full exploitation of these advantages requires a better understanding of the mechanisms controlling the separation process. As a model system, we have studied the binding of three proteins (bovine serum albumin (BSA), ovalbumin, and lysozyme) to PNIPA microgels. Binding experiments were conducted both below (25 degrees C) and above (37 degrees C) the volume phase transition temperature of the gel, T(c). The analysis of the binding isotherms has shown that although an average gel particle contained a larger amount of protein below the phase transition temperature, the concentration of the protein within the particle was higher above this temperature. These findings were attributed to changes in the binding loci due to temperature swings around T(c): whereas a sorption mechanism is dominant below this temperature, surface-adsorption was more important above it. A comparison between the three studied proteins has shown that below T(c) the binding increases with a decrease in the molecular weight. On the other hand, no significant difference in the bound protein amounts was observed above the phase transition temperature. Our results imply that, despite the increase in the gel's hydrophobicity above the phase transition temperature, the resolution in bioseparations based on PNIPA gels is not necessarily better above T(c).     

Biotinylated epidermal growth factor: a useful tool for the histochemical analysis of specific binding sites

Summary Epidermal growth factor (EGF) was labelled with biotin via modification of either the amino or carboxyl groups, using suitable reagents, namely biotinyl-N-hydroxysuccinimide ester or biotinamidocaproyl hydrazide. To assure that the specific binding capacity of EGF is retained despite its chemical modification, displacement of the EGF by biotinylated derivatives in a routine binding assay was performed. The inhibitory potency compared to unmodified EGF was only slightly reduced. This result is the prerequisite for testing the usefulness of biotinylated EGF in histochemistry. The biotinylated probes were applied to sections of human tumour tissue and of monkey organs (liver, kidney, uterus of Cynomolgus and Rhesus monkey) to localize the specific binding sites for EGF. Formalin-fixed, paraffin-embedded tissue sections were deparaffinized and incubated with the probes at a concentration of 10 g ml^–1 at room temperature for 60 min. Specific binding of the EGF was visualized by the avidin-biotin techniques (ABC). A positive reaction in conjunction with appropriate controls by competitive inhibition was seen for all monkey tissue sections and for the following number of cancer cases: breast carcinoma: 7/10; mesothelioma: 2/4; lung carcinoma: 1/3; colon carcinoma: 1/3.The staining properties were similar for both types of probes that differed in the functional group that is involved in modification by biotion attachment. However, the batches with modification of the amino groups stained more intensely and more distinctly than the carboxyl modified EGF. Overall, the data indicate that the ligand properties of the EGF are not impaired by biotinylation of the two types of functional groups. Thus, biotinylated EGF is a useful tool for histochemical detection and identification of EGF specific binding sites in mammalian tissue.     

The methanol-induced transition and the expanded helical conformation in hen lysozyme.

Methanol-induced conformational transitions of hen egg white lysozyme were investigated with a combined use of far- and near-UV CD and NMR spectroscopies, ANS binding and small-angle X-ray scattering. Addition of methanol induced no global change in the native conformation itself, but induced a transition from the native state to the denatured state which was highly cooperative, as shown by the coincidence of transition curves monitored by the far- and near-UV CD spectroscopy, by isodichroic points in the far- and near-UV CD spectra and by the concomitant disappearance of individual 1H NMR signals of the native state. The ANS binding experiments could detect no intermediate conformer similar to the molten globule state in the process of the methanol denaturation. However, at high concentration of methanol, e.g., 60% (v/v) methanol/water, a highly helical state (H) was realized. The H state had a helical content much higher than the native state, monitored by far-UV CD spectroscopy, and had no specific tertiary structure, monitored both by near-UV CD and NMR spectroscopy. The radius of gyration in the H state, 24.9 angstroms, was significantly larger than that in the native state (15.7 angstroms). The Kratky plot for the H state did not show a clear peak and was quite similar to that for the urea-denatured state, indicating a complete lack of globularity. Thus we conclude that the H state has a considerably expanded, flexible broken rod-like conformation which is clearly distinguishable from the "molten globule" state. The stability of both N and H states depends on pH and methanol concentration. Thus a phase diagram involving N and H was constructed.     

