A specific C-terminal deletion in tropomyosin results in a stronger head-to-tail interaction and increased polymerization

The structural dynamics of α-tropomyosin on F-actin shape the overlap complex between adjacent tropomyosin molecules

Coiled-coil tropomyosin, localized on actin filaments in virtually all eukaryotic cells, serves as a gatekeeper regulating access of the motor protein myosin and other actin-binding proteins onto the thin filament surface. Tropomyosin's modular pseudo-repeating pattern of approximately 39 amino acid residues is designed to allow binding of the coiled-coil to successive actin subunits along thin filaments. Even though different tropomyosin isoforms contain varying numbers of repeat modules, the pseudo-repeat length, in all cases, matches that of a single actin subunit. Thus, the seven pseudo-repeats of 42nm long muscle tropomyosin bind to seven successive actin subunits along thin filaments, while simultaneously bending into a super-helical conformation that is preshaped to the actin filament helix. In order to form a continuous cable on thin filaments that is free of gaps, adjacent tropomyosin molecules polymerize head-to-tail by means of a short (∼9 residue) overlap. Several laboratories have engineered peptides to mimic the N- and C-terminal tropomyosin association and to characterize the overlap structure. All overlapping domains examined show a compact N-terminal coiled-coil inserting into a partially opened C-terminal partner, where the opposing coiled-coils at the overlap junction face each other at up to ∼90° twist angles. Here, Molecular Dynamics (MD) simulations were carried out to determine constraints on the formation of the tropomyosin overlap complex and to assess the amount of twisting exhibited by full-length tropomyosin when bound to actin. With the exception of the last 20-40 C- and N-terminal residues, we find that the average tropomyosin structure closely resembles a "canonical" model proposed in the classic work of McLachlan and Stewart, displaying perfectly symmetrical supercoil geometry matching the F-actin helix with an integral number of coiled-coil turns, a coiled-coil helical pitch of 137Å, a superhelical pitch of 770Å, and no localized pseudo-rotation. Over the middle 70% of tropomyosin, the average twisting of the coiled-coil deviates only by 10° from the canonical model and the torsional freedom is very small (std. dev. of 7°). This small degree of twisting cannot yield the orthogonal N- and C-termini configuration observed experimentally. In marked contrast, considerable coiled-coil unfolding, splaying and twisting at N- and C-terminal ends is observed, providing the conformational plasticity needed for head-to-tail nexus formation.

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.

Structure of the tropomyosin overlap complex from chicken smooth muscle: insight into the diversity of N-terminal recognition

Tropomyosin is a stereotypical alpha-helical coiled coil that polymerizes to form a filamentous macromolecular assembly that lies on the surface of F-actin. The interaction between the C-terminal and N-terminal segments on adjacent molecules is known as the overlap region. We report here two X-ray structures of the chicken smooth muscle tropomyosin overlap complex. A novel approach was used to stabilize the C-terminal and N-terminal fragments. Globular domains from both the human DNA ligase binding protein XRCC4 and bacteriophage varphi29 scaffolding protein Gp7 were fused to 37 and 28 C-terminal amino acid residues of tropomyosin, respectively, whereas the 29 N-terminal amino acids of tropomyosin were fused to the C-terminal helix bundle of microtubule binding protein EB1. The structures of both the XRCC4 and Gp7 fusion proteins complexed with the N-terminal EB1 fusion contain a very similar helix bundle in the overlap region that encompasses approximately 15 residues. The C-terminal coiled coil opens to allow formation of the helix bundle, which is stabilized by hydrophobic interactions. These structures are similar to that observed in the NMR structure of the rat skeletal overlap complex [Greenfield, N. J., et al. (2006) J. Mol. Biol. 364, 80-96]. The interactions between the N- and C-terminal coiled coils of smooth muscle tropomyosin show significant curvature, which differs somewhat between the two structures and implies flexibility in the overlap complex, at least in solution. This is likely an important attribute that allows tropomyosin to assemble around the actin filaments. These structures provide a molecular explanation for the role of N-acetylation in the assembly of native tropomyosin.

Mg2+ ions bind at the C-terminal region of skeletal muscle alpha-tropomyosin

Tropomyosin (Tm) is a dimeric coiled-coil protein that polymerizes through head-to-tail interactions. These polymers bind along actin filaments and play an important role in the regulation of muscle contraction. Analysis of its primary structure shows that Tm is rich in acidic residues, which are clustered along the molecule and may form sites for divalent cation binding. In a previous study, we showed that the Mg(2+)-induced increase in stability of the C-terminal half of Tm is sensitive to mutations near the C-terminus. In the present report, we study the interaction between Mg(2+) and full-length Tm and smaller fragments corresponding to the last 65 and 26 Tm residues. Although the smaller Tm peptide (Tm(259-284(W269))) is flexible and to large extent unstructured, the larger Tm(220-284(W269)) fragment forms a coiled coil in solution whose stability increases significantly in the presence of Mg(2+). NMR analysis shows that Mg(2+) induces chemical shift perturbations in both Tm(220-284(W269)) and Tm(259-284(W269)) in the vicinity of His276, in which are located several negatively charged residues.

