A PKC that controls polyphosphate levels, pinocytosis and exocytosis, regulates stationary phase onset in Dictyostelium

Many cells can pause their growth cycle, a topic much enriched by studies of the stationary phase (SP) of model microorganisms. Although several kinases are implicated in SP onset, whether protein kinase C has a role remains unknown. We show that Dictyostelium discoideum cells lacking pkcA entered SPat a reduced cell density, but only in shaking conditions. Precocious SP entry occurs because levels of extracellular polyphosphate (polyP) reach the threshold needed to induce the SP onset at a lower cell density than seen in wild-type cells; adding exopolyphosphatase to pkcA- cells reverses the effect and mimics wild-type growth. PkcA-mediated regulation of polyP depended on inositol hexakisphosphate kinase and phospholipase D. PkcA- mutants also had higher F-actin levels, higher rates of exocytosis and lower pinocytosis rates. Postlysosomes were smaller and present in fewer pkcA- cells compared to the wild type. Overall, the results suggest that a reduced PkcA level triggers SP primarily because cells do not acquire or retain nutrients as efficiently, thus mimicking, or amplifying, the conditions of actual starvation.