Localized synthesis and transport of sterol at ER endocytic contact sites initiates actin polymerization

Abstract

Oxysterol Binding Proteins (OSBP) are a family of conserved lipid binding proteins, enriched at endoplasmic reticulum (ER) contacts sites. OSBPs promote non-vesicular lipid transport to other organelles and work as lipid sensors in the context of multiple cellular tasks, but the determinants of their distinct localization and function are still not understood. Using a combination of Time Resolved Electron Microscopy (TREM) and life-cell imaging in yeast, we demonstrate that the endocytic invaginations associate with the cortical endoplasmic reticulum as they mature, and that this association requires the OSBPs Osh2 and Osh3, which bridge the endocytic myosin-I Myo5 to the ER integral-membrane VAMP-associated protein (VAP) Scs2. Using mutations that specifically disrupt the myosin-I/OSBP/VAP link, as well as a reticulon mutant with extended cER-free plasma membrane subdomains, we show that ER contact to the endocytic sites has a dual function initiating actin-dependent membrane invagination and promoting vesicle scission. Further, we show that ER-induced actin polymerization requires the localized transfer of sterols by the endocytic OSBPs.