FIG 8.

Fpk1 protein kinase is a signaling node that controls membrane permeability and the efficiency of endocytosis. In cells stressed by a diminution in the supply of sphingolipids, Fpk1 is less active for two reasons: (i) lack of direct stimulation by MIPC (23) and (ii) inhibitory phosphorylation by Ypk1, which is activated in a TORC2-dependent manner when sphingolipids are limiting (6, 7). The cell cycle-regulated protein kinase Gin4 also inhibits Fpk1; thus, Fpk1 is also downregulated when Gin4 is active (31). Fpk1, in turn, phosphorylates and stimulates the flippases Dnf1 and Dnf2 and, as shown in this study, phosphorylates and negatively regulates Akl1. One function of Akl1 is to impede endocytosis by phosphorylation of multiple endocytic factors (including Sla1, Ent2, and Pan1), which disables their function. Thus, when Fpk1 activity is high, Akl1 function is downregulated and endocytosis can proceed; when Fpk1 activity is low, Akl1 function is released from inhibition, causing phosphorylation of it targets and thereby down-modulating the efficiency of endocytosis. Two concomitant changes in PM composition ensue when Fpk1 activity is decreased: aminoglycerophospholipid content in the outer leaflet remains higher due to lack of Fpk1-mediated stimulation of the flippases, and bulk membrane and lipid internalization via clathrin-mediated endocytosis is impeded. Both effects contribute to conferring elevated resistance to certain toxic xenobiotic compounds, such as doxorubicin and hygromycin B.