Effects of side-chain characteristics on stability and oligomerization state of a de novo-designed model coiled-coil: 20 amino acid substitutions in position "d"

Conservation of C4BP-binding Sequence Patterns in Streptococcus pyogenes M and Enn Proteins

Antigenically sequence variable M proteins of the major bacterial pathogen Streptococcus pyogenes (Strep A) are responsible for recruiting human C4b-binding protein (C4BP) to the bacterial surface, which enables Strep A to evade destruction by the immune system. The most sequence divergent portion of M proteins, the hypervariable region (HVR), is responsible for binding C4BP. Structural evidence points to the conservation of two C4BP-binding sequence patterns (M2 and M22) in the HVR of numerous M proteins, with this conservation applicable to vaccine immunogen design. These two patterns, however, only partially explain C4BP-binding by Strep A. Here, we identified several M proteins that lack these patterns but still bind C4BP, and determined the structures of two, M68 and M87 HVRs, in complex with a C4BP fragment. Mutagenesis of these M proteins led to identification of amino acids that are crucial for C4BP-binding, enabling formulation of new C4BP-binding patterns. Mutagenesis was also carried out on M2 and M22 proteins to refine or generate experimentally grounded C4BP-binding patterns. The M22 pattern was the most populated among M proteins, followed by the M87 and M2 patterns, while the M68 pattern was rare. These patterns, except for M68, were also evident in numerous M-like Enn proteins. Binding of C4BP via these patterns to Enn proteins was verified. We conclude that C4BP-binding patterns occur frequently in Strep A strains of differing M types, being present in their M or Enn proteins, or frequently both, providing further impetus for their use as vaccine immunogens.

A coiled-coil-based design strategy for the thermostabilization of G-protein-coupled receptors

Structure elucidation of inactive-state GPCRs still mostly relies on X-ray crystallography. The major goal of our work was to create a new GPCR tool that would provide receptor stability and additional soluble surface for crystallization. Towards this aim, we selected the two-stranded antiparallel coiled coil as a domain fold that satisfies both criteria. A selection of antiparallel coiled coils was used for structure-guided substitution of intracellular loop 3 of the β3 adrenergic receptor. Unexpectedly, only the two GPCR variants containing thermostable coiled coils were expressed. We showed that one GPCR chimera is stable upon purification in detergent, retains ligand-binding properties, and can be crystallized. However, the quality of the crystals was not suitable for structure determination. By using two other examples, 5HTR2C and α2BAR, we demonstrate that our approach is generally suitable for the stabilization of GPCRs. To provide additional surface for promoting crystal contacts, we replaced in a structure-based approach the loop connecting the antiparallel coiled coil by T4L. We found that the engineered GPCR is even more stable than the coiled-coil variant. Negative-staining TEM revealed a homogeneous distribution of particles, indicating that coiled-coil-T4L receptor variants might also be promising candidate proteins for structure elucidation by cryo-EM. Our approach should be of interest for applications that benefit from stable GPCRs.

Azobenzene-Based Amino Acids for the Photocontrol of Coiled-Coil Peptides

Coiled-coil peptides are high-affinity, selective, self-assembling binding motifs, making them attractive components for the preparation of functional biomaterials. Photocontrol of coiled-coil self-assembly allows for the precise localization of their activity. To rationally explore photoactivity in a model coiled coil, three azobenzene-containing amino acids were prepared and substituted into the hydrophobic core of the E₃/K₃ coiled-coil heterodimer. Two of the non-natural amino acids, APhe1 and APhe2, are based on phenylalanine and differ in the presence of a carboxylic acid group. These have previously been demonstrated to modulate protein activity. When incorporated into peptide K₃, coiled-coil binding strength was affected upon isomerization, with the two variants differing in their most folded state. The third azobenzene-containing amino acid, APgly, is based on phenylglycine and was prepared to investigate the effect of amino acid size on photoisomerization. When APgly is incorporated into the coiled coil, a 4.7-fold decrease in folding constant is observed upon trans-to-cis isomerization─the largest difference for all three amino acids. Omitting the methylene group between azobenzene and α-carbon was theorized to both position the diazene of APgly closer to the hydrophobic amino acids and reduce the possible rotations of the amino acid, with molecular dynamics simulations supporting these hypotheses. These results demonstrate the ability of photoswitchable amino acids to control coiled-coil assembly through disruption of the hydrophobic interface, a strategy that should be widely applicable.

Macroporous dextran hydrogels for controlled growth factor capture and delivery using coiled-coil interactions

Macroporous hydrogels possess a vast potential for various applications in the biomedical field. However, due to their large pore size allowing for unrestricted diffusion in the macropore network, macroporous hydrogels alone are not able to efficiently capture and release biomolecules in a controlled manner. There is thus a need for biofunctionalized, affinity-based gels that can efficiently load and release biomolecules in a sustained and controlled manner. For this purpose, we report here the use of a E/K coiled-coil affinity pair for the controlled capture and delivery of growth factors from highly interconnected, macroporous dextran hydrogels. By conjugating the Kcoil peptide to the dextran backbone, we achieved controlled loading and release of Ecoil-tagged Epidermal and Vascular Endothelial Growth Factors. To finely tune the behavior of the gels, we propose four control parameters: (i) macropore size, (ii) Kcoil grafting density, (iii) Ecoil valency and (iv) E/K affinity. We demonstrate that Kcoil grafting can produce a 20-fold increase in passive growth factor capture by macroporous dextran gels. Furthermore, we demonstrate that our gels can release as little as 20% of the loaded growth factors over one week, while retaining bioactivity. Altogether, we propose a versatile, highly tunable platform for the controlled delivery of growth factors in biomedical applications. STATEMENT OF SIGNIFICANCE: This work presents a highly tunable platform for growth factor capture and sustained delivery using affinity peptides in macroporous, fully interconnected dextran hydrogels. It addresses several ongoing challenges by presenting: (i) a versatile platform for the delivery of a wide range of stable, bioactive molecules, (ii) a passive, affinity-based loading of growth factors in the platform, paving the way for in situ (re)loading of the device and (iii) four different control parameters to finely tune growth factor capture and release. Altogether, our macroporous dextran hydrogels have a vast potential for applications in controlled delivery, tissue engineering and regenerative medicine.

