The double helix coiled coil structure of murein lipoprotein from Escherichia coli

Strategies for Site-Specific Labeling of Receptor Proteins on the Surfaces of Living Cells by Using Genetically Encoded Peptide Tags

Fluorescence microscopy imaging enables receptor proteins to be investigated within their biological context. A key challenge is to site-specifically incorporate reporter moieties into proteins without interfering with biological functions or cellular networks. Small peptide tags offer the opportunity to combine inducible labeling with small tag sizes that avoid receptor perturbation. Herein, we review the current state of live-cell labeling of peptide-tagged cell-surface proteins. Considering their importance as targets in medicinal chemistry, we focus on membrane receptors such as G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). We discuss peptide tags that i) are subject to enzyme-mediated modification reactions, ii) guide the complementation of reporter proteins, iii) form coiled-coil complexes, and iv) interact with metal complexes. Given our own contributions in the field, we place emphasis on peptide-templated labeling chemistry.

Peptide-PAINT Super-Resolution Imaging Using Transient Coiled Coil Interactions

Super-resolution microscopy is transforming research in the life sciences by enabling the visualization of structures and interactions on the nanoscale. DNA-PAINT is a relatively easy-to-implement single-molecule-based technique, which uses the programmable and transient interaction of dye-labeled oligonucleotides with their complements for super-resolution imaging. However, similar to many imaging approaches, it is still hampered by the subpar performance of labeling probes in terms of their large size and limited labeling efficiency. To overcome this, we here translate the programmability and transient binding nature of DNA-PAINT to coiled coil interactions of short peptides and introduce Peptide-PAINT. We benchmark and optimize its binding kinetics in a single-molecule assay and demonstrate its super-resolution capability using self-assembled DNA origami structures. Peptide-PAINT outperforms classical DNA-PAINT in terms of imaging speed and efficiency. Finally, we prove the suitability of Peptide-PAINT for cellular super-resolution imaging by visualizing the microtubule and vimentin network in fixed cells.

Coiled Coils - A Model System for the 21st Century

α-Helical coiled coils were described more than 60 years ago as simple, repetitive structures mediating oligomerization and mechanical stability. Over the past 20 years, however, they have emerged as one of the most diverse protein folds in nature, enabling many biological functions beyond mechanical rigidity, such as membrane fusion, signal transduction, and solute transport. Despite this great diversity, their structures can be described by parametric equations, making them uniquely suited for rational protein design. Far from having been exhausted as a source of structural insight and a basis for functional engineering, coiled coils are poised to become even more important for protein science in the coming decades.

Distinct regulatory mechanisms balance DegP proteolysis to maintain cellular fitness during heat stress

Intracellular proteases combat proteotoxic stress by degrading damaged proteins, but their activity must be controlled to maintain cellular fitness. DegP is a conserved periplasmic protease essential for E. coli growth at high temperatures. Kim and Sauer investigate how allosteric activation and polyhedral cage formation contribute to DegP function and cellular fitness. The results suggest that allosteric control of active and inactive conformations is the primary mechanism regulating DegP proteolysis and cellular fitness, with cage assembly providing an additional layer of protection against excessive protease activity.

Intracellular proteases combat proteotoxic stress by degrading damaged proteins, but their activity must be carefully controlled to maintain cellular fitness. The activity of Escherichia coli DegP, a highly conserved periplasmic protease, is regulated by substrate-dependent allosteric transformations between inactive and active trimer conformations and by the formation of polyhedral cages that confine the active sites within a proteolytic chamber. Here, we investigate how these distinct control mechanisms contribute to bacterial fitness under heat stress. We found that mutations that increase or decrease the equilibrium population of active DegP trimers reduce high-temperature fitness, that a mutation that blocks cage formation causes a mild fitness decrease, and that combining mutations that stabilize active DegP and block cage formation generates a lethal rogue protease. This lethality is suppressed by an extragenic mutation that prevents covalent attachment of an abundant outer-membrane lipoprotein to peptidoglycan and makes this protein an inhibitor of the rogue protease. Lethality is also suppressed by intragenic mutations that stabilize inactive DegP trimers. In combination, our results suggest that allosteric control of active and inactive conformations is the primary mechanism that regulates DegP proteolysis and fitness, with cage formation providing an additional layer of cellular protection against excessive protease activity.

