Biomedical subjects

A C Steven

Publications and source records attributed to A C Steven.

At least 19 recordsLinked to original sources

PubMed1992

Conformational changes of a viral capsid protein. Thermodynamic rationale for proteolytic regulation of bacteriophage T4 capsid expansion, co-operativity, and super-stabilization by soc binding.

We have used differential scanning calorimetry in conjunction with cryo-electron microscopy to investigate the conformational transitions undergone by the maturing capsid of phage T4. Its precursor shell is composed primarily of gp23 (521 residues): cleavage of gp23 to gp23* (residues 66 to 521) facilitates a concerted conformational change in which the particle expands substantially, and is greatly stabilized. We have now characterized the intermediate states of capsid maturation; namely, the cleaved/unexpanded, state, which denatures at tm = 60 degrees C, and the uncleaved/expanded state, for which tm = 70 degrees C. When compared with the precursor uncleaved/unexpanded state (tm = 65 degrees C), and the mature cleaved/expanded state (tm = 83 degrees C, if complete cleavage precedes expansion), it follows that expansion of the cleaved precursor (delta tm approximately +23 degrees C) is the major stabilizing event in capsid maturation. These observations also suggest an advantage conferred by capsid protein cleavage (some other phage capsids expand without cleavage): if the gp23-delta domains (residues 1 to 65) are not removed by proteolysis, they impede formation of the stablest possible bonding arrangement when expansion occurs, most likely by becoming trapped at the interface between neighboring subunits or capsomers. Icosahedral capsids denature at essentially the same temperatures as tubular polymorphic variants (polyheads) for the same state of the surface lattice. However, the thermal transitions of capsids are considerably sharper, i.e. more co-operative, than those of polyheads, which we attribute to capsids being closed, not open-ended. In both cases, binding of the accessory protein soc around the threefold sites on the outer surface of the expanded surface lattice results in a substantial further stabilization (delta tm = +5 degrees C). The interfaces between capsomers appear to be relatively weak points that are reinforced by clamp-like binding of soc. These results imply that the "triplex" proteins of other viruses (their structural counterparts of soc) are likely also to be involved in capsid stabilization. Cryo-electron microscopy was used to make conclusive interpretations of endotherms in terms of denaturation events. These data also revealed that the cleaved/unexpanded capsid has an angular polyhedral morphology and has a pronounced relief on its outer surface. Moreover, it is 14% smaller in linear dimensions than the cleaved/expanded capsid, and its shell is commensurately thicker.

Allosteric Regulation↗

PubMed1992

Distinct monoclonal antibodies separately label the hexons or the pentons of herpes simplex virus capsid.

The surface shell of the capsid of herpes simplex virus type 1 (HSV-1) is 15 nm thick and 125 nm in outer diameter and has the form of an icosahedral (T = 16) surface lattice, composed of 150 hexons and 12 pentons. Hexons are traversed by axial channels and have six-fold symmetric external protrusions, separated by triangular nodules ("triplexes"). Pentons resemble hexons morphologically, apart from their different order of symmetry. To localize VP5, the major capsid protein, in the shell structure and to investigate whether pentons are composed of the same molecules as hexons, we have performed cryo-electron microscopy and three-dimensional image reconstructions of control HSV-1 B capsids and of B capsids immunoprecipitated with two monoclonal antibodies raised against purified VP5 and purified capsids. The results clearly map the epitope of the anti-VP5 monoclonal antibody to the distal tips of the hexon protrusions. In contrast, no detectable labeling of pentons was observed. We conclude that the hexon protrusions are domains of VP5 hexamers, other parts of these molecules forming the basic matrix of the capsid shell to which the other proteins are attached at specific sites. Conversely, the anti-capsid monoclonal antibody decorates the outer rim of pentons but does not bind to hexons. These observations imply that either pentons are composed of some other protein(s) or that they also contain VP5, but in a conformation sufficiently different from that assumed in hexons as to transform its antigenic character. Other evidence leads us to favor the latter alternative.

Antibodies, Monoclonal↗

PubMed1992

Magnification mismatches between micrographs: corrective procedures and implications for structural analysis.