An M protein coiled coil unfurls and exposes its hydrophobic core to capture LL-37

Surface-associated, coiled-coil M proteins of Streptococcus pyogenes (Strep A) disable human immunity through interaction with select proteins. However, coiled coils lack features typical of protein-protein interaction sites, and it is therefore challenging to understand how M proteins achieve specific binding, for example, with the human antimicrobial peptide LL-37, leading to its neutralization. The crystal structure of a complex of LL-37 with M87 protein, an antigenic M protein variant from a strain that is an emerging threat, revealed a novel interaction mode. The M87 coiled coil unfurled and asymmetrically exposed its hydrophobic core to capture LL-37. A single LL-37 molecule was bound by M87 in the crystal, but in solution additional LL-37 molecules were recruited, consistent with a 'protein trap' neutralization mechanism. The interaction mode visualized crystallographically was verified to contribute significantly to LL-37 resistance in an M87 Strep A strain and was identified to be conserved in a number of other M protein types that are prevalent in human populations. Our results provide specific detail for therapeutic inhibition of LL-37 neutralization by M proteins.

Conformational control in a photoswitchable coiled coil

The coiled coil is a common protein tertiary structure intimately involved in mediating protein recognition and function. Due to their structural simplicity, coiled coils have served as attractive scaffolds for the development of functional biomaterials. Herein we describe the design of conformationally-defined coiled coil photoswitches as potential environmentally-sensitive biomaterials.

Structural insights into the tropomodulin assembly at the pointed ends of actin filaments

Tropomodulins are a family of important regulators of actin dynamics at the pointed ends of actin filaments. Four isoforms of tropomodulin, Tmod1-Tmod4, are expressed in vertebrates. Binding of tropomodulin to the pointed end is dependent on tropomyosin, an actin binding protein that itself is represented in mammals by up to 40 isoforms. The understanding of the regulatory role of the tropomodulin/tropomyosin molecular diversity has been limited due to the lack of a three-dimensional structure of the tropomodulin/tropomyosin complex. In this study, we mapped tropomyosin residues interacting with two tropomyosin-binding sites of tropomodulin and generated a three-dimensional model of the tropomodulin/tropomyosin complex for each of these sites. The models were refined by molecular dynamics simulations and validated via building a self-consistent three-dimensional model of tropomodulin assembly at the pointed end. The model of the pointed-end Tmod assembly offers new insights in how Tmod binding ensures tight control over the pointed end dynamics.

Nonimmune antibody interactions of Group A Streptococcus M and M-like proteins

M and M-like proteins are major virulence factors of the widespread and potentially deadly bacterial pathogen Streptococcus pyogenes. These proteins confer resistance against innate and adaptive immune responses by recruiting specific human proteins to the streptococcal surface. Nonimmune recruitment of immunoglobulins G (IgG) and A (IgA) through their fragment crystallizable (Fc) domains by M and M-like proteins was described almost 40 years ago, but its impact on virulence remains unresolved. These interactions have been suggested to be consequential under immune conditions at mucosal surfaces and in secretions but not in plasma, while other evidence suggests importance in evading phagocytic killing in nonimmune blood. Recently, an indirect effect of Fc-binding through ligand-induced stabilization of an M-like protein was shown to increase virulence. Nonimmune recruitment has also been seen to contribute to tissue damage in animal models of autoimmune diseases triggered by S. pyogenes infection. The damage was treatable by targeting Fc-binding. This and other potential therapeutic applications warrant renewed attention to Fc-binding by M and M-like proteins.

Separation of Coiled-Coil Structures in Lamin A/C Is Required for the Elongation of the Filament

Intermediate filaments (IFs) commonly have structural elements of a central α-helical coiled-coil domain consisting of coil 1a, coil 1b, coil 2, and their flanking linkers. Recently, the crystal structure of a long lamin A/C fragment was determined and showed detailed features of a tetrameric unit. The structure further suggested a new binding mode between tetramers, designated eA22, where a parallel overlap of coil 1a and coil 2 is the critical interaction. This study investigated the biochemical effects of genetic mutations causing human diseases, focusing on the eA22 interaction. The mutant proteins exhibited either weakened or augmented interactions between coil 1a and coil 2. The ensuing biochemical results indicated that the interaction requires the separation of the coiled-coils in the N-terminal of coil 1a and the C-terminal of coil 2, coupled with the structural transition in the central α-helical rod domain. This study provides insight into the role of coil 1a as a molecular regulator in the elongation of IF proteins.

P. falciparum and P. vivax Orthologous Coiled-Coil Candidates for a Potential Cross-Protective Vaccine

Over the last four decades, significant efforts have been invested to develop vaccines against malaria. Although most efforts are focused on the development of P. falciparum vaccines, the current availability of the parasite genomes, bioinformatics tools, and high throughput systems for both recombinant and synthetic antigen production have helped to accelerate vaccine development against the P. vivax parasite. We have previously in silico identified several P. falciparum and P. vivax proteins containing α-helical coiled-coil motifs that represent novel putative antigens for vaccine development since they are highly immunogenic and have been associated with protection in many in vitro functional assays. Here, we selected five pairs of P. falciparum and P. vivax orthologous peptides to assess their sero-reactivity using plasma samples collected in P. falciparum- endemic African countries. Pf-Pv cross-reactivity was also investigated. The pairs Pf27/Pv27, Pf43/Pv43, and Pf45/Pv45 resulted to be the most promising candidates for a cross-protective vaccine because they showed a high degree of recognition in direct and competition ELISA assays and cross-reactivity with their respective ortholog. The recognition of P. vivax peptides by plasma of P. falciparum infected individuals indicates the existence of a high degree of cross-reactivity between these two Plasmodium species. The design of longer polypeptides combining these epitopes will allow the assessment of their immunogenicity and protective efficacy in animal models.

Heteromeric three-stranded coiled coils designed using a Pb(II)(Cys)3 template mediated strategy

Three-stranded coiled coils are peptide structures constructed from amphipathic heptad repeats. Here we show that it is possible to form pure heterotrimeric three-stranded coiled coils by combining three distinct characteristics: (1) a cysteine sulfur layer for metal coordination, (2) a thiophilic, trigonal pyramidal metalloid (Pb(II)) that binds to these sulfurs and (3) an adjacent layer of reduced steric bulk generating a cavity where water can hydrogen bond to the cysteine sulfur atoms. Cysteine substitution in an a site yields Pb(II)A₂B heterotrimers, while d sites provide pure Pb(II)C₂D or Pb(II)CD₂ scaffolds. Altering the metal from Pb(II) to Hg(II) or shifting the relative position of the sterically less demanding layer removes heterotrimer specificity. Because only two of the eight or ten hydrophobic layers are perturbed, catalytic sites can be introduced at other regions of the scaffold. A Zn(II)(histidine)₃(H₂O) centre can be incorporated at a remote location without perturbing the heterotrimer selectivity, suggesting a unique strategy to prepare dissymmetric catalytic sites within self-assembling de novo-designed proteins.