Core structure of the outer membrane lipoprotein from Escherichia coli at 1.9 A resolution

The outer membrane lipoprotein of the Escherichia coli cell envelope has characteristic lipid modifications at an amino-terminal cysteine and can exist in a form bound covalently to the peptidoglycan through a carboxyl-terminal lysine. The 56-residue polypeptide moiety of the lipoprotein, designated Lpp-56, folds into a stable, trimeric helical structure in aqueous solution. The 1.9 A resolution crystal structure of Lpp-56 comprises a parallel three-stranded coiled coil including a novel alanine-zipper unit and two helix-capping motifs. The amino-terminal motif forms a hydrogen-bonding network anchoring an umbrella-shaped fold. The carboxyl-terminal motif uses puckering of the tyrosine side-chains as a unique docking arrangement in helix termination. The structure provides an explanation for assembly and insertion of the lipoprotein molecules into the outer membrane of gram-negative bacteria and suggests a molecular target for antibacterial drug discovery.

Modeling alpha-helical coiled coils: analytic relations between parameters

This paper deals with the alpha-helical coiled coil secondary structure of proteins, which is found not only in many fibrous proteins but also in globular proteins. The standard model used nowadays to describe a coiled coil structure is derived from the mathematical description established more than 40 years ago by F. H. C. Crick (1953, Acta Crystallogr. 6, 685-689) from geometrical arguments. In this paper, we apply stereochemical constraints to the protein chains to refine this model. We present a model based on Crick's calculations with less restrictive hypotheses than the standard model and only requiring a set of initial parameters that can be experimentally measured. In addition, the metrics equation method developed here ensures a minimization of the distortions occurring during the coiling process relating the original straight alpha-helix and the coiled coil minor helix. It leads to a modification of the widely used relation between the numbers of residues per turn in the minor and alpha-helices, mathematically demonstrating a previously semiempirical result. This method can be extended to a wide range of coiled structures.

Alignment and structure prediction of divergent protein families: periplasmic and outer membrane proteins of bacterial efflux pumps

Broad-specificity efflux pumps have been implicated in multidrug-resistant strains of Pseudomonas aeruginosa and other Gram-negative bacteria. Most Gram-negative pumps of clinical relevance have three components, an inner membrane transporter, an outer membrane channel protein, and a periplasmic protein, which together coordinate efflux from the cytoplasmic membrane across the outer membrane through an unknown mechanism. The periplasmic efflux proteins (PEPs) and outer membrane efflux proteins (OEPs) are not obviously related to proteins of known structure, and understanding the structure and function of these proteins has been hindered by the difficulty of obtaining reasonable multiple alignments. We present a general strategy for the alignment and structure prediction of protein families with low mutual sequence similarity using the PEP and OEP families as detailed examples. Gibbs sampling, hidden Markov models, and other analysis techniques were used to locate motifs, generate multiple alignments, and assign PEP or OEP function to hypothetical proteins in several species. We also developed an automated procedure which combines multiple alignments with structure prediction algorithms in order to identify conserved structural features in protein families. This process was used to identify a probable alpha-helical hairpin in the PEP family and was applied to the detection of transmembrane beta-strands in OEPs. We also show that all OEPs contain a large tandem duplication, and demonstrate that the OEP family is unlikely to adopt a porin fold, in contrast to previous predictions.

Side-chains configurations in coiled coils revealed by the 5.15-A meridional reflection on hard alpha-keratin X-ray diffraction patterns

The origin of the 5.15-A meridional reflection on hard alpha-keratin X-ray diffraction patterns is discussed in terms of side-chains conformations. We show it to reveal specific configurations of the side chains which are common to all two-stranded alpha-helical coiled coils. Combining literature data on crystallised coiled coil pieces and molecular dynamics results with our X-ray diffraction pattern simulations, we propose rules for the attribution of chi1 torsion angles for coiled coils involved in fibres whose structure cannot be resolved at atomic resolution: in a (a b c d e f g) heptad repeat, a and d residues, respectively, adopt mean t and g+ configurations, whereas statistical rules are given for the other residues.

