by Alejandra Young, Meisheng Jiang, Ying Wang, Novruz B. Ahmedli, John Ramirez, Benjamin E. Reese, Lutz Birnbaumer, Debora B. Farber
Ocular albinism type 1, an X-linked disease characterized by the presence of enlarged melanosomes in the retinal pigment epithelium (RPE) and abnormal crossing of axons at the optic chiasm, is caused by mutations in the OA1 gene. The protein product of this gene is a G-protein-coupled receptor (GPCR) localized in RPE melanosomes. The Oa1-/- mouse model of ocular albinism reproduces the human disease. Oa1 has been shown to immunoprecipitate with the Gai subunit of heterotrimeric G proteins from human skin melanocytes. However, the Gai subfamily has three highly homologous members, Gai1, Gai2 and Gai3 and it is possible that one or more of them partners with Oa1. We had previously shown by in-vivo studies that Gai3-/- and Oa1-/- mice have similar RPE phenotype and decussation patterns. In this paper we analyze the specificity of the Oa1-Gai interaction. Methodology
By using the genetic mouse models Gai1-/-, Gai2-/-, Gai3-/- and the double knockout Gai1-/-, Gai3-/- that lack functional Gai1, Gai2, Gai3, or both Gai1 and Gai3 proteins, respectively, we show that Gai3 is critical for the maintenance of a normal melanosomal phenotype and that its absence is associated with changes in melanosomal size and density. GST-pull-down and immunoprecipitation assays conclusively demonstrate that Gai3 is the only Gai that binds to Oa1. Western blots show that Gai3 expression is barely detectable in the Oa1-/- RPE, strongly supporting a previously unsuspected role for Gai3 in melanosomal biogenesis. Conclusion
Our results identify the Oa1 transducer Gai3 as the first downstream component in the Oa1 signaling pathway.