Quantitative structural analysis from electron micrographs of biological macromolecules inevitably requires the synthesis of data from many parts of the same micrograph and, ultimately, from multiple micrographs. Higher resolutions require the inclusion of progressively more data, and for the particles analyzed to be consistent to within ever more stringent limits. Disparities in magnification between micrographs or even within the field of one micrograph, arising from lens hysteresis or distortions, limit the resolution of such analyses. A quantitative assessment of this effect shows that its severity depends on the size of the particle under study: for particles that are 100 nm in diameter, for example, a 2% discrepancy in magnification restricts the resolution to approximately 5 nm. In this study, we derive and describe the properties of a family of algorithms designed for cross-calibrating the magnifications of particles from different micrographs, or from widely differing parts of the same micrograph. This approach is based on the assumption that all of the particles are of identical size: thus, it is applicable primarily to cryo-electron micrographs in which native dimensions are precisely preserved. As applied to icosahedral virus capsids, this procedure is accurate to within 0.1-0.2%, provided that at least five randomly oriented particles are included in the calculation. The algorithm is stable in the presence of noise levels typical of those encountered in practice, and is readily adaptable to non-isometric particles. It may also be used to discriminate subpopulations of subtly different sizes.

Algorithms↗

PubMed1991

HIV-1 Rev expressed in recombinant Escherichia coli: purification, polymerization, and conformational properties.

The high-level expression of HIV-1 Rev in Escherichia coli is described. Protein in crude bacterial extracts was dissociated from bound nucleic acid with urea. A simple purification and renaturation protocol, monitored by circular dichroism, is described which results in high yields of pure protein. The purified protein binds with high affinity to the Rev-responsive element mRNA and has nativelike spectroscopic properties. The protein exhibits concentration-dependent self-association as judged by analytical ultracentrifugation and gel filtration measurements. Purified Rev showed reversible heat-induced aggregation over the temperature range 0-30 degrees C. This hydrophobic-driven and nonspecific protein association was inhibited by low concentrations of sulfate ions. Rev solutions at greater than 80 micrograms/mL, incubated at 0-4 degrees C, slowly polymerized to form long hollow fibers of 20-nm diameter. Filament formation occurs at a lower protein concentration and more rapidly in the presence of Rev-responsive mRNA. The nucleic acid containing filaments are about 8 nm in diameter and up to 0.4 micron in length. On the basis of physical properties of the purified protein, we have suggested that in the nucleus of infected cells, Rev binding to the Rev-responsive region of env mRNA may be followed by helical polymerization of the protein which results in coating of the nucleic acid. Coated nucleic acid could be protected from splicing in the nucleus and exported to the cytoplasm.

Circular Dichroism↗

PubMed1991

Liquid-crystalline, phage-like packing of encapsidated DNA in herpes simplex virus.

The organization of DNA within the HSV-1 capsid has been determined by cryoelectron microscopy and image reconstruction. Purified C-capsids, which are fully packaged, were compared with A-capsids, which are empty. Unlike A-capsids, C-capsids show fine striations and punctate arrays with a spacing of approximately 2.6 nm. The packaged DNA forms a uniformly dense ball, extending radially as far as the inner surface of the icosahedral (T = 16) capsid shell, whose structure is essentially identical in A-capsids and C-capsids. Thus we find no evidence for the inner T = 4 shell previously reported by Schrag et al. to be present in C-capsids. Encapsidated HSV-1 DNA closely resembles that previously visualized in bacteriophages T4 and lambda, thus supporting the idea of a close parallelism between the respective assembly pathways of a major family of animal viruses (the herpesviruses) and a major family of bacterial viruses.

Animals↗

PubMed1991

Glycine loops in proteins: their occurrence in certain intermediate filament chains, loricrins and single-stranded RNA binding proteins.

Quasi-repetitive, glycine-rich peptide sequences are widespread in at least three distinct families of proteins: the keratins and other intermediate filament proteins, including nuclear lamins; loricrins, which are major envelope components of terminally differentiated epithelial cells; and single-stranded RNA binding proteins. We propose that such sequences comprise a new structural motif termed the 'glycine loop'. The defining characteristics of glycine loop sequences are: (1) they have the form x(y)n, where x is usually an aromatic or occasionally a long-chain aliphatic residue; y is usually glycine but may include polar residues such as serine, asparagine, arginine, cysteine, and rarely other residues; and the value of n is highly variable, ranging from 1 to 35 in examples identified to date. (2) Glycine-loop-containing domains are thought to form when at least two and to date, as many as 18, such quasi-repeats are configured in tandem, so that the entire domain in a protein may be 50-150 residues long. (3) The average value of n, the pattern of residues found in the x position and the non-glycine substitutions in the y position appear to be characteristic of a given glycine loop containing domain, whereas the actual number of repeats is less constrained. (4) Glycine loop sequences display a high degree of evolutionary sequence variability and even allelic variations among different individuals of the same vertebrate species. (5) Glycine loop sequences are expected to be highly flexible, but possess little other regular secondary structure.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1991

Spatial disorders and computational cures.

