The activity of smooth muscle/non-muscle myosin II is regulated by the phosphorylation of MRLC that is modulated by the antagonistic activity of myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP). MLCP

Myosins are a superfamily of actin-dependent molecular motor proteins involved in a variety of essential processes that include muscular contraction, cytokinesis, vesicle transport, cell movement and cell shape change (reviewed by Mermall et al., 1998; Oliver et al., 1999; Sellers, 2000; Sokac and Bement, 2000; Wu et al., 2000). Among the 17 subclasses of myosins, conventional myosins, known as myosin IIs, have been the most studied. Myosin IIs form bipolar filaments that drive contractile events by bringing together actin filaments of opposite polarity. Myosin II molecules are hexameric enzymes consisting of two heavy chains, two regulatory light chains (MRLC), and two essential light chains. They can be subclassified into four groups based on their motor domain (or tail) sequences: sarcomeric myosins, vertebrate smooth muscle/non-muscle myosins, Dictyostelium/Acanthamoeba type myosins and yeast type myosins. The activity of smooth muscle/non-muscle myosin II is regulated by the phosphorylation of MRLC that is modulated by the antagonistic activity of myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP). MLCP is composed of three subunits: a catalytic subunit made up of protein phosphatase 1c β (also called δ), a myosin binding or targeting subunit (MYPT), and a small subunit of unknown function. MYPT binds and confers the selectivity of PP1c for myosin (Hartshorne et al., 1998). The phosphatase activity of MLCP can be regulated in several ways (reviewed by Hartshorne et al., 1998; Somlyo and Somlyo, 2000). Rho-kinase (ROCK) phosphorylates an inhibitory phosphorylation site on MYPT and inhibits the phosphatase activity in smooth muscle. This phosphorylation may occur through ZIPK (leucine zipper interacting protein kinase)-like kinase (MacDonald et al., 2001) or integrin-linked kinase (Kiss et al., 2002). Myotonic dystrophy protein kinase phosphorylates the same inhibitory phosphorylation site (Muranyi et al., 2001), although it is not clear whether this phosphorylation event also goes through ZIPK. In addition, protein kinase C (PKC) can phosphorylate the ankyrin repeat region of MYPT, and thus attenuate the interaction of MYPT with PP1c and MRLC (Toth et al., 2000). Furthermore, CPI17, a smooth muscle-specific inhibitor of MLCP, can also regulate the phosphatase activity of MLCP. Phosphorylation of CPI-17 by PKC, or ROCK, or protein kinase N, or p21activated kinase (PAK) dramatically enhances the inhibition ability of CPI-17 (Eto et al., 1997; Koyama et al., 2000; Senba et al., 1999; Takizawa et al., 2002a; Takizawa et al., 2002b). Finally, MRLC can also be phosphorylated by ROCK and PAK, which itself is a substrate of Rac and Cdc42. Thus ROCK can regulate MRLC phosphorylation both through direct phosphorylation of MRLC and through inactivation of MLCP. Importantly, although the biochemistry of these phosphorylation events is well characterized, the physiological significance of these regulatory steps in vivo remains to be explored. The in vivo function of non-muscle myosin II has been extensively analyzed in Drosophila melanogaster, Dictyostelium discoideum and Saccharomyces cerevisiae. Drosophila has a single non-muscle myosin II heavy chain encoded by zipper (zip), as well as a single non-muscle myosin 671 Development 130, 671-681 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00298