The ER membrane protein complex promotes biogenesis of sterol-related enzymes maintaining cholesterol homeostasis.

Norbert Volkmar,Szymon Juszkiewicz,Maria-Laetitia Thezenas,John C Christianson,Ramanujan S Hegde,Daniel K Nomura,Sharon M Louie,Benedikt M Kessler

doi:10.1242/jcs.223453

Abstract

ABSTRACTThe eukaryotic endoplasmic reticulum (ER) membrane contains essential complexes that oversee protein biogenesis and lipid metabolism, impacting nearly all aspects of cell physiology. The ER membrane protein complex (EMC) is a newly described transmembrane domain (TMD) insertase linked with various phenotypes, but whose clients and cellular responsibilities remain incompletely understood. We report that EMC deficiency limits the cellular boundaries defining cholesterol tolerance, reflected by diminished viability with limiting or excessive extracellular cholesterol. Lipidomic and proteomic analyses revealed defective biogenesis and concomitant loss of the TMD-containing ER-resident enzymes sterol-O-acyltransferase 1 (SOAT1) and squalene synthase (SQS, also known as FDFT1), which serve strategic roles in the adaptation of cells to changes in cholesterol availability. Insertion of the weakly hydrophobic tail-anchor (TA) of SQS into the ER membrane by the EMC ensures sufficient flux through the sterol biosynthetic pathway while biogenesis of polytopic SOAT1 promoted by the EMC provides cells with the ability to store free cholesterol as inert cholesteryl esters. By facilitating insertion of TMDs that permit essential mammalian sterol-regulating enzymes to mature accurately, the EMC is an important biogenic determinant of cellular robustness to fluctuations in cholesterol availability.This article has an associated First Person interview with the first author of the paper.

Highlights

Almost all ER membrane protein complex (EMC) subunits were lost in EMC6 knockdowns, their corresponding mRNA levels were not significantly changed (Fig. S1D), suggesting that the remaining subunits are degraded post-translationally
AVA further enhanced sensitivity of ΔEMC6 cells to Chol:MBCD-induced cell death, indicating that sterol-Oacyltransferase 1 (SOAT1) activity might be limiting but not entirely absent. These findings indicate a post-transcriptional defect in SOAT1 biogenesis coinciding with EMC deficiency that results in degradation and compromised esterification capacity
Critical roles for subunits in EMC assembly Individual subunits of the EMC have been implicated in regulation of various proteins and processes, but how those activities are linked to the fully assembled, mature EMC oligomer had not been fully appreciated

Summary

Introduction

Its major roles in the cell include the biogenesis of most secretory and membrane proteins, lipid biosynthesis, Ca2+ storage and the regulation of various metabolic pathways. The maintenance of ER homeostasis is of central importance to overall cellular fitness, and identifying the requisite factors and pathways has been a major goal of contemporary cell biology. Largescale genetic interaction analyses in yeast have identified several ER-resident proteins and complexes with currently unknown functions (Jonikas et al, 2009; Schuldiner et al, 2005). One such complex is the ER membrane protein complex (EMC), an abundant, multi-subunit protein complex, present in all eukaryotic kingdoms (Wideman, 2015)

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