The Effect of Tropomyosin Mutations on Actin-Tropomyosin Binding: In Search of Lost Time

Abstract

The initial binding of tropomyosin onto actin filaments and then its polymerization into continuous cables on the filament surface must be precisely tuned to overall thin-filament structure, function, and performance. Low-affinity interaction of tropomyosin with actin has to be sufficiently strong to localize the tropomyosin on actin, yet not so tight that regulatory movement on filaments is curtailed. Likewise, head-to-tail association of tropomyosin molecules must be favorable enough to promote tropomyosin cable formation but not so tenacious that polymerization precedes filament binding. Arguably, little molecular detail on early tropomyosin binding steps has been revealed since Wegner's seminal studies on filament assembly almost 40 years ago. Thus, interpretation of mutation-based actin-tropomyosin binding anomalies leading to cardiomyopathies cannot be described fully. In vitro, tropomyosin binding is masked by explosive tropomyosin polymerization once cable formation is initiated on actin filaments. In contrast, in silico analysis, characterizing molecular dynamics simulations of single wild-type and mutant tropomyosin molecules on F-actin, is not complicated by tropomyosin polymerization at all. In fact, molecular dynamics performed here demonstrates that a midpiece tropomyosin domain is essential for normal actin-tropomyosin interaction and that this interaction is strictly conserved in a number of tropomyosin mutant species. Elsewhere along these mutant molecules, twisting and bending corrupts the tropomyosin superhelices as they ''lose their grip'' on F-actin. We propose that residual interactions displayed by these mutant tropomyosin structures with actin mimic ones that occur in early stages of thin-filament generation, as if the mutants are recapitulating the assembly process but in reverse. We conclude therefore that an initial binding step in tropomyosin assembly onto actin involves interaction of the essential centrally located domain.

Cardiomyopathy Mutation Alters End-to-End Junction of Tropomyosin and Reduces Calcium Sensitivity

“…The three conformational states of Tpm in the sarcomere result from a cooperative, azimuthally directed shift, which is fundamental to the on-off switching mechanism (5,13,14) that ultimately controls heart muscle contraction during each heartbeat. Previous studies on DCM-linked Tpm mutations have shown both local and delocalized structural alterations in Tpm-Tpm and in Tpm-actin interactions (15)(16)(17). However, these studies indicate that such alterations may be mutation specific and not a result of an effect common to the different mutations.…”

“…Our earlier work showed that, in addition to the flexural variance described above, subtle cumulative twisting of the Tpm coiled coil is critical to assure strong alignment between Tpm-actin electrostatic contacts (17,39). Fig.…”

“…The A277V-induced overtwist is located near Tpm E139, a charged residue thought to be critical for binding a cluster of positively charged amino acid side chains on actin (17,40). The overtwist, which can be clearly seen in Fig.…”

Special Issue: The Actin-Myosin Interaction in Muscle: Background and Overview

“…Actin (thin) filaments are composed mainly of G-actin monomers ([37]: Figure 5) that have aggregated to form long helical assemblies (filamentous actin; F-actin [38]) along which run strands of the regulatory proteins tropomyosin and troponin (Figure 2d). Tropomyosin molecules are rather like part of the myosin rod in that they are parallel 2-chain α–helical molecules, but they are only about 400 Å long [3,39,40,41]. Tropomyosin molecules link end-to-end to form long strands along the actin filaments; they follow the long-pitched helical symmetry of the actin filaments.…”

Structural basis underlying specific biochemical activities of non-muscle tropomyosin isoforms

“…S4). It corresponds to residues Lys326 and Lys328, which have been predicted to bind tropomyosins via salt bridges, alongside negatively charged Asp311 (Lehman et al, 2019; Pavadai et al, 2020) (see below). Most of the actin-facing side of both Tpm1.6 (Fig.…”