The mouse ocular albinism 1 gene product is an endolysosomal protein

To gain insight into the role of Oa1, the mouse homolog of the human X-link ed ocular albinism 1 protein, its properties and subcellular localization w ere investigated. Antiserum raised against an expressed segment of the Oa1 protein recognized a band of similar to 48 kDa in immunoblots of extracts o f cultured mouse melan-a melanocytes, but not of cells of non-melanocyte or igin. When melanocyte extracts were treated with glycopeptidase F. a simila r to 44 kDa band appeared. Like the melanogenic enzyme tyrosinase, expressi on of Oa1 was stimulated by alpha -melanocyte stimulating hormone and inhib ited by agouti signal protein. Upon density gradient centrifugation of orga nelles of melan-a cells. Oa1 protein colocalized with the late endosomal/ly sosomal marker Lamp1, but only partial overlap was observed with melanosoma l proteins in the high density region of the gradient. Immunofluorescence s taining revealed that neither endogenous Oa1 nor an Oa1-green fluorescent p rotein fusion product colocalized with the melanosomal protein tyrosinase r elated protein-1 in the cell periphery. In contrast, colocalization of Oa1 and Oa1-green fluorescent protein fusion product with Lamp1 was extensive t hroughout the cell. These results indicate that Oa1 is a melanocyte-specifi c integral membrane glycoprotein localized to late endosomes/lysosomes but not mature melanosomes. Considering the microscopic findings in patients wi th X-linked ocular albinism 1, we speculate that Oa1 may play a role in the trafficking of vesicles to developing melanosomes. (C) 2001 Academic Press .