ER membrane protein complex is required for the insertions of late-synthesized transmembrane helices of Rh1 in Drosophila photoreceptors.

Most membrane proteins are synthesized on and inserted into the membrane of the endoplasmic reticulum (ER), in eukaryote. The widely conserved ER membrane protein complex (EMC) facilitates the biogenesis of a wide range of membrane proteins. In this study, we investigated the EMC function using Drosophila photoreceptor as a model system. We found that the EMC was necessary only for the biogenesis of a subset of multipass membrane proteins such as rhodopsin (Rh1), TRP, TRPL, Csat, Cni, SERCA, and Na ⁺ K ⁺ ATPase α, but not for that of secretory or single-pass membrane proteins. Additionally, in EMC-deficient cells, Rh1 was translated to its C terminus but degraded independently from ER-associated degradation. Thus, EMC exerted its effect after translation but before or during the membrane integration of transmembrane domains (TMDs). Finally, we found that EMC was not required for the stable expression of the first three TMDs of Rh1 but was required for that of the fourth and fifth TMDs. Our results suggested that EMC is required for the ER membrane insertion of succeeding TMDs of multipass membrane proteins.