Structural Basis of Myosin 1C Ca2+ Regulation

Abstract

Myosins are actin-based motor proteins that hydrolyze ATP to generate force and movement. Class I myosins are single-headed and can be subdivided into short and long-tailed isoforms. Myosin 1c is a short-tailed isoform involved in the insulin-dependent transport of GLUT4-storage vesicles and the adaptation response of the stereocilia in the inner ear. Both these processes are Ca2+-regulated. Here, we show how myosin 1c motor activity is regulated by calcium ion binding to calmodulin at the first IQ motif. We present the crystal structure of the human myosin 1c motor domain in complex with calmodulin bound to the first IQ motif in the pre-power stroke state at 2.7 A. The structure reveals a lever arm orientation that positions the Ca2+-binding lobes of calmodulin closely to the HO-linker of the motor domain. Deletion of a unique 6 amino acids insertion in the HO-linker abolished Ca2+-regulation of myosin 1c, as evidenced by a detailed comparative kinetic analysis of wild-type and deletion mutants. Our findings reveal an allosteric communication pathway, where the extended HO-linker senses Ca2+-induced changes in calmodulin. These changes are transduced via the HO-helix and the central β-sheet to residues that mediate coupling between the actin and nucleotide binding sites.