Binding of brilliant red compound to lysozyme: insights into the enzyme toxicity of water-soluble aromatic chemicals

The non-covalent interaction of brilliant red (BR) with lysozyme was investigated by the UV spectrometry, circular dichroism (CD) and isothermal titration calorimetry (ITC). The thermodynamic characterization of the interaction was performed and the assembly complexes were formed: lysozyme(BR)₁₇ at pH 2.03, lysozyme(BR)₁₅ at pH 3.25 and lysozyme(BR)₁₂ at pH 4.35, which corresponded to the physiological acidities. The ionic interaction induces a combination of multiple non-covalent bonds including hydrogen bond, hydrophobic interaction and van der Waals force. The two-step binding model of BR was found, in which one or two BR molecules entered the hydrophobic intracavity of lysozyme and the others bound to the hydrophilic outer surface of lysozyme. Moreover, BR binding resulted in change of the lysozyme conformation and inhibition of the lysozyme activity. The possible binding site and type of BR and the conformational transition of lysozyme were speculated and illustrated. This work provided a useful approach for study on enzyme toxicity of aromatic azo chemicals.     

Mechanism of lysozyme catalysis: role of ground-state strain in subsite D in hen egg-white and human lysozymes.

The association constants for the binding of various saccharides to hen egg-white lysozyme and human lysozyme have been measured by fluorescence titration. Among these are the oligosaccharides GlcNAc-beta(1 leads to 4)-MurNAc-beta(1 leads to 4)-GlcNAc-beta(1 leads to 4)-GlcNAc, GlcNAc-beta(1 leads to 4)-MurNAc-beta(1 leads to 4)-GlcNAc-beta(1 leads to 4)-N-acetyl-D-xylosamine, and GlcNAc-beta(1 leads to 4-GlcNAc-beta(1 leads to 4)-MurNAc, prepared here for the first time. The binding constants for saccharides which must have N-acetylmuramic acid, N-acetyl-D-glucosamine, or N-acetyl-D-xylosamine bound in subsite D indicate that there is no strain involved in the binding of N-acetyl-D-glycosamine in this site, and that the lactyl group of N-acetylmuramic acid (rather than the hydroxymethyl group) is responsible for the apparent strain previously reported for binding at this subsite. For hen egg-white lysozyme, the dependence of saccharide binding on pH or on a saturating concentration of Gd(III) suggests that the conformation of several of the complexes are different from one another and from that proposed for a productive complex. This is supported by fluorescence difference spectra of the various hen egg-white lysozyme-saccharide complexes. Human lysozyme binds most saccharides studied more weakly than the hen egg-white enzyme, but binds GlcNAc-beta(1 leads to 4)-MurNAc-beta(1leads to 4)-GlcNAc-beta(1 leads to 4)-MurNAc more strongly. It is suggested that subsite C of the human enzyme is "looser" than the equivalent site in the hen egg enzyme, so that the rearrangement of a saccharide in this subsite in response to introduction of an N-acetylmuramic acid residue into subsite D destabilizes the saccharide complexes of human lysozyme less than it does the corresponding hen egg-white lysozyme complexes. This difference and the differences in the fluorescence difference spectra of hen egg-white lysozyme and human lysozyme are ascribed mainly to the replacement of Trp-62 in hen egg-white lysozyme by Tyr-63 in the human enzyme. The implications of our findings for the assumption of superposition and additivity of energies of binding in individual subsites, and for the estimation of the role of strain in lysozyme catalysis, are discussed.     

Binding of lysozyme to phospholipid bilayers: evidence for protein aggregation upon membrane association

Biological functions of lysozyme, including its antimicrobial, antitumor, and immune-modulatory activities have been suggested to be largely determined by the lipid binding properties of this protein. To gain further insight into these interactions on a molecular level the association of lysozyme to liposomes composed of either 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine or its mixtures with 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-glycerol, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-phosphatidylserine, or bovine heart cardiolipin was studied by a combination of fluorescence techniques. The characteristics of the adsorption of lysozyme to lipid bilayers were investigated using fluorescein 5'-isothiocyanate labeled protein, responding to membrane association by a decrease in fluorescence. Upon increasing the content of anionic phospholipids in lipid vesicles, the binding isotherms changed from Langmuir-like to sigmoidal. Using adsorption models based on scaled particle and double-layer theories, this finding was rationalized in terms of self-association of the membrane-bound protein. The extent of quenching of lysozyme tryptophan fluorescence by acrylamide decreased upon membrane binding, revealing a conformational transition for the protein upon its surface association, resulting in a diminished access of the fluorophore to the aqueous phase. Steady-state fluorescence anisotropy of bilayer-incorporated probe 1,6-diphenyl-1,3,5-hexatriene was measured at varying lipid-to-protein molar ratios. Lysozyme was found to increase acyl-chain order for liposomes with the content of acidic phospholipid exceeding 10 mol %. Both electrostatic and hydrophobic protein-lipid interactions can be concluded to modulate the aggregation behavior of lysozyme when bound to lipid bilayers. Modulation of lysozyme aggregation propensity by membrane binding may have important implications for protein fibrillogenesis in vivo. Disruption of membrane integrity by the aggregated protein species is likely to be the mechanism responsible for the cytotoxicity of lysozyme.     