A novel SPECC1L mutation causing Teebi hypertelorism syndrome: Expanding phenotypic and genetic spectrum

Only eleven SPECC1L mutations have been reported worldwide which were associated with autosomal dominant oblique facial clefts, Opitz G/BBB Syndrome and Teebi hypertelorism syndrome. In this study, we reported the first Chinese patient with Teebi hypertelorism syndrome. Utilizing whole exome sequencing and Sanger sequencing, we identified a de novo missense mutation NM_015330.3: c.1249A > C, p.(Thr417Pro) in SPECC1L gene. With common manifestations in Teebi hypertelorism syndrome such as special facial appearance, umbilical malformations and congenital heart defects, the patient also had unusual symptoms including recurrent infections, febrile seizures and widely opened anterior fontanelle. Furthermore, all the recorded SPECC1L mutations were analyzed by in silico analysis. Coiled-coil domain 2 was the most frequently mutated domain and positions e and g might be more important than other positions. This paper expanded the phenotypic spectrum of Teebi hypertelorism syndrome and elaborated molecular characteristics of SPECC1L mutations.

Instability in a coiled-coil signaling helix is conserved for signal transduction in soluble guanylyl cyclase

How nitric oxide (NO) activates its primary receptor, α1/β1 soluble guanylyl cyclase (sGC or GC-1), remains unknown. Likewise, how stimulatory compounds enhance sGC activity is poorly understood, hampering development of new treatments for cardiovascular disease. NO binding to ferrous heme near the N-terminus in sGC activates cyclase activity near the C-terminus, yielding cGMP production and physiological response. CO binding can also stimulate sGC, but only weakly in the absence of stimulatory small-molecule compounds, which together lead to full activation. How ligand binding enhances catalysis, however, has yet to be discovered. Here, using a truncated version of sGC from Manduca sexta, we demonstrate that the central coiled-coil domain, the most highly conserved region of the ~150,000 Da protein, not only provides stability to the heterodimer but is also conformationally active in signal transduction. Sequence conservation in the coiled coil includes the expected heptad-repeating pattern for coiled-coil motifs, but also invariant positions that disfavor coiled-coil stability. Full-length coiled coil dampens CO affinity for heme, while shortening of the coiled coil leads to enhanced CO binding. Introducing double mutation αE447L/βE377L, predicted to replace two destabilizing glutamates with leucines, lowers CO binding affinity while increasing overall protein stability. Likewise, introduction of a disulfide bond into the coiled coil results in reduced CO affinity. Taken together, we demonstrate that the heme domain is greatly influenced by coiled-coil conformation, suggesting communication between heme and catalytic domains is through the coiled coil. Highly conserved structural imperfections in the coiled coil provide needed flexibility for signal transduction.

Critical Roles for Coiled-Coil Dimers of Butyrophilin 3A1 in the Sensing of Prenyl Pyrophosphates by Human Vγ2Vδ2 T Cells

Vγ2Vδ2 T cells play important roles in human immunity to pathogens and tumors. Their TCRs respond to the sensing of isoprenoid metabolites, such as (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate and isopentenyl pyrophosphate, by butyrophilin (BTN) 3A1. BTN3A1 is an Ig superfamily protein with extracellular IgV/IgC domains and intracellular B30.2 domains that bind prenyl pyrophosphates. We have proposed that intracellular α helices form a coiled-coil dimer that functions as a spacer for the B30.2 domains. To test this, five pairs of anchor residues were mutated to glycine to destabilize the coiled-coil dimer. Despite maintaining surface expression, BTN3A1 mutagenesis either abrogated or decreased stimulation by (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate. BTN3A2 and BTN3A3 proteins and orthologs in alpacas and dolphins are also predicted to have similar coiled-coil dimers. A second short coiled-coil region dimerizes the B30.2 domains. Molecular dynamics simulations predict that mutation of a conserved tryptophan residue in this region will destabilize the dimer, explaining the loss of stimulation by BTN3A1 proteins with this mutation. The juxtamembrane regions of other BTN/BTN-like proteins with B30.2 domains are similarly predicted to assume α helices, with many predicted to form coiled-coil dimers. An exon at the end of this region and the exon encoding the dimerization region for B30.2 domains are highly conserved. We propose that coiled-coil dimers function as rod-like helical molecular spacers to position B30.2 domains, as interaction sites for other proteins, and as dimerization regions to allow sensing by B30.2 domains. In these ways, the coiled-coil domains of BTN3A1 play critical roles for its function.

An Intrinsic Hydrophobicity Scale for Amino Acids and Its Application to Fluorinated Compounds

More than 100 hydrophobicity scales have been introduced, with each being based on a distinct condensed-phase approach. However, a comparison of the hydrophobicity values gained from different techniques, and their relative ranking, is not straightforward, as the interactions between the environment and the amino acid are unique to each method. Here, we overcome this limitation by studying the properties of amino acids in the clean-room environment of the gas phase. In the gas phase, entropic contributions from the hydrophobic effect are by default absent and only the polarity of the side chain dictates the self-assembly. This allows for the derivation of a novel hydrophobicity scale, which is based solely on the interaction between individual amino acid units within the cluster and thus more accurately reflects the intrinsic nature of a side chain. This principle can be further applied to classify non-natural derivatives, as shown here for fluorinated amino acid variants.

Self-assembly in elastin-like recombinamers: a mechanism to mimic natural complexity

The topic of self-assembled structures based on elastin-like recombinamers (ELRs, i.e., elastin-like polymers recombinantly bio-produced) has released a noticeable amount of references in the last few years. Most of them are intended for biomedical applications. In this review, a complete revision of the bibliography is carried out. Initially, the self-assembly (SA) concept is considered from a general point of view, and then ELRs are described and characterized based on their intrinsic disorder. A classification of the different self-assembled ELR-based structures is proposed based on their morphologies, paying special attention to their tentative modeling. The impact of the mechanism of SA on these biomaterials is analyzed. Finally, the implications of ELR SA in biological systems are considered.

