EMC is required for biogenesis and membrane insertion of Xport-A, an essential chaperone of rhodopsin-1 and the TRP channel

Insertion of hydrophobic transmembrane domains (TMDs) into the endoplasmic reticulum (ER) lipid bilayer is an essential step during eukaryotic membrane protein biogenesis. The ER membrane complex (EMC) functions as an insertase for TMDs of low hydrophobicity and is required for the biogenesis of a subset of tail-anchored (TA) and polytopic membrane proteins, including rhodopsin-1 (Rh1) and the TRP channel. To better understand the physiological implications of membrane protein biogenesis dependent on the EMC, we performed a bioinformatic analysis to predict TA proteins present in the Drosophila proteome. From 254 predicted TA proteins, subsequent genetic screening in Drosophila larval eye discs led to the identification of 2 proteins that require EMC for their biogenesis: farinelli (fan) and Xport-A. Fan is required for sperm individualization and male fertility in Drosophila and we now show that EMC is also required for these important biological processes. Interestingly, Xport-A is essential for the biogenesis of both Rh1 and TRP, raising the possibility that disruption of Rh1 and TRP biogenesis in EMC loss of function mutations is secondary to the Xport-A defect. We show that EMC is required for Xport-A TMD membrane insertion and increasing the hydrophobicity of Xport-A TMD rendered its membrane insertion to become EMC-independent. Moreover, these EMC-independent Xport-A mutants rescued Rh1 and TRP biogenesis in EMC mutants. Our data establish that EMC can impact the biogenesis of polytopic membrane proteins indirectly, by controlling the biogenesis and membrane insertion of an essential protein co-factor.

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