Boehringer Mannheim award lecture 1995. La conference Boehringer Mannheim 1995. De novo design of alpha-helical proteins: basic research to medical applications

The two-stranded alpha-helical coiled-coil is a universal dimerization domain used by nature in a diverse group of proteins. The simplicity of the coiled-coil structure makes it an ideal model system to use in understanding the fundamentals of protein folding and stability and in testing the principles of de novo design. The issues that must be addressed in the de novo design of coiled-coils for use in research and medical applications are (i) controlling parallel versus antiparallel orientation of the polypeptide chains, (ii) controlling the number of helical strands in the assembly (iii) maximizing stability of homodimers or heterodimers in the shortest possible chain length that may require the engineering of covalent constraints, and (iv) the ability to have selective heterodimerization without homodimerization, which requires a balancing of selectivity versus affinity of the dimerization strands. Examples of our initial inroads in using this de novo design motif in various applications include: heterodimer technology for the detection and purification of recombinant peptides and proteins; a universal dimerization domain for biosensors; a two-stage targeting and delivery system; and coiled-coils as templates for combinatorial helical libraries for basic research and drug discovery and as synthetic carrier molecules. The universality of this dimerization motif in nature suggests an endless number of possibilities for its use in de novo design, limited only by the creativity of peptide-protein engineers.

Repacking protein cores with backbone freedom: structure prediction for coiled coils

Progress in homology modeling and protein design has generated considerable interest in methods for predicting side-chain packing in the hydrophobic cores of proteins. Present techniques are not practically useful, however, because they are unable to model protein main-chain flexibility. Parameterization of backbone motions may represent a general and efficient method to incorporate backbone relaxation into such fixed main-chain models. To test this notion, we introduce a method for treating explicitly the backbone motions of alpha-helical bundles based on an algebraic parameterization proposed by Francis Crick in 1953 [Crick, F. H. C. (1953) Acta Crystallogr. 6, 685-689]. Given only the core amino acid sequence, a simple calculation can rapidly reproduce the crystallographic main-chain and core side-chain structures of three coiled coils (one dimer, one trimer, and one tetramer) to within 0.6-A root-mean-square deviations. The speed of the predictive method [approximately 3 min per rotamer choice on a Silicon Graphics (Mountain View, CA) 4D/35 computer] permits it to be used as a design tool.

Characterization of lipoprotein EnvA in Chlamydia psittaci 6BC

The primary sequence of the small cysteine-rich protein (EnvA) of Chlamydia psittaci 6BC has been shown to possess a potential lipid modification/signal peptidase II-processing site, and the mature protein was labeled by a [3H]palmitic acid precursor. We further characterized the mature EnvA, showing that it lacks the N-terminal methionine of the primary peptide, is hydrophobic despite a peptide sequence that is predicted to be hydrophilic, and appears to be lipid modified at an N-terminal cysteine in a manner analogous to that of murein lipoproteins of gram-negative bacteria. We also report the fatty acid content of the small cysteine-rich proteins of C. psittaci and Chlamydia trachomatis L2 as determined by combined gas chromatography-mass spectrometry.

Rational design of a three-heptad coiled-coil protein and comparison by molecular dynamics simulation with the GCN4 coiled coil: presence of interior three-center hydrogen bonds

alpha-Helical coiled coils have a 7-residue repeating pattern (abcdefg) where a and d are usually hydrophobic. We have designed a 2-stranded 44-residue coiled-coil protein (P44) consisting of 2 22-residue alpha-helices linked by 2 terminal disulfide groups to test whether the disulfide bridges could stabilize a 3-heptad coiled coil. P44 should be stabilized by intrahelical hydrogen bonds, interhelical disulfide and salt bridges, and interior hydrophobic interactions. A computer model of P44 was built and its stability was studied by molecular dynamics simulation with explicit water. This doubly crosslinked 3-heptad coiled coil did not unfold during a 300-ps simulation with explicit water. This doubly crosslinked 3-heptad coiled coil did not unfold during a 300-ps simulation. But reduced P44 with 4 thiol groups did unfold. For comparison, the 62-residue crystal structure of the 4-heptad coiled coil of transcription activator GCN4 did not unfold during a 300-ps simulation. Thus P44 may be a stable folded protein in aqueous solution. These simulations revealed the presence of 2 local hydrogen bond networks involving intra-helical 3-center hydrogen bonds in the hydrophobic interior of the coiled coils of GCN4 and P44. The NH hydrogen at d makes a 3-center hydrogen bond whose major component is to the i - 4 C = O oxygen at g and minor component is to the solvent-inaccessible i - 3 C = O oxygen at a. Likewise, the NH hydrogen at g makes a 3-center hydrogen bond with the i - 4 C = O oxygen at c and the buried i - 3 C = O oxygen at d.

