Role of myosin-II phosphorylation in V12Cdc42-mediated disruption of Drosophila cellularization

Microinjection of constitutively active Cdc42 (V12Cdc42) disrupts the actom yosin cytoskeleton during cellularization (Crawford et al., Dev. Biol., 204 , 151-164 (1998)). The p21-activated kinase (PAK) family of Ser/Thr kinases are effecters of GTP-bound forms of the small GTPases, Cdc42 and Rac. Dros ophila PAK, which colocalizes with actin and myosin-II during cellularizati on, concentrates at sites of V12Cdc42-induced actomyosin disruption. In vit ro biochemical analyses demonstrate that PAK phosphorylates the regulatory light Chain (RLC) of Drosophila nonmuscle myosin-ll on Seral, a site known to activate myosin-II function. Although activated PAK does not disrupt the actomyosin cytoskeleton, it induces increased levels of Ser21 phosphorylat ed RLC. These findings suggest that increased levels of RLC phosphorylation do not contribute to disruption of the actomyosin hexagonal array.