Image averaging provides a powerful method for enhancing the yield of interpretable information from electron micrographs of biological macromolecules. However, as originally conceived, the full benefit of averaging is achieved only with perfectly ordered two-dimensional crystals. More recent developments, reviewed here, allow one to rectify disordered lattices, straighten randomly bent filaments, and combine multiple images of free-standing particles, thus extending the advantages of image averaging to virtually every class of macromolecular specimen.

Animals↗

PubMed1991

Image averaging of flexible fibrous macromolecules: the clathrin triskelion has an elastic proximal segment.

We have developed computational techniques that allow image averaging to be applied to electron micrographs of filamentous molecules that exhibit tight and variable curvature. These techniques, which involve straightening by cubic-spline interpolation, image classification, and statistical analysis of the molecules' curvature properties, have been applied to purified brain clathrin. This trimeric filamentous protein polymerizes, both in vivo and in vitro, into a wide range of polyhedral structures. Contrasted by low-angle rotary shadowing, dissociated clathrin molecules appear as distinctive three-legged structures, called "triskelions" (E. Ungewickell and D. Branton (1981) Nature 289, 420). We find triskelion legs to vary from 35 to 62 nm in total length, according to an approximately bell-shaped distribution (mu = 51.6 nm). Peaks in averaged curvature profiles mark hinges or sites of enhanced flexibility. Such profiles, calculated for each length class, show that triskelion legs are flexible over their entire lengths. However, three curvature peaks are observed in every case: their locations define a proximal segment of systematically increasing length (14.0-19.0 nm), a mid-segment of fixed length (approximately 12 nm), and a rather variable end-segment (11.6-19.5 nm), terminating in a hinge just before the globular terminal domain (approximately 7.3 nm diameter). Thus, two major factors contribute to the overall variability in leg length: (1) stretching of the proximal segment and (2) stretching of the end-segment and/or scrolling of the terminal domain. The observed elasticity of the proximal segment may reflect phosphorylation of the clathrin light chains.

Animals↗

PubMed1991

The maturation-dependent conformational change of phage T4 capsid involves the translocation of specific epitopes between the inner and the outer capsid surfaces.

After polymerization of the phage T4 prohead is complete, its capsid expands by approximately 16%, is greatly stabilized, and acquires the capacity to bind accessory proteins. These effects are manifestations of a large-scale, irreversible, conformational change undergone by the major capsid protein, gp23 (521 residues) which is cleaved to gp23* (residues 66-521) during this maturation process. In order to explore its structural basis, we have performed immunoelectron microscopy with antibodies raised against synthetic peptides that correspond to precisely defined segments of the amino acid sequence of gp23. These antibodies were used to label purified polyheads (tubular polymorphic variants of the normal icosahedral capsid), in experiments designed to impose constraints on the possible foldings of the gp23/gp23* polypeptide chains in their successive conformational states. Peptide 1 (residues 48-57), part of the gp23-delta domain that is excised when gp23 is converted to gp23*, resides on the inner surface of the precursor surface lattice, but--if not proteolyzed--is found on the outer surface of the mature surface lattice. Peptide 2 (residues 65-73), immediately distal to the cleavage site, is located on the inside of the precursor surface lattice, and remains there subsequent to expansion. Peptide 3 (residues 139-146) is translocated in the opposite direction from peptide 1, i.e., from the outer to the inner surface upon expansion; moreover, expansion greatly increases the polyheads' affinity for these antibodies. Peptide 5 (residues 301-308) is located on the inside in both the precursor and the mature states. Taking into account data from other sources, these observations imply that the conformational change that underlies capsid expansion involves a radical reorganization of the proteins' structure, in which at least three distinct epitopes, situated in widely differing parts of the polypeptide chain, are translocated from one side to the other. Moreover, the amino-terminal portion of gp23/gp23*, around the cleavage site, is particularly affected.

Amino Acid Sequence↗

PubMed1991

Surface topography and molecular stoichiometry of the mitochondrial channel, VDAC, in crystalline arrays.