Packing and hydrophobicity effects on protein folding and stability: effects of beta-branched amino acids, valine and isoleucine, on the formation and stability of two-stranded alpha-helical coiled coils/leucine zippers

The aim of this study was to examine the differences between hydrophobicity and packing effects in specifying the three-dimensional structure and stability of proteins when mutating hydrophobes in the hydrophobic core. In DNA-binding proteins (leucine zippers), Leu residues are conserved at positions "d," and beta-branched amino acids, Ile and Val, often occur at positions "a" in the hydrophobic core. In order to discern what effect this selective distribution of hydrophobes has on the formation and stability of two-stranded alpha-helical coiled coils/leucine zippers, three Val or three Ile residues were simultaneously substituted for Leu at either positions "a" (9, 16, and 23) or "d" (12, 19, and 26) in both chains of a model coiled coil. The stability of the resulting coiled coils was monitored by CD in the presence of Gdn.HCl. The results of the mutations of Ile to Val at either positions "a" or "d" in the reduced or oxidized coiled coils showed a significant hydrophobic effect with the additional methylene group in Ile stabilizing the coiled coil (delta delta G values range from 0.45 to 0.88 kcal/mol/mutation). The results of mutations of Leu to Ile or Val at positions "a" in the reduced or oxidized coiled coils showed a significant packing effect in stabilizing the coiled coil (delta delta G values range from 0.59 to 1.03 kcal/mol/mutation). Our results also indicate the subtle control hydrophobic packing can have not only on protein stability but on the conformation adopted by the amphipathic alpha-helices. These structural findings correlate with the observation that in DNA-binding proteins, the conserved Leu residues at positions "d" are generally less tolerant of amino acid substitutions than the hydrophobic residues at positions "a."

Design, synthesis and structural characterization of model heterodimeric coiled-coil proteins

We report the design and synthesis of model heterodimeric coiled-coil proteins and the packing contribution of interchain hetero-hydrophobic side-chains to coiled-coil stability. The heterodimeric coiled-coils are obtained by oxidizing two 35-residue polypeptide chains, each containing a cysteine residue at position 2 and differing in amino acid sequences in the hydrophobic positions ("a" and "d") responsible for the formation and stabilization of the coiled-coil. In each peptide, a single Ala residue was substituted for Leu at position "a" or "d". The formation and stability of heterodimeric coiled-coils were investigated by circular dichroism studies in the presence and absence of guanidine hydrochloride and compared to the corresponding homodimeric coiled-coils. The coiled-coil proteins with an Ala substitution at position "a" were less stable than those with an Ala substitution at position "d" in both the homodimeric (Ala-Ala interchain interactions) and heterodimeric (Leu-Ala interchain interactions ) coiled-coils. The 70-residue disulfide bridged peptides (homo- and heterodimeric coiled-coils) can be readily separated by reversed-phase chromatography (RPC) even though they have identical amino acid compositions as well as in the hydrophobic "a" and "d" positions. The elution of the 70-residue peptides prior to their corresponding 35-residue monomers suggests that these proteins are retaining a large portion of their coiled-coil structure during RPC at pH2 and their retention behavior correlates with protein stability.

X-ray structure of the GCN4 leucine zipper, a two-stranded, parallel coiled coil

The x-ray crystal structure of a peptide corresponding to the leucine zipper of the yeast transcriptional activator GCN4 has been determined at 1.8 angstrom resolution. The peptide forms a parallel, two-stranded coiled coil of alpha helices packed as in the "knobs-into-holes" model proposed by Crick in 1953. Contacts between the helices include ion pairs and an extensive hydrophobic interface that contains a distinctive hydrogen bond. The conserved leucines, like the residues in the alternate hydrophobic repeat, make side-to-side interactions (as in a handshake) in every other layer of the dimer interface. The crystal structure of the GCN4 leucine zipper suggests a key role for the leucine repeat, but also shows how other features of the coiled coil contribute to dimer formation.

The wac gene product of bacteriophage T4 contains coiled-coil structural patterns

The bacteriophage T4 late gene wac (whisker antigen control) encodes the protein which forms the fibrous structure on the neck of the virion called whiskers. Amino acid sequence analysis of wac gene product, as deduced from the nucleotide sequence, indicate ten alpha-helical domains (19-40 residues long) with coiled-coil structural patterns. These regions comprise about 70% of the entire 486 amino acid sequence. The alpha-helices are separated by short stretches of polypeptide chain which are similar to the loop regions of the globular protein sequences. We propose a structural model for the dimer of wac gene product molecule, that we call fibritin in which two polypeptide chains associate in a parallel fashion and form a segmented alpha-helical coiled-coil rod similar to epidermal keratins.

