Electron microscopy and persistence length analysis of semi-rigid smooth muscle tropomyosin strands

The structural mechanics of tropomyosin are essential determinants of its affinity and positioning on F-actin. Thus, tissue-specific differences among tropomyosin isoforms may influence both access of actin-binding proteins along the actin filaments and the cooperativity of actin-myosin interactions. Here, 40 nm long smooth and striated muscle tropomyosin molecules were rotary-shadowed and compared by means of electron microscopy. Electron microscopy shows that striated muscle tropomyosin primarily consists of single molecules or paired molecules linked end-to-end. In contrast, smooth muscle tropomyosin is more a mixture of varying-length chains of end-to-end polymers. Both isoforms are characterized by gradually bending molecular contours that lack obvious signs of kinking. The flexural stiffness of the tropomyosins was quantified and evaluated. The persistence lengths along the shaft of rotary-shadowed smooth and striated muscle tropomyosin molecules are equivalent to each other (approximately 100 nm) and to values obtained from molecular-dynamics simulations of the tropomyosins; however, the persistence length surrounding the end-to-end linkage is almost twofold higher for smooth compared to cardiac muscle tropomyosin. The tendency of smooth muscle tropomyosin to form semi-rigid polymers with continuous and undampened rigidity may compensate for the lack of troponin-based structural support in smooth muscles and ensure positional fidelity on smooth muscle thin filaments.