Deciphering the role of the electrostatic interactions in the alpha-tropomyosin head-to-tail complex

Skeletal alpha-tropomyosin (Tm) is a dimeric coiled-coil protein that forms linear assemblies under low ionic strength conditions in vitro through head-to-tail interactions. A previously published NMR structure of the Tm head-to-tail complex revealed that it is formed by the insertion of the N-terminal coiled-coil of one molecule into a cleft formed by the separation of the helices at the C-terminus of a second molecule. To evaluate the contribution of charged residues to complex stability, we employed single and double-mutant Tm fragments in which specific charged residues were changed to alanine in head-to-tail binding assays, and the effects of the mutations were analyzed by thermodynamic double-mutant cycles and protein-protein docking. The results show that residues K5, K7, and D280 are essential to the stability of the complex. Though D2, K6, D275, and H276 are exposed to the solvent and do not participate in intermolecular contacts in the NMR structure, they may contribute to head-to-tail complex stability by modulating the stability of the helices at the Tm termini.

Different effects of trifluoroethanol and glycerol on the stability of tropomyosin helices and the head-to-tail complex

Tropomyosin (Tm) is a dimeric coiled-coil protein, composed of 284 amino acids (410 A), that forms linear homopolymers through head-to-tail interactions at low ionic strength. The head-to-tail complex involves the overlap of approximately nine N-terminal residues of one molecule with nine C-terminal residues of another Tm molecule. In this study, we investigate the influence of 2,2,2-trifluoroethanol (TFE) and glycerol on the stability of recombinant Tm fragments (ASTm1-142, Tm143-284(5OHW269)) and of the dimeric head-to-tail complex formed by the association of these two fragments. The C-terminal fragment (Tm143-284(5OHW269)) contains a 5-hydroxytryptophan (5OHW) probe at position 269 whose fluorescence is sensitive to the head-to-tail interaction and allows us to accompany titrations of Tm143-284(5OHW269) with ASTm1-142 to calculate the dissociation constant (Kd) and the interaction energy at TFE and glycerol concentrations between 0% and 15%. We observe that TFE, but not glycerol, reduces the stability of the head-to-tail complex. Thermal denaturation experiments also showed that the head-to-tail complex increases the overall conformational stability of the Tm fragments. Urea and thermal denaturation assays demonstrated that both TFE and glycerol increase the stability of the isolated N- and C-terminal fragments; however, only TFE caused a significant reduction in the cooperativity of unfolding these fragments. Our results show that these two cosolvents stabilize the structures of individual Tm fragments in different manners and that these differences may be related to their opposing effects on head-to-tail complex formation.

Thermal unfolding of smooth muscle and nonmuscle tropomyosin alpha-homodimers with alternatively spliced exons

We used differential scanning calorimetry (DSC) and circular dichroism (CD) to investigate thermal unfolding of recombinant fibroblast isoforms of alpha-tropomyosin (Tm) in comparison with that of smooth muscle Tm. These two nonmuscle Tm isoforms 5a and 5b differ internally only by exons 6b/6a, and they both differ from smooth muscle Tm by the N-terminal exon 1b which replaces the muscle-specific exons 1a and 2a. We show that the presence of exon 1b dramatically decreases the measurable calorimetric enthalpy of the thermal unfolding of Tm observed with DSC, although it has no influence on the alpha-helix content of Tm or on the end-to-end interaction between Tm dimers. The results suggest that a significant part of the molecule of fibroblast Tm (but not smooth muscle Tm) unfolds noncooperatively, with the enthalpy no longer visible in the cooperative thermal transitions measured. On the other hand, both DSC and CD studies show that replacement of muscle exons 1a and 2a by nonmuscle exon 1b not only increases the thermal stability of the N-terminal part of Tm, but also significantly stabilizes Tm by shifting the major thermal transition of Tm to higher temperature. Replacement of exon 6b by exon 6a leads to additional increase in the alpha-Tm thermal stability. Thus, our data show for the first time a significant difference in the thermal unfolding between muscle and nonmuscle alpha-Tm isoforms, and indicate that replacement of alternatively spliced exons alters the stability of the entire Tm molecule.