Nodal modulator is required to sustain endoplasmic reticulum morphology

Abstract

Nodal Modulator (NOMO) is a widely conserved type I transmembrane protein of unknown function, with three nearly identical orthologs specified in the human genome. We identified NOMO1 in a proteomics approach aimed at the identification of proteins that support the structural integrity of the endoplasmic reticulum (ER). Overexpression of NOMO1 imposes a sheet morphology on the ER, while depletion of NOMO1 and its orthologs causes a collapse of ER morphology concomitant with the formation of membrane-delineated holes in the ER network. These structures are positive for the autophagy marker LAMP1, and LC3 is profoundly upregulated upon NOMO depletion. In vitro reconstitution of NOMO1 revealed a dimeric state that is mediated by the cytosolic tail domain, with each monomer featuring a "beads on a string" structure likely representing bacterial Ig-like folds. Based on these observations and a genetic epistasis analysis including the known ER-shaping proteins Atlastin2 and Climp63, we propose a role for NOMO1 in the functional network of ER-shaping proteins.