Functional studies of individual myosin molecules.

The "conventional" isoform of myosin that polymerizes into filaments (myosin II) is the molecular motor powering contraction in all three types of muscle. Considerable attention has been paid to the developmental progression, isoform distribution, and mutations that affect myocardial development, function, and adaptation. Optical trap (laser tweezer) experiments and various types of high-resolution fluorescence microscopy, capable of interrogating individual protein motors, are revealing novel and detailed information about their functionally relevant nanometer motions and pico-Newton forces. Single-molecule laser tweezer studies of cardiac myosin isoforms and their mutants have helped to elucidate the pathogenesis of familial hypertrophic cardiomyopathies. Surprisingly, some disease mutations seem to enhance myosin function. More broadly, the myosin superfamily includes more than 20 nonfilamentous members with myriad cellular functions, including targeted organelle transport, endocytosis, chemotaxis, cytokinesis, modulation of sensory systems, and signal transduction. Widely varying genetic, developmental and functional disorders of the nervous, pigmentation, and immune systems have been described in accordance with these many roles. Compared to the collective nature of myosin II, some myosin family members operate with only a few partners or even alone. Individual myosin V and VI molecules can carry cellular vesicular cargoes much farther distances than their own size. Laser tweezer mechanics, single-molecule fluorescence polarization, and imaging with nanometer precision have elucidated the very different mechano-chemical properties of these isoforms. Critical contributions of nonsarcomeric myosins to myocardial development and adaptation are likely to be discovered in future studies, so these techniques and concepts may become important in cardiovascular research.