Functional overlap between retinitis pigmentosa 2 protein and the tubulin-specific chaperone cofactor C

Mutations in the X-linked retinitis pigmentosa 2 gene cause progressive degeneration of photoreceptor cells. The retinitis pigmentosa 2 protein (RP2) is similar in sequence to the tubulin-specific chaperone cofactor C. Together with cofactors D and E, cofactor C stimulates the GTPase activity of native tubulin, a reaction regulated by ADP-ribosylation factor-like 2 protein. Here we show that in the presence of cofactor D, RP2 protein also stimulates the GTPase activity of tubulin. We find that this function is abolished by mutation in an arginine residue that is conserved in both cofactor C and RP2. Notably, mutations that alter this arginine codon cause familial retinitis pigmentosa. Our data imply that this residue acts as an "arginine finger" to trigger the tubulin GTPase activity and suggest that loss of this function in RP2 contributes to retinal degeneration. We also show that in Saccharomyces cerevisiae, both cofactor C and RP2 partially complement the microtubule phenotype resulting from deletion of the cofactor C homolog, demonstrating their functional overlap in vivo. Finally, we find that RP2 interacts with GTP-bound ADP ribosylation factor-like 3 protein, providing a link between RP2 and several retinal-specific proteins, mutations in which also cause retinitis pigmentosa.