Mutations in the N-terminus of the X-linked retinitis pigmentosa protein RP2 interfere with the normal targeting of the protein to the plasma membrane

Abstract

The X-linked retinitis pigmentosa (XLRP) gene, RP2, codes for a novel 350 amino acid protein of unknown function. We have identified putative sites for N-terminal acyl modification by myristoylation and palmitoylation in the RP2 protein. The RP2 protein is expressed ubiquitously in human tissues at relatively low levels (0.01% of total protein) and has a predominantly plasma membrane localization in cultured cells, as would be expected if the protein was subject to dual N-terminal acylation. Furthermore, mutagenesis of residues potentially required for N-terminal acylation prevents targeting of RP2 to the plasma membrane and the N-terminal 15 amino acids of the protein appear to be sufficient for this targeting. Our data suggest that the protein is dually acylated and that the palmitoyl moiety is responsible for targeting of the myristoylated protein from intracellular membranes to the plasma membrane. The effect of two mutations, which have been reported as causes of XLRP, R118H and DeltaS6, were investigated. The R118H mutation does not affect the normal plasma membrane localization of RP2; in contrast, the DeltaS6 mutation interferes with the targeting of the protein to the plasma membrane. Therefore, the DeltaS6 mutation may cause XLRP because it prevents normal amounts of RP2 reaching the correct cellular locale, whereas the R118H mutation is in a region of the protein that is vital for another aspect of RP2 function in the retina.