Sphingoid base signaling via Pkh kinases is required for endocytosis in yeast

In yeast, sphingoid base synthesis is required for the internalization step of endocytosis and organization of the actin cytoskeleton. We show that ov erexpression of either one of the two kinases Pkh1p or Pkh2p, that are homo logous to mammalian 3-phosphoinositide-dependent kinase-1 (PDK1), can speci fically suppress the sphingoid base synthesis requirement for endocytosis. Pkh1p and Pkh2p have an overlapping function because only a mutant with imp aired function of both kinases is defective for endocytosis. Pkh1/2p kinase s are activated in vitro by nanomolar concentrations of sphingoid base. The se results suggest that Pkh1/2p kinases are part of a sphingoid base-mediat ed signaling pathway that is required for the internalization step of endoc ytosis. The Pkc1p kinase that is phosphorylated by Pkh1/2p kinases and play s a role in endocytosis was identified as one of the downstream effectors o f this signaling cascade.