Purification and light-dependent phosphorylation of a candidate fusion protein, the photoreceptor cell peripherin/rds

The proteins peripherin/rds and rom-1 form a protein complex in the rims of photoreceptor outer segment disk membranes. Peripherin/rds plays an essential role in the morphogenesis and maintenance of disk membrane structure, with peripherin/rds gene mutations resulting in photoreceptor cell degeneration. We report two different chromatographic procedures for the purification of native peripherin/rds from bovine photoreceptor cell outer segments and show that the protein is a phosphoprotein that promotes membrane fusion in vitro. During one procedure, peripherin/rds was copurified in association with rom-1 by hyroxylapatite and Mono Q FPLC. During the other, it was purified free from rom-1 by concanavalin-A affinity chromatography and chromatofocusing. Analysis of homogeneous peripherin/rds from the second procedure showed that exposure of photoreceptor outer segments to light resulted in the incorporation of nearly 2 mol of phosphate per mole of peripherin/rds and a concomitant shift in the isoelectric point of the protein. In addition, we found that recombination of purified peripherin/rds into lipid vesicles increased membrane fusion, with more rapid fusion detected with phosphorylated peripherin/rds. In conclusion, studies with purified peripherin/rds reveal that the protein undergoes light-dependent phosphorylation and that it may function in membrane fusion.