Smooth Muscle Tropomyosin Forms Semi-Rigid End to End Polymers

Abstract

Variation in the structural mechanics of tropomyosin isoforms may govern differences in their affinity and positioning on F-actin. Such differences may influence the access of actin-binding proteins along the sides of actin filaments and also the cooperativity of actin-myosin interactions. Here, smooth and striated muscle tropomyosin were rotary shadowed and compared by electron microscopy. EM shows that cardiac and skeletal tropomyosin primarily consist of 40 nm long single molecules, whereas smooth muscle tropomyosin is a mixture of varying length chains of end-to-end linked molecules found together with single molecules. The tendency of smooth muscle tropomyosin to polymerize reflects greater end-to-end interaction, possibly required on smooth muscle thin filaments, which lack troponin to stabilize this interaction. Measurement of the apparent persistence length (PL) of single smooth muscle tropomyosin molecules and the chain-like polymers yield indistinguishable values, which are comparable to those that we find for cardiac tropomyosin. The semi-rigidity of smooth muscle tropomyosin polymers may ensure a high degree of positional fidelity of tropomyosin on smooth muscle thin filaments, despite the lack of troponin (cf. Lehman et al., 2009). It is unlikely, however, that stiff, polymerized superhelical chains of tropomyosin can bind directly to F-actin. However, in vitro an equilibrium may yield sufficient single smooth muscle tropomyosin molecules or short chains to bind. In vivo, actin and smooth muscle (or cytoskeletal) tropomyosin may copolymerize or, alternatively, G-actin may polymerize on a scaffold of tropomyosin chains. Thus differing mechanisms of thin filaments assembly may be related to tropomyosin end-to-end binding strength.