A Conserved, Lipid-Mediated Sorting Mechanism of Yeast Ist2 and Mammalian STIM Proteins to the Peripheral ER

Sorting of yeast Ist2 to the plasma membrane (PM) or the cortical endoplasmic reticulum (ER) requires a cortical sorting signal (CSSIst2) that interacts with lipids including phosphatidylinositol-4,5-bisphosphate (PI(4,5)P-2) at the PM. Here, we show that the expression of Ist2 in mammalian cells resulted in a peripheral patch-like localization without any detection of Ist2 at the cell surface. Attached to C-termini of mammalian integral membrane proteins, the CSSIst2 targeted these proteins to PM-associated domains of the ER and abolished trafficking via the classical secretory pathway. The interaction of integral membrane proteins with PI(4,5)P-2 at the PM created ER-PM contacts. This process is similar to the regulated coupling of ER domains to the PM via stromal interaction molecule (STIM) proteins during store-operated Ca2+ entry (SOCE). The CSSIst2 and the C-terminus of the ER-located Ca2+ sensor STIM2 were sufficient to bind PI(4,5)P-2 and PI(3,4,5)P-3 at the PM, showing that an evolutionarily conserved mechanism is involved in the sorting of integral membrane proteins to PM-associated domains of the ER. Yeast Ist2 and STIM2 share a common basic and amphipathic signal at their extreme C-termini. STIM1 showed binding preference for liposomes containing PI(4,5)P-2, suggesting a specific contribution of lipids to the recruitment of ER domains to the PM during SOCE.