MCTP1 and MCTP2 promote ER-PM contact sites and regulate PI4P homeostasis and cell migration.

Endoplasmic reticulum-plasma membrane (ER-PM) contact sites play important roles in maintaining lipid homeostasis at plasma membrane (PM), cellular calcium homeostasis and cell signaling. Here, we show that MCTP1 and MCTP2 are at ER subdomains that form membrane contact sites (MCS) with multiple organelles using proximity labelling assay. MCTPs are three C2 domain-containing transmembrane proteins. We show that upon overexpression, MCTPs promote ER-PM contact sites in C2 domain dependent manner. MCTP C2 domains bind to PI(4)P and PI(4,5)P 2 , phosphoinositides that are enriched in the PM. Furthermore, we show that deletion of MCTP1 or MCTP2 increases PI(4)P levels in the PM and promote cell migration. Thus, our study identifies MCTPs as multiple ER-organelle contact site proteins and establishes its role at ER-PM contact sites in regulating lipid homeostasis and cell migration.