The mitochondrial outer membrane contains a protein, called VDAC, that forms large aqueous pores. In Neurospora crassa outer membranes, VDAC forms two-dimensional crystalline arrays whose size and frequency can be greatly augmented by lipase treatment of these membranes (C. Mannella, Science 224, 165, 1984). Fourier filtration and surface reconstruction of freeze-dried/shadowed (45 degrees) arrays produced detailed images of two populations of crystals, whose lattices are mirror images of each other. Most likely, this technique has revealed both surfaces of the same two-dimensional crystal with lattice parameters: a = 12.3 +/- 0.1 nm, b = 11.2 +/- 0.1 nm, and theta = 109 +/- 1 degree. Three-dimensional reconstructions of the surface reliefs on both sides of the crystal show them to be very similar. The majority of the protein forming the channel appears to be at or below the level of the membrane. To address the issue of the number of 30-kDa polypeptides that form a VDAC channel, measurements of mass per unit area were carried out by analyzing scanning transmission electron micrographs of unstained, freeze-dried arrays. The crystal form used for mass analysis contained the same motif of six stain-accumulating centers per unit cell, with p2 symmetry as in the oblique configuration, but it had a different orientation relative to the lattice lines. These data yielded a surface density of 1.9 +/- 0.2 kDa/nm2, indicating that there is a one-to-one ratio between VDAC polypeptides and the channels visualized in filtered electron micrographs, and that VDAC membrane crystals contain 68% protein and 32% lipid by mass.

Crystallization↗

PubMed1991

gp160, the envelope glycoprotein of human immunodeficiency virus type 1, is a dimer of 125-kilodalton subunits stabilized through interactions between their gp41 domains.

The molecular masses, carbohydrate contents, oligomeric status, and overall molecular structure of the env glycoproteins of human immunodeficiency virus type 1--gp120, gp160, and gp41--have been determined by quantitative electron microscopy. Using purified gp160s, a water-soluble form of env purified from a recombinant vaccinia virus expression system, we have measured the masses of several hundred individual molecules by dark-field scanning transmission electron microscopy. When combined with sequence-based information, these mass measurements establish that gp160s is a dimer of subunits with an average monomer mass of 123 kDa, of which approximately 32 kDa is carbohydrate and 91 kDa is protein. Similarly, gp120 was found to be a monomer of 89 kDa and to contain virtually all of env's glycosylation. gp41 is glycosylated only slightly, if at all, and is responsible for the interactions that stabilize the gp160s dimer. A molecular mass map of gp160s derived by image processing depicts an asymmetric dumbbell whose two domains have masses of approximately 173 and approximately 73 kDa, corresponding to a gp120 dimer and a gp41 dimer, respectively. We infer that the average monomer mass of native gp160 is 125 kDa and that in situ, env is either a dimer or a tetramer but is most unlikely to be a trimer.

Gene Products, env↗

PubMed1990

Identification of a major keratinocyte cell envelope protein, loricrin.

During epidermal cell cornification, the deposition of a layer of covalently cross-linked protein on the cytoplasmic face of the plasma membrane forms the cell envelope. We have isolated and characterized cDNA clones encoding a major differentiation product of mouse epidermal cells, which has an amino acid composition similar to that of purified cell envelopes. Transcripts of this gene are restricted to the granular layer and are as abundant as the differentiation-specific keratins, K1 and K10. An antiserum against a C-terminal peptide localizes this protein in discrete granules in the stratum granulosum and subsequently at the periphery of stratum corneum cells. Immunofluorescence and immunoelectron microscopy detect this epitope only on the inner surface of purified cell envelopes. Taken together, these results suggest that it is a major component of cell envelopes. On the basis of its presumed function, this protein is named loricrin.

Amino Acid Sequence↗

PubMed1990

The maturation-dependent conformational change of the major capsid protein of bacteriophage T4 involves a substantial change in secondary structure.