Post-assembly α-helix to β-sheet structural transformation within SAF-p1/p2a peptide nanofibers

We report an unanticipated helix-to-sheet structural transformation within an assembly of SAF-p1 and SAF-p2a designer peptides. Solid-state NMR spectroscopic data support the assembled structure that was targeted by rational peptide design: an α-helical coiled-coil co-assembly of both peptides. Subsequent to assembly, however, the system converts to a β-sheet structure that continues to exhibit nearest-neighbor interactions between the two peptide components. The structural transition occurs at pH 7.4 and exhibits strongly temperature-dependent kinetics between room temperature (weeks) and 40 °C (minutes). We further observed evidence of reversibility on the timescale of months at 4 °C. The structural conversion from the anticipated structure to an unexpected structure highlights an important aspect to the challenge of designing peptide assemblies. Furthermore, the conformational switching mechanism mediated by a prerequisite α-helical nanostructure represents a previously unknown route for β-sheet designer peptide assembly.

Polyserine repeats promote coiled coil-mediated fibril formation and length-dependent protein aggregation

Short polyserine (polyS) repeats are frequently found in proteins and longer ones are produced in neurological disorders such as Huntington disease (HD) owing to translational frameshifting or non-ATG-dependent translation, together with polyglutamine (polyQ) and polyalanine (polyA) repeats, forming intracellular aggregates. However, the physiological and pathological structures of polyS repeats are not clearly understood. Early studies highlighted their structural versatility, similar to other homopolymers whose conformation is influenced by the surrounding protein context. As polyS stretches are frequently near polyQ and polyA repeats, which can be part of coiled coil (CC) structures, and the frameshift-derived polyS repeats in HD directly flank CC heptads important for aggregation, we investigate here the structural and aggregation properties of polyS in the context of CC structures. We have taken advantage of peptide models, previously used to study polyQ and polyA in CCs, in which we inserted polyS repeats of variable length and studied them in comparison with polyQ and polyA peptides. We found that polyS repeats promote CC-mediated polymerization and fibrillization as revealed by circular dichroism, chemical crosslinking, and atomic force microscopy. Furthermore, they promote CC-based, length-dependent intracellular aggregation, which is negligible with 7 and widespread with 49 serines. These findings show that polyS repeats can participate in the formation of CCs, as previously found for polyQ and polyA, conferring to peptides distinctive structural properties with aggregation kinetics that are intermediate between those of polyA and polyQ CCs, and contribute to an overall structural definition of the pathophysiogical roles of homopolymeric repeats in CC structures.

Tropomyosin Structure, Function, and Interactions: A Dynamic Regulator

Tropomyosin is the archetypal-coiled coil, yet studies of its structure and function have proven it to be a dynamic regulator of actin filament function in muscle and non-muscle cells. Here we review aspects of its structure that deviate from canonical leucine zipper coiled coils that allow tropomyosin to bind to actin, regulate myosin, and interact directly and indirectly with actin-binding proteins. Four genes encode tropomyosins in vertebrates, with additional diversity that results from alternate promoters and alternatively spliced exons. At the same time that periodic motifs for binding actin and regulating myosin are conserved, isoform-specific domains allow for specific interaction with myosins and actin filament regulatory proteins, including troponin. Tropomyosin can be viewed as a universal regulator of the actin cytoskeleton that specifies actin filaments for cellular and intracellular functions.

Role of positions e and g in the fibrous assembly formation of an amphipathic α-helix-forming polypeptide

We previously characterized α3, a polypeptide that has a three times repeated sequence of seven amino acids (abcdefg: LETLAKA) and forms fibrous assemblies composed of amphipathic α-helices. Upon comparison of the amino acid sequences of α3 with other α-helix forming polypeptides, we proposed that the fibrous assemblies were formed due to the alanine (Ala) residues at positions e and g. Here, we characterized seven α3 analog polypeptides with serine (Ser), glycine (Gly), or charged residues substituted for Ala at positions e and g. The α-helix forming abilities of the substituted polypeptides were less than that of α3. The polypeptides with amino acid substitutions at position g and the polypeptide KEα3, in which Ala was substituted with charged amino acids, formed few fibrous assemblies. In contrast, polypeptides with Ala replaced by Ser at position e formed β-sheets under several conditions. These results show that Ala residues at position e and particularly at position g are involved in the formation of fibrous assemblies.

Platform technology to generate broadly cross-reactive antibodies to α-helical epitopes in hemagglutinin proteins from influenza A viruses

We have utilized a de novo designed two‐stranded α‐helical coiled‐coil template to display conserved α‐helical epitopes from the stem region of hemagglutinin (HA) glycoproteins of influenza A. The immunogens have all the surface‐exposed residues of the native α‐helix in the native HA protein of interest displayed on the surface of the two‐stranded α‐helical coiled‐coil template. This template when used as an immunogen elicits polyclonal antibodies which bind to the α‐helix in the native protein. We investigated the highly conserved sequence region 421–476 of HA by inserting 21 or 28 residue sequences from this region into our template. The cross‐reactivity of the resulting rabbit polyclonal antibodies prepared to these immunogens was determined using a series of HA proteins from H1N1, H2N2, H3N2, H5N1, H7N7, and H7N9 virus strains which are representative of Group 1 and Group 2 virus subtypes of influenza A. Antibodies from region 449–476 were Group 1 specific. Antibodies to region 421–448 showed the greatest degree of cross‐reactivity to Group 1 and Group 2 and suggested that this region has a great potential as a “universal” synthetic peptide vaccine for influenza A. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 144–159, 2016.

The Elucidation of the Interactome of 16 Arabidopsis bZIP Factors Reveals Three Independent Functional Networks

The function of the bZIP transcription factors is strictly dependent on their ability to dimerize. Heterodimerization has proven to be highly specific and is postulated to operate as a combinatorial mechanism allowing the generation of a large variety of dimers with unique qualities by specifically combining a small set of monomers; an assumption that has not yet been tested systematically. Here, the interaction pattern and the transactivation properties of 16 Arabidopsis thaliana bZIPs are examined in transiently transformed Arabidopsis protoplasts to deliver a perspective on the relationship between bZIP dimerization and function. An interaction matrix of bZIPs belonging to the C, G, H, and S1 bZIP groups was resolved by Bimolecular Fluorescent Complementation (BiFC) coupled to quantitative flow cytometric analysis, while an extensive GUS reporter gene assay was carried out to determine the effect of different bZIP pairs on the expression of four different known bZIP-targeted promoters. Statistical data treatment and complementary bioinformatic analysis were performed to substantiate the biological findings. According to these results, the 16 bZIPs interact in three isolated networks, within which their members dimerize non-specifically and exhibit a significant level of functional redundancy. A coherent explanation for these results is supported by in silico analysis of differences in the length, structure and composition of their leucine zippers and appears to explain their dimerization specificity and dynamics observed in vivo quite well. A model in which the bZIP networks act as functional units is proposed.