X-ray scattering indicates that the leucine zipper is a coiled coil

Dimerization of the bZIP class of eukaryotic transcriptional control proteins requires a sequence motif called the leucine zipper. We have grown two distinct crystal forms of a 33-amino acid peptide corresponding to the leucine zipper of the yeast transcriptional activator GCN4. This peptide is known to form a dimer of parallel helices in solution. X-ray scattering from both crystal forms shows reflections that are diagnostic of coiled coils. The most notable reflections occur at approximately 5.2 A resolution and correspond to the pitch of helices in coiled coils. There is no diffraction maximum near 5.4 A, the characteristic pitch of straight helices. Our results provide direct evidence that the leucine zipper of GCN4 is a coiled coil.

Evidence that the leucine zipper is a coiled coil

Recently, a hypothetical structure called a leucine zipper was proposed that defines a new class of DNA binding proteins. The common feature of these proteins is a region spanning approximately 30 amino acids that contains a periodic repeat of leucines every seven residues. A peptide corresponding to the leucine zipper region of the yeast transcriptional activator GCN4 was synthesized and characterized. This peptide associates in the micromolar concentration range to form a very stable dimer of alpha helices with a parallel orientation. Although some features of the leucine zipper model are supported by our experimental data, the peptide has the characteristics of a coiled coil.

Conserved lipoprotein H.8 of pathogenic Neisseria consists entirely of pentapeptide repeats

The pathogenic Neisseria, N. gonorrhoeae and N. meningitidis, possess an outer membrane protein (OMP), designated H.8, with a conserved monoclonal antibody (MAb)-binding epitope. We determined the DNA sequence of a gonococcal H.8 gene, and confirmed the relationship between the cloned gene and the H.8 OMP by constructing a gonococcal mutant lacking H.8. The predicted H.8 OMP is a lipoprotein 71 amino acids in length, composed of 13 repeats of a consensus sequence AAEAP with perfect 5-residue periodicity. The AAEAP units form a repeating epitope that comprises the entire predicted sequence of the protein.

Structure of the ColE1 rop protein at 1.7 A resolution

Structural details of the Rop protein from plasmid ColE1 are presented, with a description of the X-ray crystal structure determination and refinement at a nominal resolution of 1.7 A. The 63 amino acid protein is a dimer. Each monomer consists almost entirely of two alpha helices, the whole molecule forming a highly regular four-alpha-helix bundle. This may be approximated by a four-stranded rope with a radius of 7.0 A, a left-handed helical twist and a pitch of 172.5 A. The packing constraints for this novel type of coiled-coil structure are given. The protein acts in the control of plasmid replication via regulation of an RNA-RNA interaction in a manner not yet understood in atomic detail.

Ultrastructure of native lipoprotein from Escherichia coli envelopes

The free form of the major lipoprotein from Escherichia coli cells envelopes has been purified to homogeneity by gentle extraction procedures and conventional chromatographic separations in a non-ionic detergent. The morphology of paracrystals obtained from homogeneous protein was investigated by low-dose electron microscopy. Electron diffraction of the paracrystals was consistent with alpha-helices arranged perpendicularly to the main cross-band with a periodicity of 20 nm.

Presence of two distinct regions in the coiled-coil structure of the streptococcal Pep M5 protein: relationship to mammalian coiled-coil proteins and implications to its biological properties

The complete amino acid sequence of Pep M5, a biologically active 197-residue fragment comprising nearly half of the group A streptococcal M5 protein, has structural features characteristic of an alpha-helical coiled-coil protein. Fourier analyses of the nonpolar residues show strong periodicities based on repeats of 7 residues (7/2 and 7/3). Except for the nonhelical NH2-terminal 12-residue segment, the 7-residue periodicity in the distribution of nonpolar residues extends through the remainder of the Pep M5 molecule, with some discontinuities and irregularities. The molecule contains two distinct regions that differ in the pattern of distribution of the nonpolar and charged residues. The 7-residue pattern "a, b, c, d, e, f, g" in region 13-121 is atypical in that position "a" is predominantly occupied by asparagine, rather than nonpolar residues. On the other hand, the periodicity in region 122-196 is more typical of that found in other coiled-coil proteins, such as the myosin rod region, keratin, desmin, and vimentin, rather than tropomyosin. Although the periodicity in nonpolar residues is not highly regular, the predominance of basic and acidic residues in the inner "e" and "g" positions, respectively, suggests that ionic interactions between chains may contribute significantly to the stability of the coiled-coil. The distribution of charged residues in the outer positions within the two regions of the molecule is also distinct. The NH2-terminal region carries a significantly higher net negative charge than the COOH-terminal region, suggesting that the former region may play an important role in some of the biological functions of the Pep M5 molecule.

