Voeltz, G. K. | bioRxiv Science

bioRxiv2023

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.

cell biology↗

bioRxiv2022

An ER phospholipid hydrolase drives ER-associated mitochondrial constriction for fission and fusion

Mitochondria are dynamic organelles that undergo cycles of fission and fusion at a unified platform defined by endoplasmic reticulum (ER)-mitochondria membrane contact sites (MCSs). These MCSs or nodes co-localize fission and fusion machinery. We set out to identify how ER-associated mitochondrial nodes can regulate both fission and fusion machinery assembly. We have used a promiscuous biotin ligase linked to the fusion machinery, Mfn1, and proteomics to identify an ER membrane protein, Aphyd, as a major regulator of node formation. In the absence of Aphyd, fission and fusion machineries fail to recruit to ER-associated mitochondrial nodes and fission and fusion rates are significantly reduced. Aphyd contains an acyltransferase motif and an /{beta} hydrolase domain and point mutations in critical residues of these regions fail to rescue the formation of ER-associated mitochondrial hot spots. These data suggest a mechanism whereby Aphyd functions by altering phospholipid composition at ER-mitochondria MCSs. Our data present the first example of an ER membrane protein that regulates the recruitment of both fission and fusion machineries to mitochondria.

cell biology↗