ACTIVATION OF MYOSIN-I BY MEMBERS OF THE STE20P PROTEIN-KINASE FAMILY

The heavy chain of myosin-ID isolated from Dictyostelium was identifie d as an in vitro substrate for members of the Ste20p family of serine/ threonine protein kinases which are thought to regulate conserved mito gen-activated protein kinase pathways. Yeast Ste20p and Cla4p and mamm alian p21-activated protein kinase (PAK) phosphorylated the heavy chai n to 0.5-0.6 mol of P-i/mol and stimulated the actin-dependent Mg2+-AT Pase activity to an extent equivalent to that of the Ste20p-like myosi n-I heavy chain kinase isolated from Dictyostelium. PAK purified from rat brain required GTP gamma S-Cdc42 to express full activity, whereas recombinant mouse mPAK3 fused to glutathione S-transferase and purifi ed from bacteria, and Ste20p and Cla4p purified from yeast extracts we re fully active without GTP gamma S-Cdc42. These results suggest, toge ther with the high degree of structural and functional conservation of Ste20p family members and myosin-I isoforms, that myosin-I activation by Ste20p family protein kinases may contribute to the regulation of morphogenetic processes in organisms ranging from yeast to mammalian c ells.