The mammalian EMC in membrane protein biogenesis

The mammalian Endoplasmic Reticulum Membrane protein Complex (mEMC) has previously been implicated in a range of cellular activities, including the biogenesis of oligomeric plasma membrane receptors, endoplasmic reticulum (ER)-mitochondrial phospholipid transfer, and as a host factor modulating viral pathogenicity. Despite its evolutionary conservation in all eukaryotes, the biological functions and molecular mechanism of the EMC remain poorly understood. Here, the requirement of the mEMC for integral membrane protein biogenesis is investigated. It is reported that the mEMC is involved in the biogenesis of squalene synthase (SQS), an integral ER-resident cholesterogenic enzyme with tail-anchor protein topology. The mEMC is assembled from 10 subunits (EMC1 - 10). Quantitative proteomics on mEMC-deficient ΔEMC5 and ΔEMC6 U2OS Flp-In cells revealed reduced steady-state levels of select cell surface and intracellular proteins. In particular, both EMC5 and EMC6 knockout cells exhibited substantial co-depletion of SQS. The enzyme was significantly destabilized and rapidly degraded by the ubiquitin/proteasome system (UPS) in ΔEMC6 cells. Cells lacking the mEMC were viable under replete media conditions, but were prone to enhanced cell death upon acute cholesterol depletion. Sensitivity to cholesterol loss in EMC6 knockout cells was reproduced by direct deletion or pharmacological inhibition of SQS. mEMC depletion also caused imbalances in the mevalonate pathway, likely leading to increased shunting of the farnesyl pyrophosphate precursor into the isoprenoid pathway, increasing the detection of prenylated proteins. It is proposed that the mEMC is a maturation factor for SQS, and potentially other transmembrane proteins sharing critical molecular features with the biogenesis of this enzyme.