Mammalian homolog of Drosophila retinal degeneration B rescues the mutant fly phenotype.

Mutations in the Drosophila rdgB gene, which encodes a transmembrane phosphatidylinositol transfer protein (PITP), cause a light-enhanced retinal degeneration. Cloning of mammalian rdgB orthologs (mrdgB) reveal predicted proteins that are 39% identical to rdgB, with highest homology in the N-terminal PITP domain (62%) and in a region near the C terminus (65%). The human mrdgB gene spans approximately 12 kb and maps to 11q13.1, a locus where several retinal diseases have also been mapped. Murine mrdgB maps to a syntenic region on the proximal region of chromosome 19. MrdgB is specifically expressed in the retina and brain. In the retina, MrdgB protein is localized to photoreceptor inner segments and the outer and inner plexiform layers. Expression of murine mrdgB in mutant flies fully rescues both the rdgB-dependent retinal degeneration and abnormal electroretinogram. These results suggest the existence of similarities between the invertebrate and mammalian retina that were not previously appreciated and also identify mrdgB as a candidate gene for retinal diseases that map to 11q13.1.

Rin, a neuron-specific and calmodulin-binding small G-protein, and Rit define a novel subfamily of ras proteins.

cDNAs encoding two novel 25 kDa Ras-like proteins, Rit and Rin, were isolated from mouse retina using a degenerate PCR-based cloning strategy. Using the expressed sequence tag database, human orthologs were also obtained and sequenced. The protein sequences of Rit and Rin, which are 64% identical, are more similar to each other than to any known Ras protein. Their closest homologs in the databases are Mucor racemosus Ras2 and Ras3, to which they show approximately 48% identity. Rit and Rin both bind GTP in vitro. An unusual feature of their structure is that they lack a known recognition signal for C-terminal lipidation, a modification that is generally necessary for plasma membrane association among the Ras subfamily of proteins. Nonetheless, transiently expressed Rit and Rin are plasma membrane-localized. Both proteins contain a C-terminal cluster of basic amino acids, which could provide a mechanism for membrane association. Deletion analysis suggested that this region is important for Rit membrane binding but is not necessary for Rin. Rit, like most Ras-related proteins, is ubiquitously expressed. Rin, however, is unusual in that it is expressed only in neurons. In addition, Rin binds calmodulin through a C-terminal binding motif. These results suggest that Rit and Rin define a novel subfamily of Ras-related proteins, perhaps using a new mechanism of membrane association, and that Rin may be involved in calcium-mediated signaling within neurons.