Once correctly folded, the transmembrane proteins can be transported to the cell membrane. Here, they mature and ‘fold’ into their correct three-dimensional shape with help from chaperone proteins. Transmembrane proteins are made inside the cell and are inserted into the membrane surrounding a compartment called the endoplasmic reticulum. Rhodopsin is one such transmembrane protein that is found in the light-sensitive ‘photoreceptor’ cells of the eye, where it plays an essential role in vision. The membranes that surround cells contain many proteins, and those that span the entire width of the membrane are known as transmembrane proteins. These results collectively indicate that EMC is a key factor in the biogenesis of multi-pass transmembrane proteins, including Rh1, and its loss causes retinal degeneration. Furthermore, we found that dPob/EMC3 deficiency induces rhabdomere degeneration in a light-independent manner. Moreover, EMC is required for the stable expression of other multi-pass transmembrane proteins such as minor rhodopsins Rh3 and Rh4, transient receptor potential, and Na +K +-ATPase, but not for a secreted protein or type I single-pass transmembrane proteins. dPob/EMC3 localizes to the ER and associates with EMC1 and calnexin. We screened the mutants affecting rhabdomeric expression of rhodopsin 1 (Rh1) in the Drosophila photoreceptors and found that dPob/EMC3, EMC1, and EMC8/9, Drosophila homologs of subunits of ER membrane protein complex (EMC), are essential for stabilization of immature Rh1 in an earlier step than that at which another Rh1-specific chaperone (NinaA) acts. In eukaryotes, most integral membrane proteins are synthesized, integrated into the membrane, and folded properly in the endoplasmic reticulum (ER).
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