The transition state in the folding-unfolding reaction of four species of three-disulfide variant of hen lysozyme: the role of each disulfide bridge

The effects of lacking a specific disulfide bridge on the transition state in folding were examined in order to explore the folding-unfolding mechanism of lysozyme. Four species of three-disulfide variant of hen lysozyme (3SS-lysozyme) were prepared by replacing two Cys residues with Ala or Ser: C6S/C127A, C30A/C115A, C64A/C80A and C76A/C94A. The recombinant hen lysozyme was studied as the standard reference containing four authentic disulfide bridges and the extra N-terminal Met: the recombinant hen lysozyme containing the extra N-terminal. Folding rates were measured by monitoring the change in fluorescence intensity associated with tri-N-acetyl-d-glucosamine binding to the active site of refolded lysozyme. It was confirmed that the folding rate of the recombinant hen lysozyme containing the extra N-terminal was the same as that of wild-type lysozyme, and that the folding rate was little affected by the presence of tri-N-acetyl-d-glucosamine (triNAG). The folding rate of C64A/C80A was found to be the fastest and almost the same as that of the recombinant hen lysozyme containing the extra N-terminal, and that of C30A/C115A the second, and that of C6S/C127A the third. The folding rate of C76A/C94A was particularly slow. On the other hand, the unfolding rates which were measured in the presence of triNAG showed the dependence on the concentration of triNAG. The intrinsic unfolding rate in the absence of triNAG was determined by extrapolation. Also in the unfolding rate, C76A/C94A was markedly slower than the others. It was found from the analysis of binding constants of triNAG to C64A/C80A during the unfolding process that the active site of C64A/C80A partly unfolds already prior to the unfolding transition. On the basis of these kinetic data, we suggest that C64A/C80A folding transition can occur with leaving the loop region around SS3 (C64-C80) flexible, while cross-linking by SS4 (C76-C94) is important for the promotion of folding, because it is an indispensable constraint on the way towards the folding transition state.     

Selective accumulation of the high molecular weight neurofilament subunit within the distal region of growing axonal neurites.

Axonal maturation in situ is accompanied by the transition of neurofilaments (NFs) comprised of only NF-M and NF-L to those also containing NF-H. Since NF-H participates in interactions of NFs with each other and with other cytoskeletal constituents, its appearance represents a critical event in the stabilization of axons that accompanies their maturation. Whether this transition is effected by replacement of "doublet" NFs with "triplet" NFs, or by incorporation of NF-H into existing doublet NFs is unclear. To address this issue, we examined the distribution of NF subunit immunoreactivity within axonal cytoskeletons of differentiated NB2a/d1 cell and DRG neurons between days 3-7 of outgrowth. Endogenous immunoreactivity either declined in a proximal-distal gradient or was relatively uniform along axons. This distribution was paralleled by microinjected biotinylated NF-L. By contrast, biotinylated NF-H displayed a bipolar distribution, with immunoreactivity concentrated within the proximal- and distal-most axonal regions. Proximal biotinylated NF-H accumulation paralleled that of endogenous NF immunoreactivity; however, distal-most biotinylated NF-H accumulation dramatically exceeded that of endogenous NFs and microinjected NF-L. This phenomenon was not due to co-polymerization of biotin-H with vimentin or alpha-internexin. This phenomenon declined with continued time in culture. These data suggest that NF-H can incorporate into existing cytoskeletal structures, and therefore suggest that this mechanism accounts for at least a portion of the accumulation of triplet NFs during axonal maturation. Selective NF-H accumulation into existing cytoskeletal structures within the distal-most region may provide de novo cytoskeletal stability for continued axon extension and/or stabilization.     

Rapid isolation of flanking genomic DNA using biotin-RAGE, a variation of single-sided polymerase chain reaction.

A method is described for quickly and reproducibly isolating genomic DNA contiguous with known DNA sequence by means of the polymerase chain reaction (PCR). Flanking genomic DNA is isolated using a biotinylated sequence-specific primer in combination with a generic hybrid primer that binds to a deoxyoligonucleotide sequence artificially added to the ends of the genomic DNA. Amplified sequences that include the biotinylated primer are purified from nonbiotinylated amplification products by binding to a solid-phase streptavidin matrix. The biotinylated amplification product(s) are subjected to a further round of amplification, after which they can be subcloned and analyzed. This technique was applied to the isolation of three intron-exon junctions. Verification of the identify of these junction sequences was accomplished by designing primers based on the intron sequences isolated by Biotin-RAGE, amplifying across the exon using these intron primers, and sequencing the PCR-generated product.