A Sac7-Rho1 axis at the plasma membrane controls clathrin-independent endocytosis

Abstract

In eukaryotes, our understanding of clathrin-independent endocytosis (CIE) lags far behind that of clathrin-mediated endocytosis (CME). CIE plays key roles in internalizing receptors, viruses, bacterial toxins, and pathogens; thus, deeper mechanistic insights are critical for understanding cellular strategies for plasma membrane regulation. Yeast CIE requires a signal relay between the stress sensor Mid2, the guanine nucleotide exchange factor (GEF) Rom1, the Rho1 GTPase, and the formin Bni1. While GEFs promote GTPase activity, GTPase-activating proteins (GAPs) conversely stimulate nucleotide hydrolysis and GTPase inactivation. Here, we provide new insight into CIE, adding the RhoGAP Sac7 as a regulator. SAC7 deletion in CME-deficient cells improved cargo internalization, and Sac7 localizes primarily to the mother cortex. Cells lacking SAC7 accumulate active Rho1 and retain Bni1 at the plasma membrane, where Bni1 retention may subsequently enhance actin assembly needed for CIE. Our results thus demonstrate that Sac7 negatively regulates CIE by restricting cortical Rho1 activity.