Amphiphilic Elastin-Like Block Co-Recombinamers Containing Leucine Zippers: Cooperative Interplay between Both Domains Results in Injectable and Stable Hydrogels

Many biological processes are regulated by reversible binding events, with these interactions between macromolecules representing the core of dynamic chemistry. As such, any attempt to gain a better understanding of such interactions, which would pave the way to the extrapolation of natural designs to create new advanced systems, is clearly of interest. This work focuses on the development of a leucine zipper-elastin-like recombinamer (ZELR) in order to elucidate the behavior of such domains when coexisting along the same molecule and to engineer reversible, injectable and stable hydrogels. The unique propensity of the Z-moiety selected to dimerize, together with the thermosensitive behavior of the ELR, which has been constructed as a thermosensitive amphiphilic tetrablock, has been engineered into a single recombinant molecule. In this molecular design, the Z-moieties are unable to form a network, while the ELR is below its Tt, thus, guaranteeing the liquid-like state of the system. However, this situation changes rapidly as the temperature increases above Tt, where a stable hydrogel is formed, as demostrated by rheological tests. The inability of the ELR molecule (without Z-domains) to form such a stable hydrogel above Tt clearly points to a positive cooperative effect between these two domains (Z and EL), and no conformational changes in the former are involved, as demonstrated by circular dichroism analysis. AFM shows that Z-motifs seem to induce the aggregation of micelles, which supports the enhanced stability displayed by ZELRs when compared to ELR at the macroscale level. To the best of our knowledge, this is the first time that such an interplay between these two domains has been reported. Furthermore, the cytocompatibility of the resulting hydrogels opens the door to their use in biomedical applications.

Coiled-coils in phage display screening: insight into exceptional selectivity provided by molecular dynamics

Involved in numerous key biological functions, protein helix-helix interactions follow a well-defined intermolecular recognition pattern. The characteristic structure of the α-helical coiled-coil allows for the specific randomization of clearly defined interaction partners within heteromeric systems. In this work, a rationally designed heterodimeric coiled-coil was used to investigate potential factors influencing the sequence selectivity in interhelical interactions.

Sequence-resolved free energy profiles of stress-bearing vimentin intermediate filaments

Intermediate filaments (IFs) are key to the mechanical strength of metazoan cells. Their basic building blocks are dimeric coiled coils mediating hierarchical assembly of the full-length filaments. Here we use single-molecule force spectroscopy by optical tweezers to assess the folding and stability of coil 2B of the model IF protein vimentin. The coiled coil was unzipped from its N and C termini. When pulling from the C terminus, we observed that the coiled coil was resistant to force owing to the high stability of the C-terminal region. Pulling from the N terminus revealed that the N-terminal half is considerably less stable. The mechanical pulling assay is a unique tool to study and control seed formation and structure propagation of the coiled coil. We then used rigorous theory-based deconvolution for a model-free extraction of the energy landscape and local stability profiles. The data obtained from the two distinct pulling directions complement each other and reveal a tripartite stability of the coiled coil: a labile N-terminal half, followed by a medium stability section and a highly stable region at the far C-terminal end. The different stability regions provide important insight into the mechanics of IF assembly.

Tropomyosin dynamics

Tropomyosin is a two chained α-helical coiled coil protein that binds actin filaments and interacts with various actin binding proteins. Tropomyosin function depends on its ability to move to distinct locations on the surface of actin in response to the binding of different thin filament effectors. Tropomyosin dynamics plays an important role in these fluctuating interactions with actin and is thought to be fundamental to many of its biological activities. For example tropomyosin concerted movement on the surface of actin triggered by Ca(2+) binding to troponin or myosin head binding to actin has been argued to be key to the cooperative allosteric regulation of muscle contraction. These large-scale motions are affected by tropomyosin internal dynamics and mechanical properties. Tropomyosin internal dynamics corresponding to smaller and more localised structural fluctuations are increasingly recognised to play an important role in its function. A thorough understanding of the coupling between local and global structural fluctuations in tropomyosin is required to understand how time dependent structural fluctuations in tropomyosin contribute to the overall thin filament dynamics and dictate their various biological activities.

Transmission of stability information through the N-domain of tropomyosin is interrupted by a stabilizing mutation (A109L) in the hydrophobic core of the stability control region (residues 97-118)

Tropomyosin (Tm) is an actin-binding, thin filament, two-stranded α-helical coiled-coil critical for muscle contraction and cytoskeletal function. We made the first identification of a stability control region (SCR), residues 97-118, in the Tm sequence that controls overall protein stability but is not required for folding. We also showed that the individual α-helical strands of the coiled-coil are stabilized by Leu-110, whereas the hydrophobic core is destabilized in the SCR by Ala residues at three consecutive d positions. Our hypothesis is that the stabilization of the individual α-helices provides an optimum stability and allows functionally beneficial dynamic motion between the α-helices that is critical for the transmission of stabilizing information along the coiled-coil from the SCR. We prepared three recombinant (rat) Tm(1-131) proteins, including the wild type sequence, a destabilizing mutation L110A, and a stabilizing mutation A109L. These proteins were evaluated by circular dichroism (CD) and differential scanning calorimetry. The single mutation L110A destabilizes the entire Tm(1-131) molecule, showing that the effect of this mutation is transmitted 165 Å along the coiled-coil in the N-terminal direction. The single mutation A109L prevents the SCR from transmitting stabilizing information and separates the coiled-coil into two domains, one that is ∼9 °C more stable than wild type and one that is ∼16 °C less stable. We know of no other example of the substitution of a stabilizing Leu residue in a coiled-coil hydrophobic core position d that causes this dramatic effect. We demonstrate the importance of the SCR in controlling and transmitting the stability signal along this rodlike molecule.

