A protein containing a serine-rich domain with vesicle fusing properties mediates cell cycle-dependent cytosolic pH regulation

Initial differentiation in Dictyostelium involves both asymmetric cell divi sion and a cell cycle-dependent mechanism, We previously identified a gene, rtoA, which when disrupted randomizes the cell cycle-dependent mechanism w ithout affecting either the underlying cell cycle or asymmetric differentia tion. We find that in wild-type cells, RtoA levels vary during the cell cyc le, Cytosolic pH, which normally varies with the cell cycle, is randomized in rtoA cells. The middle 60% of the RtoA protein is 10 tandem repeats of a n 11 peptide long serine-rich motif, which we find has a random coil struct ure. This domain catalyzes the fusion of phospholipid vesicles in vitro. Co nversely, rtoA cells have a defect in the fusion of endocytic vesicles. The y also have a decreased exocytosis rate, a decreased pH of endocytic/ exocy tic vesicles, and an increased average cytosolic pH. Our data indicate that the serine-rich domain of RtoA can mediate membrane fusion and that RtoA c an increase the rate of vesicle fusion during processing of endocytic vesic les. We hypothesize that RtoA modulates initial cell type choice by linking vegetative cell physiology to the cell cycle.