Unconventional myosins in cell movement, membrane traffic, and signal transduction

In the past few years genetic, biochemical, and cytolocalization data have implicated members of the myosin superfamily of actin-based molecular motors in a variety of cellular functions including membrane trafficking, cell movements, and signal transduc-tion. The importance of myosins is illustrated by the identification of myosin genes as targets for disease-causing mutations. The task at hand is to decipher how the multitude of myosins function at both the molecular and cellular level—a task facilitated by our understanding of myosin structure and function in muscle. Over the past several years, evidence has emerged for the existence of a large super-family of myosins. To date, at least 13 struc-turally distinct classes (1) of myosin heavy chains have been identified in addition to the well-characterized myosins-II of muscle and nonmuscle cells (Figs. 1 and 2). These classes are empirically defined on the basi