Complementary interhelical interactions between three buried Glu-Lys pairs within three heptad repeats are essential for Hec1-Nuf2 heterodimerization and mitotic progression

Hec1 and Nuf2, core components of the NDC80 complex, are essential for kinetochore-microtubule attachment and chromosome segregation. It has been shown that both Hec1 and Nuf2 utilize their coiled-coil domains to form a functional dimer; however, details of the consequential significance and structural requirements to form the dimerization interface have yet to be elucidated. Here, we showed that Hec1 required three contiguous heptad repeats from Leu-324 to Leu-352, but not the entire first coiled-coil domain, to ensure overall stability of the NDC80 complex through direct interaction with Nuf2. Substituting the hydrophobic core residues, Leu-331, Val-338, and Ile-345, of Hec1 with alanine completely eliminated Nuf2 binding and blocked mitotic progression. Moreover, unlike most coiled-coil proteins, where the buried positions are composed of hydrophobic residues, Hec1 possessed an unusual distribution of glutamic acid residues, Glu-334, Glu-341, and Glu-348, buried within the interior dimerization interface, which complement with three Nuf2 lysine residues: Lys-227, Lys-234, and Lys-241. Substituting these corresponding residues with alanine diminished the binding affinity between Hec1 and Nuf2, compromised NDC80 complex formation, and adversely affected mitotic progression. Taken together, these findings demonstrated that three buried glutamic acid-lysine pairs, in concert with hydrophobic interactions of core residues, provide the major specificity and stability requirements for Hec1-Nuf2 dimerization and NDC80 complex formation.

Two coiled-coil domains of Chlamydia trachomatis IncA affect membrane fusion events during infection

Chlamydia trachomatis replicates in a parasitophorous membrane-bound compartment called an inclusion. The inclusions corrupt host vesicle trafficking networks to avoid the degradative endolysosomal pathway but promote fusion with each other in order to sustain higher bacterial loads in a process known as homotypic fusion. The Chlamydia protein IncA (Inclusion protein A) appears to play central roles in both these processes as it participates to homotypic fusion and inhibits endocytic SNARE-mediated membrane fusion. How IncA selectively inhibits or activates membrane fusion remains poorly understood. In this study, we analyzed the spatial and molecular determinants of IncA’s fusogenic and inhibitory functions. Using a cell-free membrane fusion assay, we found that inhibition of SNARE-mediated fusion requires IncA to be on the same membrane as the endocytic SNARE proteins. IncA displays two coiled-coil domains showing high homology with SNARE proteins. Domain swap and deletion experiments revealed that although both these domains are capable of independently inhibiting SNARE-mediated fusion, these two coiled-coil domains cooperate in mediating IncA multimerization and homotypic membrane interaction. Our results support the hypothesis that Chlamydia employs SNARE-like virulence factors that positively and negatively affect membrane fusion and promote infection.

Anti-antimicrobial peptides: folding-mediated host defense antagonists

Antimicrobial or host defense peptides are innate immune regulators found in all multicellular organisms. Many of them fold into membrane-bound α-helices and function by causing cell wall disruption in microorganisms. Herein we probe the possibility and functional implications of antimicrobial antagonism mediated by complementary coiled-coil interactions between antimicrobial peptides and de novo designed antagonists: anti-antimicrobial peptides. Using sequences from native helical families such as cathelicidins, cecropins, and magainins we demonstrate that designed antagonists can co-fold with antimicrobial peptides into functionally inert helical oligomers. The properties and function of the resulting assemblies were studied in solution, membrane environments, and in bacterial culture by a combination of chiroptical and solid-state NMR spectroscopies, microscopy, bioassays, and molecular dynamics simulations. The findings offer a molecular rationale for anti-antimicrobial responses with potential implications for antimicrobial resistance.

Effects of chain length of an amphipathic polypeptide carrying the repeated amino acid sequence (LETLAKA)(n) on α-helix and fibrous assembly formation

Polypeptide α3 (21 residues), with three repeats of a seven-amino-acid sequence (LETLAKA)(3), forms an amphipathic α-helix and a long fibrous assembly. Here, we investigated the ability of α3-series polypeptides (with 14-42 residues) of various chain lengths to form α-helices and fibrous assemblies. Polypeptide α2 (14 residues), with two same-sequence repeats, did not form an α-helix, but polypeptide α2L (15 residues; α2 with one additional leucine residue on its carboxyl terminal) did form an α-helix and fibrous assembly. Fibrous assembly formation was associated with polypeptides at least as long as polypeptide α2L and with five leucine residues, indicating that the C-terminal leucine has a critical element for stabilization of α-helix and fibril formation. In contrast, polypeptides α5 (35 residues) and α6 (42 residues) aggregated easily, although they formed α-helices. A 15-35-residue chain was required for fibrous assembly formation. Electron microscopy and X-ray fiber diffraction showed that the thinnest fibrous assemblies of polypeptides were about 20 Å and had periodicities coincident with the length of the α-helix in a longitudinal direction. These results indicated that the α-helix structures were orientated along the fibrous axis and assembled into a bundle. Furthermore, the width and length of fibrous assemblies changed with changes in the pH value, resulting in variations in the charged states of the residues. Our results suggest that the formation of fibrous assemblies of amphipathic α-helices is due to the assembly of bundles via the hydrophobic faces of the helices and extension with hydrophobic noncovalent bonds containing a leucine.

A SWI/SNF chromatin-remodeling complex acts in noncoding RNA-mediated transcriptional silencing

RNA-mediated transcriptional silencing prevents deleterious effects of transposon activity and controls the expression of protein-coding genes. It involves long noncoding RNAs (lncRNAs). In Arabidopsis thaliana, some of those lncRNAs are produced by a specialized RNA Polymerase V (Pol V). The mechanism by which lncRNAs affect chromatin structure and mRNA production remains mostly unknown. Here we identify the SWI/SNF ATP-dependent nucleosome-remodeling complex as a component of the RNA-mediated transcriptional silencing pathway. We found that SWI3B, an essential subunit of the SWI/SNF complex, physically interacts with a lncRNA-binding protein, IDN2. SWI/SNF subunits contribute to lncRNA-mediated transcriptional silencing. Moreover, Pol V mediates stabilization of nucleosomes on silenced regions. We propose that Pol V-produced lncRNAs mediate transcriptional silencing by guiding the SWI/SNF complex and establishing positioned nucleosomes on specific genomic loci. We further propose that guiding ATP-dependent chromatin-remodeling complexes may be a more general function of lncRNAs.

A de novo germline mutation in MYH7 causes a progressive dominant myopathy in pigs

Background

About 9% of the offspring of a clinically healthy Piétrain boar named ‘Campus’ showed a progressive postural tremor called Campus syndrome (CPS). Extensive backcross experiments suggested a dominant mode of inheritance, and the founder boar was believed to be a gonadal mosaic. A genome-scan mapped the disease-causing mutation to an 8 cM region of porcine chromosome 7 containing the MHY7 gene. Human distal myopathy type 1 (MPD1), a disease partially resembling CPS in pigs, has been associated with mutations in the MYH7 gene.

