A Flat Protein Complex Shapes Rough ER Membrane Sheets

Rough ER sheets are a fundamental domain of the ER and the gateway into the secretory pathway. While reticulon proteins stabilize high-curvature ER tubules, it is unclear if 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. Our results demonstrate an unexpected way for proteins to sense and propagate flat membrane sheets.