Nonmuscle Myosin II Moves in New Directions

For many years, analyses of the role of the actomyosin cytoskeleton in many basic cellular processes have centered on actin. Increasingly, however, a number of investigators are examining proteins that are proximal to actin; in particular, nonmuscle myosin II (NMII). Recent experiments have increased our understanding of the role of NMII in three related cellular activities: generation of cell polarity, cell migration and cell-cell adhesion. Progress has been particularly promising thanks to the use of new microscopic, genetic and biochemical techniques. In mammalian systems, generation of transgenic mice and the introduction of specific siRNAs have been useful in deciphering the role of the three different isoforms of NMII: NMIIA, NMIIB and NMIIC. Studies in Drosophila and Aplysia, which are informative model systems for investigating the function of NMII, have also shed light on NMII. Recent work examines the contractile and structural roles that NMII plays at cell-cell boundaries, and both its contractile and actin-crosslinking roles in cell migration. In addition, NMII might also function as a scaffold molecule, anchoring signaling molecules, such as kinases and Rho GTPase guanine nucleotide exchange factors.