Results

The porcine MYH7 gene structure was predicted based on porcine reference genome sequence, porcine mRNA, and in comparison to the human ortholog. The gene structure was highly conserved with the exception of the first exon. Mutation analysis of a contiguous genomic interval of more than 22 kb spanning the complete MYH7 gene revealed an in-frame insertion within exon 30 of MYH7 (c.4320_4321insCCCGCC) which was perfectly associated with the disease phenotype and confirmed the dominant inheritance. The mutation is predicted to insert two amino acids (p.Ala1440_Ala1441insProAla) in a very highly conserved region of the myosin tail. The boar ‘Campus’ was shown to be a germline and somatic mosaic as assessed by the presence of the mutant allele in seven different organs.

Conclusion

This study illustrates the usefulness of recently established genomic resources in pigs. We have identified a spontaneous mutation in MYH7 as the causative mutation for CPS. This paper describes the first case of a disorder caused by a naturally occurring mutation in the MYH7 gene of a non-human mammalian species. Our study confirms the previous classification as a primary myopathy and provides a defined large animal model for human MPD1. We provide evidence that the CPS mutation occurred during the early development of the boar ‘Campus’. Therefore, this study provides an example of germline mosaicism with an asymptomatic founder.

The size and conservation of a coiled-coil structure in the ectodomain of human BST-2/tetherin is dispensable for inhibition of HIV-1 virion release

BST-2/CD317/tetherin is a host factor that inhibits HIV-1 release and is counteracted by HIV-1 Vpu. Structural studies indicate that the BST-2 ectodomain assumes a coiled-coil conformation. Here we studied the role of the BST-2 ectodomain for tethering function. First, we addressed the importance of the length and structure of the ectodomain by adding or substituting heterologous coiled-coil or non-coiled-coil sequences. We found that extending or replacing the BST-2 ectodomain using non-coiled-coil sequences resulted in loss of BST-2 function. Doubling the size of the BST-2 ectodomain by insertion of a heterologous coiled-coil motif or substituting the BST-2 coiled-coil domain with a heterologous coiled-coil motif maintained tethering function. Reductions in the size of the BST-2 coiled-coil domain were tolerated as well. In fact, deletion of the C-terminal half of the BST-2 ectodomain, including a series of seven consecutive heptad motifs did not abolish tethering function. However, slight changes in the positioning of deletions affecting the relative placing of charged or hydrophobic residues on the helix severely impacted the functional properties of BST-2. Overall, we conclude that the size of the BST-2 ectodomain is highly flexible and can be reduced or extended as long as the positioning of residues important for the stability of the dimer interface is maintained.

Strong contributions from vertical triads to helix-partner preferences in parallel coiled coils

Pairing preferences in heterodimeric coiled coils are determined by complementarities among side chains that pack against one another at the helix-helix interface. However, relationships between dimer stability and interfacial residue identity are not fully understood. In the context of the "knobs-into-holes" (KIH) packing pattern, one can identify two classes of interactions between side chains from different helices: "lateral", in which a line connecting the adjacent side chains is perpendicular to the helix axes, and "vertical", in which the connecting line is parallel to the helix axes. We have previously analyzed vertical interactions in antiparallel coiled coils and found that one type of triad constellation (a'-a-a') exerts a strong effect on pairing preferences, while the other type of triad (d'-d-d') has relatively little impact on pairing tendencies. Here, we ask whether vertical interactions (d'-a-d') influence pairing in parallel coiled-coil dimers. Our results indicate that vertical interactions can exert a substantial impact on pairing specificity, and that the influence of the d'-a-d' triad depends on the lateral a' contact within the local KIH motif. Structure-informed bioinformatic analyses of protein sequences reveal trends consistent with the thermodynamic data derived from our experimental model system in suggesting that heterotriads involving Leu and Ile are preferred over homotriads involving Leu and Ile.

Structural attributes for the recognition of weak and anomalous regions in coiled-coils of myosins and other motor proteins

BACKGROUND

Coiled-coils are found in different proteins like transcription factors, myosin tail domain, tropomyosin, leucine zippers and kinesins. Analysis of various structures containing coiled-coils has revealed the importance of electrostatic and hydrophobic interactions. In such domains, regions of different strength of interactions need to be identified since they could be biologically relevant.

FINDINGS

We have updated our coiled-coil validation webserver, now called COILCHECK+, where new features were added to efficiently identify the strength of interaction at the interface region and measure the density of charged residues and hydrophobic residues. We have examined charged residues and hydrophobic ladders, using a new algorithm called CHAHO, which is incorporated within COILCHECK + server. CHAHO permits the identification of spatial charged residue patches and the continuity of hydrophobic ladder which stabilizes and destabilizes the coiled-coil structure.

CONCLUSIONS

The availability of such computational tools should be useful to understand the importance of spatial clustering of charged residues and the continuity of hydrophobic residues at the interface region of coiled-coil dimers. COILCHECK + is a structure based tool to validate coiled-coil stability; it can be accessed at http://caps.ncbs.res.in/coilcheckplus.

Tropomyosin: double helix from the protein world

This review concerns the structure and functions of tropomyosin (TM), an actin-binding protein that plays a key role in the regulation of muscle contraction. The TM molecule is a dimer of α-helices, which form a coiled-coil. Recent views on the TM structure are analyzed, and special attention is concentrated on those structural traits of the TM molecule that distinguish it from the other coiled-coil proteins. Modern data are presented on TM functional properties, such as its interaction with actin and ability to move on the surface of actin filaments, which underlies the regulation of the actin-myosin interaction upon contraction of skeletal and cardiac muscles. Also, part of the review is devoted to analysis of the effects of mutations in TM genes associated with muscle diseases (myopathies) on the structure and functions of TM.

Investigation of the network of preferred interactions in an artificial coiled-coil association using the peptide array technique

We screened a randomized library and identified natural peptides that bound selectively to a chimeric peptide containing α-, β- and γ-amino acids. The SPOT arrays provide a means for the systematic study of the possible interaction space accessible to the αβγ-chimera. The mutational analysis reveals the dependence of the binding affinities of α-peptides to the αβγ-chimera, on the hydrophobicity and bulkiness of the side chains at the corresponding hydrophobic interface. The stability of the resulting heteroassemblies was further confirmed in solution by CD and thermal denaturation.

