CONSTITUTIVE ACTIVATION OF ENDOCYTOSIS BY MUTATION OF MYOA, THE MYOSIN-I GENE OF ASPERGILLUS-NIDULANS

Class I myosins function in cell motility, intracellular vesicle traff icking and endocytosis, Recently, it was shown that class I myosins ar e phosphorylated by a member of the pal-activated kinase (PAK) family. PAK phosphorylates a conserved serine or threonine residue in the myo sin heavy chain. Phosphorylation at this site is required for maximal activation of the actin-activated Mg2+-ATPase activity in vitro. This serine or threonine residue is conserved En all known class I myosins of microbial origin and in the human and mouse class VI myosins. We ha ve investigated the in vivo significance of this phosphorylation by mu tating serine 371 of the class I myosin heavy chain gene myoA of Asper gillus nidulans. Mutation to glutamic acid, which mimics phosphorylati on and therefore activation of the myosin, results in ale accumulation of membranes in growing hyphae. This accumulation of membranes result s from an activation of endocytosis, In contrast, mutation of serine 3 71 to alanine had no discernible effect ore endocytosis. These studies are the first to demonstrate the in vivo significance of a regulatory phosphorylation on a class I myosin. Furthermore, our results suggest that MYOA has two functions, one dependent and one independent of pho sphorylation.