Tails of unconventional myosins

In addition to the conventional myosins (class II) required for processes s uch as muscle contraction and cytokinesis, the myosin superfamily of actin- based motor proteins includes at least 14 'unconventional' classes. These u nconventional myosins are defined by myosin-like head (motor) domains attac hed to class-specific tail domains that differ greatly from those of myosin -II. The unconventional myosins account for almost:two-thirds of the 28 or more myosin genes currently believed to be expressed in humans and 80-90% o f the similar to 10 or more myosin genes expressed in a typical nonmuscle c ell. Although these members of the myosin superfamily have not been as inte nsively investigated as the conventional myosins, unconventional myosins ar e known or believed to power many forms of actin-based motility and organel le trafficking. The presence of signaling domains such as kinase domains, S H3 domains, PH domains or GTPase-activating domains in the tails of unconve ntional myosins indicates that these proteins can also be components of sig nal transduction pathways. Since several classes of the myosin superfamily hale been found only in lower eukaryotes or plants (VIII,:XI,:XIII and XIV) , in this review we will focus on the structures and properties of the unco nventional myosins found in multicellular animals (excluding classes I and V, which have been reviewed elsewhere recently). Special attention will be focused on the three classes of unconventional myosins that can cause deafn ess in mouse or humans when mutated. In addition, we discuss the discovery of a pair of intriguing domains, the Myosin Tail Homology 4 (MyTH4) and FER M (band 4.1, Ezrin, Radixin, Moesin) domains, that are present in the tails of otherwise very different myosins as well as a plant kinesin-like protei n. Recent progress in the identification of novel unconventional myosins wi ll also be summarized.