A two-dimensional representation of relative orientations of alpha-helix residues

Simple rules for constructing a two-dimensional representation of relative orientations of alpha-helix residues are presented. The repeat units in tropomyosin and haemagglutinin, together with their interrelationships, are also illustrated.

Streptococcal M protein: alpha-helical coiled-coil structure and arrangement on the cell surface

The conformation and molecular dimensions of purified type 6 streptococcal M proteins establish the close structural relationship of these molecules to tropomyosin. Ultracentrifuge studies reveal that the M molecules exist as stable dimers; circular dichroism spectra indicate that the molecules contain about 70% alpha helix; and fiber x-ray diffraction diagrams show the characteristic reflections of the alpha-helical pattern. Electron microscopic images of M protein shadowed with platinum reveal rod-shaped molecules having the same width as tropomyosin. However, the lengths of the M molecules are about 30% shorter than lengths predicted by assuming a completely alpha-helical molecule. These findings indicate that the structure of the M6 protein is primarily alpha-helical coiled coil. Comparison of the lengths of the fibers on the surface of the streptococcus and the isolated M proteins suggests that each fiber on the cell wall consists of a single M-protein molecule approximately 500 A long. The structure determined for these fimbriae is the first alpha-helical coiled-coil conformation to be demonstrated for bacterial surface projections.

Conversion of free lipoprotein to the murein-bound form

The time-course of the conversion of free lipoprotein to the murein-bound form was followed after pulsing growing cells of Escherichia coli for 2 min with [35S]methionine. The conversion of the free to the bound form was measured by two independent methods. First, the appearance of murein-bound lipoprotein was determined in isolated murein sacculi after extraction with hot 4% sodium dodecylsulphate. Second, the ratio of various forms of the bound lipoprotein to the free lipoprotein was determined in outer membranes. Both methods indicated that the half-maximal amount of bound lipoprotein was reached within one-sixth of a generation time.

Outer-membrane vesicles released by normally growing Escherichia coli contain very little lipoprotein

The lipoprotein content of the outer-membrane medium vesicles, which are released from Escherichia coli during normal growth, was compared to the lipoprotein content of the corresponding cellular outer membranes. It was found that the medium vesicles contained only 35% free lipoprotein and almost none of the bound lipoprotein when compared with cellular outer membranes. Medium vesicles also had reduced amounts of protein II and a protein V (Mr = 16 000), while they contained large amounts of pore-forming proteins I and lamB. A mechanism is proposed in which outer membrane vesicles are formed when the outer membrane expands faster than the underlying peptidoglycan layer. The lack or enrichment of individual proteins in medium vesicles may be determined by their interactions with the peptidoglycan-bound lipoprotein complex.

Tropomyosin-like seven residue periodicity in three immunologically distinct streptococal M proteins and its implications for the antiphagocytic property of the molecule

Partial sequences of three immunologically distinct group A streptococcal M proteins (M5, M6, and M24) revealed significant homology with each other, certain amino acid residues being conserved within the three molecules. In addition, a common feature of the sequenced regions of these M proteins was their high alpha-helical potential and the presence of a repeating seven residue periodicity that is characteristic of the double helical coiled-coil molecule, tropomyosin. The existence of a tropomyosin-like seven residue periodicity strongly suggests that regions of these three M proteins may participate in intra- and/or intermolecular coiled-coil interactions. Because of the constraints imposed by such a repeating periodicity, certain conserved residues within the M proteins would occupy spatially equivalent positions in the tertiary structure of these molecules. This common characteristic could play an important role in the common antiphagocytic property of the immunologically diverse M molecules. In addition to similarities in the secondary structure of M proteins and tropomyosin, significant sequence homology has also been observed between certain regions of these molecules with up to 50% identical residues. As a result of the striking structural similarity with tropomyosin, M proteins may play a regulatory role in the contractile mechanisms involved in phagocytosis.