RESUMEN
Rough endoplasmic reticulum (ER) sheets are a fundamental domain of the ER and the gateway into the secretory pathway. Although reticulon proteins stabilize high-curvature ER tubules, it is unclear whether other proteins scaffold the flat membranes of rough ER sheets. Through a proteomics screen using ER sheet-localized RNA-binding proteins as bait, we identify the sigma-1 receptor (SigmaR1) as an ER sheet-shaping factor. High-resolution live cell imaging and electron tomography assign SigmaR1 as an ER sheet-localized factor whose levels determine the amount of rough ER sheets in cells. Structure-guided mutagenesis and in vitro reconstitution on giant unilamellar vesicles further support a mechanism whereby SigmaR1 oligomers use their extended arrays of amphipathic helices to bind and flatten the lumenal leaflet of ER membranes to oppose membrane curvature and stabilize rough ER sheets.