Mutations in RP2 cause the second most frequent form of X-linked retinitis
pigmentosa, a severe retinal degeneration that leads to loss of visual acui
ty and blindness, The RP2 gene encodes a protein with homology to cofactor
C, a tubulin-folding chaperone, By searching protein sequence databases, we
identified a whole set of similar molecules from diverse organisms. Protei
n sequence alignments show that RP2 and cofactor C represent members of two
distinct orthologous groups. All previously identified missense mutations
affect amino acid residues which are conserved in all RP2 orthologues or bo
th orthologous groups. Intracellular localization of the wild-type protein
and mutated variants was determined by fluorescence microscopy of cells exp
ressing RP2 with a green fluorescent protein tag. A mutation in the N-termi
nus of RP2 abolishes localization to the plasma membrane in HeLa cells. C-t
erminal protein truncation mutations, which account for 2/3 of the pathogen
ic RP2 variants, lead to scattered fluorescent foci in the cytoplasm of COS
-7 and HeLa cells, Analysis of protein extracts from the respective cells w
ith anti-RP2 antibodies identified truncated proteins of expected size in a
low-speed centrifugation pellet while the wild-type protein appeared in th
e supernatant, Moreover, no protein was detected in immortalized cell lines
from patients with protein truncation mutations while mRNA was still prese
nt. Thus, loss of the protein and/or aberrant intracellular distribution mi
ght be the basis for the photoreceptor cell degeneration in most RP2 cases.