The N-terminal tropomyosin- and actin-binding sites are important for leiomodin 2's function

• Published in: Molecular biology of the cell 2016-08, Vol.27 (16), p.2565-2575
• Main Authors: Ly, Thu, Moroz, Natalia, Pappas, Christopher T, Novak, Stefanie M, Tolkatchev, Dmitri, Wooldridge, Dayton, Mayfield, Rachel M, Helms, Gregory, Gregorio, Carol C, Kostyukova, Alla S
• Format: Article
• Language: English
• Subjects: Actin Cytoskeleton - metabolism; Actins - metabolism; Amino Acid Sequence; Animals; Binding Sites; Carrier Proteins - metabolism; Chickens; Cytoskeletal Proteins - genetics; Cytoskeletal Proteins - metabolism; Mice; Microfilament Proteins - metabolism; Muscle Proteins - genetics; Muscle Proteins - metabolism; Myocytes, Cardiac - metabolism; Protein Binding; Protein Domains; Sarcomeres - metabolism; Tropomodulin - genetics; Tropomodulin - metabolism; Tropomyosin - metabolism
• ISSN: 1059-1524; 1939-4586
• DOI: 10.1091/mbc.E16-03-0200

Leiomodin is a potent actin nucleator related to tropomodulin, a capping protein localized at the pointed end of the thin filaments. Mutations in leiomodin-3 are associated with lethal nemaline myopathy in humans, and leiomodin-2-knockout mice present with dilated cardiomyopathy. The arrangement of the N-terminal actin- and tropomyosin-binding sites in leiomodin is contradictory and functionally not well understood. Using one-dimensional nuclear magnetic resonance and the pointed-end actin polymerization assay, we find that leiomodin-2, a major cardiac isoform, has an N-terminal actin-binding site located within residues 43-90. Moreover, for the first time, we obtain evidence that there are additional interactions with actin within residues 124-201. Here we establish that leiomodin interacts with only one tropomyosin molecule, and this is the only site of interaction between leiomodin and tropomyosin. Introduction of mutations in both actin- and tropomyosin-binding sites of leiomodin affected its localization at the pointed ends of the thin filaments in cardiomyocytes. On the basis of our new findings, we propose a model in which leiomodin regulates actin poly-merization dynamics in myocytes by acting as a leaky cap at thin filament pointed ends.