We have investigated the conformational basis of the expansion transformation that occurs upon maturation of the bacteriophage T4 prohead, by using laser Raman spectroscopy to determine the secondary structure of the major capsid protein in both the precursor and the mature states of the surface lattice. This transformation involves major changes in the physical, chemical, and immunological properties of the capsid and is preceded in vivo by processing of its major protein, gp23 (56 kDa), to gp23* (49 kDa), by proteolysis of its N-terminal gp23-delta domain. The respective secondary structures of gp23 in the unexpanded state, and of gp23* in the expanded state, were determined from the laser Raman spectra of polyheads, tubular polymorphic variants of the capsid. Similar measurements were also made on uncleaved polyheads that had been expanded in vitro and, for reference, on thermally denatured polyheads. We find that, with or without cleavage of gp23, expansion is accompanied by substantial changes in secondary structure, involving a major reduction in alpha-helix content and an increase in beta-sheet. The beta-sheet contents of gp23* or gp23 in the expanded state of the surface lattice, and even of gp23 in the unexpanded state, are sufficient for a domain with the "jellyroll" fold of antiparallel beta-sheets, previously detected in the capsid proteins of other icosahedral viruses.(ABSTRACT TRUNCATED AT 250 WORDS)

Capsid↗

PubMed1990

Three-dimensional structures of maturable and abortive capsids of equine herpesvirus 1 from cryoelectron microscopy.

Cryoelectron microscopy and three-dimensional computer reconstruction techniques have been used to compare the structures of two types of DNA-free capsids of equine herpesvirus 1 at a resolution of 4.5 nm. "Light" capsids are abortive, whereas "intermediate" capsids are related to maturable intracellular precursors. Their T = 16 icosahedral outer shells, approximately 125 nm in diameter, are indistinguishable and may be described in terms of three layers of density, totalling 15 nm in thickness. The outermost layer consists of protruding portions of both the hexon and the penton capsomers, rising approximately 5 nm above a midlayer of density. The innermost layer, or "floor," is a 4-nm-thick sheet of virtually continuous density except for the orifices of the channels that traverse each capsomer. Hexon protrusions are distinctly hexagonal in shape, and penton protrusions are pentagonal. The structures of the three kinds of hexons (distinguished according to their positions on the surface lattice) are closely similar but differ somewhat in their respective orientations and in the shapes of their channels. The most prominent features of the midlayer are threefold nodules ("triplexes") at the trigonal lattice points. By analogy with other viral capsids, the triplexes may represent trimers of another capsid protein, possibly VP23 (36 kilodaltons [kDa]) or VP26 (12 kDa). Intermediate capsids differ from light capsids, which are empty, in having one or more internal components. In individual images from which the shell structure has been filtered away, these components are seen to have dimensions of 20 to 30 nm but to lack a visible substructure. This material--which is smeared out in the reconstruction, implying that its distribution is not icosahedrally symmetric or necessarily consistent from particle to particle--consists of aggregates of VP22 (46 kDa). From several lines of evidence, we conclude that this protein is located entirely within the capsid shell. These aggregates may be the remnants of morphogenetic cores retained in capsids interrupted in the process of DNA packaging.

Animals↗

PubMed1990

Molecular structure of the cell-attachment protein of reovirus: correlation of computer-processed electron micrographs with sequence-based predictions.

The receptor-recognition interaction that initiates reovirus infection is mediated by the sigma 1 protein, located at the vertices of the icosahedral virion. We have applied computer-based image-averaging techniques to electron micrographs of negatively stained preparations of sigma 1 purified from virions (serotype 2 Jones). Combining these results with inferences based on the amino acid sequence has led to a molecular model in which the overall folding of the chains is described; its conformation embodies motifs, coiled-coil alpha-helices and nodular multichain elements rich in beta-sheets, previously detected in the corresponding proteins of other viruses, but with some novel variations. Sigma 1 is a filamentous lollipop-shaped molecule with an overall length of approximately 48 nm; it has a flexible "tail," approximately 40 nm long by 4 to 6 nm wide, terminating at its distal end in a globular "head," approximately 9.5 nm in diameter. The purified protein is a tetramer (4 by 50 kilodaltons) consisting of two similarly oriented dimers bonded side by side and in register. For each chain, a cluster of hydrophobic residues at its amino terminus resides at the proximal end of the tail; next, an alpha-helical domain (residues 25 to 172) participates in a two-chained coiled coil, 22 nm long, with two such coiled coils pairing laterally to form the proximal half of the tail. The remainder of the tail (residues 173 to approximately 316) is less uniform in width and is expected to be rich in beta-sheet; the interdimer bonding is evidently sustained through this portion of the molecule. Finally, the globular head consists of the carboxy-terminal domains (which contain the receptor-binding sites) folded into compact globular conformations; in appropriate side views, the head is resolved into two subunits, presumably contributed by the respective dimers. This model for how the four sigma 1 polypeptide chains are threaded in parallel through the fiber is supported by the observed match between an empirical curvature profile, which identifies the locations of relatively flexible sites along the tail, and the flexibility profile predicted on the basis of the model. Appraisal of the interactions that stabilize the coiled coils suggests that (i) the alpha-helices are individually only marginally stable, a property that may be of significance with regard to the retracted conformation in which sigma 1 is accommodated in the intact virion, and (ii) the predominant interactions between the two coiled coils are likely to involve hydrogen bonding between patches of uncharged residues.