Design of peptide standards with the same composition and minimal sequence variation to monitor performance/selectivity of reversed-phase matrices

The present manuscript extends our de novo peptide design approach to the synthesis and evaluation of a new generation of reversed-phase HPLC peptide standards with the same composition and minimal sequence variation (SCMSV). Thus, we have designed and synthesized four series of peptide standards with the sequences Gly-X-Leu-Gly-Leu-Ala-Leu-Gly-Gly-Leu-Lys-Lys-amide, where the N-terminal is either N(α)-acetylated (Series 1) or contains a free α-amino group (Series 3); and Gly-Gly-Leu-Gly-Gly-Ala-Leu-Gly-X-Leu-Lys-Lys-amide, where the N-terminal is either N(α)-acetylated (Series 2) or contains a free α-amino group (Series 4). In this initial study, the single substitution position, X, was substituted with alkyl side-chains (Ala Collapse

Key Words Collapse

MESH Headings

• Amino Acid Sequence
• Chromatography, High Pressure Liquid/standards
• Chromatography, Reverse-Phase/standards
• Peptides/chemistry
• Reference Standards

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Grants

• R01 GM061855 NIGMS NIH HHS
• R01 GM061855-10 NIGMS NIH HHS
• R01 GM61855 NIGMS NIH HHS

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Affiliation(s)

Colin T. Mant Department of Biochemistry and Molecular Genetics, University of Colorado Denver, School of Medicine, Aurora, CO 80045, USA
Robert S. Hodges Department of Biochemistry and Molecular Genetics, University of Colorado Denver, School of Medicine, Aurora, CO 80045, USA

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The d'--d--d' vertical triad is less discriminating than the a'--a--a' vertical triad in the antiparallel coiled-coil dimer motif

Elucidating relationships between the amino-acid sequences of proteins and their three-dimensional structures, and uncovering non-covalent interactions that underlie polypeptide folding, are major goals in protein science. One approach toward these goals is to study interactions between selected residues, or among constellations of residues, in small folding motifs. The α-helical coiled coil has served as a platform for such studies because this folding unit is relatively simple in terms of both sequence and structure. Amino acid side chains at the helix-helix interface of a coiled coil participate in so-called "knobs-into-holes" (KIH) packing whereby a side chain (the knob) on one helix inserts into a space (the hole) generated by four side chains on a partner helix. The vast majority of sequence-stability studies on coiled-coil dimers have focused on lateral interactions within these KIH arrangements, for example, between an a position on one helix and an a' position of the partner in a parallel coiled-coil dimer, or between a--d' pairs in an antiparallel dimer. More recently, it has been shown that vertical triads (specifically, a'--a--a' triads) in antiparallel dimers exert a significant impact on pairing preferences. This observation provides impetus for analysis of other complex networks of side-chain interactions at the helix-helix interface. Here, we describe a combination of experimental and bioinformatics studies that show that d'--d--d' triads have much less impact on pairing preference than do a'--a--a' triads in a small, designed antiparallel coiled-coil dimer. However, the influence of the d'--d--d' triad depends on the lateral a'--d interaction. Taken together, these results strengthen the emerging understanding that simple pairwise interactions are not sufficient to describe side-chain interactions and overall stability in antiparallel coiled-coil dimers; higher-order interactions must be considered as well.

Improvement of neural stem cell survival in collagen hydrogels by incorporating laminin-derived cell adhesive polypeptides

Cell transplantation is a potential methodology for the treatment of Parkinson's disease. However, the therapeutic effect is limited by poor viability of transplanted cells. To overcome this problem, we hypothesized that a dual step approach, whereby providing an adhesive substrate for transplanted cells and, at the same time, by preventing the infiltration of activated microglia into the site of transplantation promotes the cell survival. To establish above conditions, attempts were made to prepare 3-D matrices using collagen hydrogels that incorporated integrin-binding polypeptides derived from laminin-1. Tandem combinations of laminin globular domains as well as a single globular domain 3 were prepared using recombinant DNA technology as a fusion with hexahistidine and bound to metal chelated surfaces to screen for the adhesion and proliferation of neural stem cells (NSCs). In addition, a small peptide derived from laminin γ1 chain was prepared and heterodimerized with the globular domain-containing chimeric proteins to evaluate for the enhancement of integrin-mediated cell adhesion. As a result, a heterodimer consisting of the globular domain 3 of the laminin α1 chain and the peptide from the laminin γ1 chain was selected as the best candidate among the polypeptides studied here for the incorporation into a collagen hydrogel. It was shown that the survival of NSCs was indeed promoted in the collagen hydrogel incorporating the heterodimer compared to the pure collagen hydrogel.

The coiled coil motif in polymer drug delivery systems

The coiled coil is a superhelical structural protein motif that has been thoroughly investigated in recent years. Because of the relatively well-understood principles that determine the properties of coiled coil peptides and proteins, macromolecular systems containing the coiled coil motif have been suggested for various applications. This short review focuses on hybrid polymer coiled coil systems designed for drug delivery purposes. After a short introduction, the most important features of the coiled coils (stability, association number, oligomerization selectivity and orientation of helices) are described, and the factors influencing these characteristics are discussed. Several examples of the most interesting biomedical applications of the polymer-coiled coil systems (according to the authors' opinion) are presented.

Structural analysis of smooth muscle tropomyosin α and β isoforms

A large number of tropomyosin (Tm) isoforms function as gatekeepers of the actin filament, controlling the spatiotemporal access of actin-binding proteins to specialized actin networks. Residues ∼40-80 vary significantly among Tm isoforms, but the impact of sequence variation on Tm structure and interactions with actin is poorly understood, because structural studies have focused on skeletal muscle Tmα. We describe structures of N-terminal fragments of smooth muscle Tmα and Tmβ (sm-Tmα and sm-Tmβ). The 2.0-Å structure of sm-Tmα81 (81-aa) resembles that of skeletal Tmα, displaying a similar super-helical twist matching the contours of the actin filament. The 1.8-Å structure of sm-Tmα98 (98-aa) unexpectedly reveals an antiparallel coiled coil, with the two chains staggered by only 4 amino acids and displaying hydrophobic core interactions similar to those of the parallel dimer. In contrast, the 2.5-Å structure of sm-Tmβ98, containing Gly-Ala-Ser at the N terminus to mimic acetylation, reveals a parallel coiled coil. None of the structures contains coiled-coil stabilizing elements, favoring the formation of head-to-tail overlap complexes in four of five crystallographically independent parallel dimers. These complexes show similarly arranged 4-helix bundles stabilized by hydrophobic interactions, but the extent of the overlap varies between sm-Tmβ98 and sm-Tmα81 from 2 to 3 helical turns. The formation of overlap complexes thus appears to be an intrinsic property of the Tm coiled coil, with the specific nature of hydrophobic contacts determining the extent of the overlap. Overall, the results suggest that sequence variation among Tm isoforms has a limited effect on actin binding but could determine its gatekeeper function.