THE PHOSPHORYLATION SITE FOR STE20P-LIKE PROTEIN-KINASES IS ESSENTIALFOR THE FUNCTION OF MYOSIN-I IN YEAST

The budding yeast Saccharomyces cerevisiae has two functionally redund ant myosin-I isoforms encoded by the MYO3 and MYO5 genes. The function shared by these myosin proteins is required for proper yeast budding, Serine residue 357 in the head domain of Myo3p, conserved among myosi n-I proteins including yeast Myo5p, was identified as a unique phospho rylation site for the serine/threonine protein kinase Ste20p and its c losely related isoform Cla4p. These protein kinases share a function t hat is also essential for budding. Replacement of serine 357 with alan ine disrupted the in vivo function of Myo3p, whereas this function was maintained by changing the serine residue to aspartate, This mutant v ersion failed to compensate the growth defect of cells which lack both Ste20p and Cla4p, suggesting that myosin-I is not the only essential target of these protein kinases, Our results suggest that phosphorylat ion of the head domain by Ste20p-like protein kinases plays an essenti al role in the function of myosin-I in yeast cells.