Amino Acid Sequence↗

PubMed1989

Interactions between actin and myosin filaments in skeletal muscle visualized in frozen-hydrated thin sections.

For the purpose of determining net interactions between actin and myosin filaments in muscle cells, perhaps the single most informative view of the myofilament lattice is its averaged axial projection. We have studied frozen-hydrated transverse thin sections with the goal of obtaining axial projections that are not subject to the limitations of conventional thin sectioning (suspect preservation of native structure) or of equatorial x-ray diffraction analysis (lack of experimental phases). In principle, good preservation of native structure may be achieved with fast freezing, followed by low-dose electron imaging of unstained vitrified cryosections. In practice, however, cryosections undergo large-scale distortions, including irreversible compression; furthermore, phase contrast imaging results in a nonlinear relationship between the projected density of the specimen and the optical density of the micrograph. To overcome these limitations, we have devised methods of image restoration and generalized correlation averaging, and applied them to cryosections of rabbit psoas fibers in both the relaxed and rigor states. Thus visualized, myosin filaments appear thicker than actin filaments by a much smaller margin than in conventional thin sections, and particularly so for rigor muscle. This may result from a significant fraction of the myosin S1-cross-bridges averaging out in projection and thus contributing only to the baseline of projected density. Entering rigor incurs a loss of density from an annulus around the myosin filament, with a compensating accumulation of density around the actin filament. This redistribution of mass represents attachment of the fraction of cross-bridges that are visible above background. Myosin filaments in the "nonoverlap" zone appear to broaden on entering rigor, suggesting that on deprivation of ATP, cross-bridges in situ move outwards even without actin in their immediate proximity.

Actins↗

PubMed1989

Nucleocapsid mass and capsomer protein stoichiometry in equine herpesvirus 1: scanning transmission electron microscopic study.

The Brookhaven scanning transmission electron microscope was employed to measure the masses of two nucleocapsid species (of light and intermediate densities) of equine herpesvirus 1. These were found to be 196.7 +/- 9.2 and 229.0 +/- 9.5 megadaltons (MDa), respectively. Biochemical assays showed that neither nucleocapsid contained any significant amount of DNA (less than 0.2% [wt/wt]). Taking into account data on protein composition, we conclude that the difference between their masses is essentially contributed by viral protein 22 (46 kDa), which is an integral component of the maturable intermediate nucleocapsid but not of the abortive light nucleocapsid. In view of earlier ultrastructural information on capsomer symmetry, our mass determinations are consistent only with the 150 hexavalent capsomers being hexamers of the 148-kDa major capsid protein.

Animals↗

PubMed1988

The complete sequence of the human intermediate filament chain keratin 10. Subdomainal divisions and model for folding of end domain sequences.

We present the complete amino acid sequence of the human keratin 10 (type I) intermediate filament chain expressed in terminally differentiated epidermal cells. Comparisons of this sequence with its mouse and bovine counterparts allow us to describe structural features of the functional end domains. First, sections of their respective end domains are highly conserved and permit a redefinition of earlier models for their subdomainal organization. The amino-terminal end domain consists of El, the first 57-58 residues that are basic, glycine-rich, and have been highly conserved among the three species; V1, a region of well-defined quasi repeats of the motif aliphatic-serine/glycinen; and H1, a newly recognized short acidic sequence that has been conserved among the type I keratin family. The carboxyl-terminal end consists of V2 and E2 whose properties but not sequence resemble V1 and E1, respectively. Second, since the E1, H1, and E2 sequences have been highly conserved between the three species, we suggest they are critical elements in defining intermediate filament function. Third, we note that the E and V sequences of the keratin 10 (and other keratin) chains share many properties in common with protein chain turns found in globular proteins. We therefore propose a model in which these sequences form omega loop-like structures (Leszczynski, J. N. & Rose, G. D. (1986) Science 234, 849-855) on the surface of keratin intermediate filaments. This represents the first specific proposal for the end domain structure of any intermediate filament chain.

Amino Acid Sequence↗