Role of asparagine-linked oligosaccharides in rhodopsin maturation and association with its molecular chaperone, NinaA

Many proteins require N-linked glycosylation for conformational maturation and interaction with their molecular chaperones. In Drosophila, rhodopsin ( Rh1), the most abundant rhodopsin, is glycosylated in the endoplasmic retic ulum (ER) and requires its molecular chaperone, NinaA, for exit from the ER and transport through the secretory pathway, Studies of vertebrate rhodops ins have generated several conflicting proposals regarding the role of glyc osylation in rhodopsin maturation. We investigated the role of Rh1 glycosyl ation and Rh1/NinaA interactions under in vivo conditions by analyzing tran sgenic flies expressing Rh1 with isoleucine substitutions at each of the tw o consensus sites for N-linked glycosylation (N20I and N196I). We show that Asn(20) is the sole site for glycosylation. The Rh1(N20I) protein is retai ned within the secretory pathway, causing an accumulation of ER cisternae a nd dilation of the Golgi complex. NinaA associates with nonglycosylated Rh1 (N20I); therefore, retention of nonglycosylated rhodopsin within the ER is not due to the lack of Rh1(N20I)/NinaA interaction. We further show that Rh 1(N20I) interferes with wild type Rh1 maturation and triggers a dominant fo rm of retinal degeneration. We conclude that during maturation Rh1 is prese nt in protein complexes containing NinaA and that Rh1 glycosylation is requ ired for transport of the complexes through the secretory pathway. Failure of this transport process leads to retinal degeneration.