P21-ACTIVATED KINASE HAS SUBSTRATE-SPECIFICITY SIMILAR TO ACANTHAMOEBA MYOSIN-I HEAVY-CHAIN KINASE AND ACTIVATES ACANTHAMOEBA MYOSIN-I

Acanthamoeba class I myosins are unconventional, single-headed myosins that express actin-activated Mg2+-ATPase and in vitro motility activi ties only when a single serine or threonine in the heavy chain is phos phorylated by myosin I heavy chain kinase (MIHCK). Some other, but not most, class I myosins have the same consensus phosphorylation site se quence, and the two known class VI myosins have a phosphorylatable res idue in the homologous position, where most myosins have an aspartate or glutamate residue, Recently, we found that the catalytic domain of Acanthamoeba MIHCK has extensive sequence similarity to the p21-activa ted kinase (PAK)/STE20 family of kinases from mammals and yeast, which are activated by small GTP-binding proteins, The physiological substr ates of the PAK/STE20 kinases are not well characterized, In this pape r we show that PAK1 has similar substrate specificity as MIHCK when as sayed against synthetic substrates and that PAK1 phosphorylates the he avy chain (1 mol of P-i per mol) and activates Acanthamoeba myosin I a s MIHCK does, These results, together with the known involvement of Ac anthamoeba myosin I, yeast myosin I, STE20, PAK, and small GTP-binding proteins in membrane- and cytoskeleton-associated morphogenetic trans formations and activities, suggest that myosins may be physiological s ubstrates for the PAK/STE20 family and thus mediators of these events.