Crystal structures of cables formed by the acetylated and unacetylated forms of the Schizosaccharomyces pombe tropomyosin ortholog Tpm.

Cables formed by head-to-tail polymerization of tropomyosin, localized along the length of sarcomeric and cytoskeletal actin filaments, play a key role in regulating a wide range of motile and contractile processes. The stability of tropomyosin cables, their interaction with actin filaments and the functional properties of the resulting co-filaments are thought to be affected by N-terminal acetylation of tropomyosin. Here, we present high-resolution structures of cables formed by acetylated and unacetylated Schizosaccharomyces pombe tropomyosin ortholog Tpm. The crystal structures represent different types of cables, each consisting of Tpm homodimers in a different conformation. The structures show how the interactions of the residues in the overlap junction contribute to cable formation and how local structural perturbations affect the conformational dynamics of the protein and its ability to transmit allosteric signals. In particular, N-terminal acetylation increases the helicity of the adjacent region, which leads to a local reduction in conformational dynamics and consequently to less fraying of the N-terminal region. This creates a more consistent complementary surface facilitating the formation of specific interactions across the overlap junction.