Increased protein kinase or decreased PP2A activity bypasses sphingoid base requirement in endocytosis

Lipids have been implicated in signal transduction and in several stages of membrane trafficking, but these two functions have not been functionally l inked. In yeast, sphingoid base synthesis is required for the internalizati on step of endocytosis and organization of the actin cytoskeleton. We show that inactivation of a protein phosphatase 2A (PP2A) or overexpression of o ne of two kinases, Yck2p or Pkc1p, can specifically suppress the sphingoid base synthesis requirement for endocytosis. The two kinases have an overlap ping function because only a mutant with impaired function of both kinases is defective in endocytosis. An ultimate target of sphingoid base synthesis may be the actin cytoskeleton, because overexpression of the kinases and i nactivation of PP2A substantially corrected the actin defect due to the abs ence of sphingoid base. These results suggest that sphingoid base controls protein phosphorylation, perhaps by activating a signal transduction pathwa y that is required for endocytosis and proper actin cytoskeleton organizati on in yeast.