Tricalbin-Mediated Contact Sites Control ER Curvature to Maintain Plasma Membrane Integrity

Membrane contact sites (MCS) between the endoplasmic reticulum (ER) and the plasma membrane (PM) play fundamental roles in all eukaryotic cells. ER-PM MCS are particularly abundant in Saccharomyces cerevisiae, where approximately half of the PM surface is covered by cortical ER (cER). Several proteins, including Ist2, Scs2/22, and Tcb1/2/3 are implicated in cER formation, but the specific roles of these molecules are poorly understood. Here, we use cryo-electron tomography to show that ER-PM tethers are key determinants of cER morphology. Notably, Tcb proteins (tricalbins) form peaks of extreme curvature on the cER membrane facing the PM. Combined modeling and functional assays suggest that Tcb-mediated cER peaks facilitate the transport of lipids between the cER and the PM, which is necessary to maintain PM integrity under heat stress. ER peaks were also present at other MCS, implying that membrane curvature enforcement may be a widespread mechanism to regulate MCS function. [Display omitted] •Tethers of ER-plasma membrane (PM) contact sites shape cortical ER (cER) morphology•Tricalbins create peaks of extreme curvature on the cER membrane facing the PM•cER peaks are important to maintain PM integrity under heat stress•cER peaks may facilitate ER-to-PM lipid transport Using cryo-electron tomography, Collado et al. show that tricalbins generate peaks of extreme curvature on the cortical ER (cER) membrane at ER-plasma membrane (PM) contact sites. Functional assays and theoretical modeling indicate that cER peaks are important to maintain PM integrity under heat stress, possibly by facilitating cER-to-PM lipid transport.