IDENTIFICATION OF A MAMMALIAN MELANOSOMAL MATRIX GLYCOPROTEIN

Antiserum raised in rabbits against the Triton X-100-insoluble fractio n of melanosomes from mouse melanoma cells specifically decorates the internal matrix of melanosomes in immunoelectron microscopy. In metabo lic labeling studies, the antiserum recognizes a protein of 94 kDa, wh ich is processed to a band of 53 kDa Whereas the precursor's relativel y soluble in buffers containing Triton X-100, the processed protein re quires the addition of sodium dodecyl sulfate for effective solubiliza tion, as would be expected for a melanosomal matrix constituent. Tunic amycin reduces the Mr of the nascent protein to 75 kDa, but deoxymanno jirimycin and swainsonine have no effect, suggesting that following in itial glycosylation in the endoplasmic reticulum, the protein is not s ubject to processing by glycosidases in the Golgi apparatus or may byp ass it entirely. Subcellular fractionation followed by immunoblotting confirms that the protein is present in the melanosome-rich, large gra nule fraction. Expression of the protein is regulated differently from that of the tyrosinase-related protein family. Conditions that greatl y stimulate expression of tyrosinase-related proteins do not affect ma trix protein expression, nor is the protein immunologically related to the tyrosinase-related protein family. Our results suggest that we ha ve identified an authentic component of the mammalian melanosomal matr ix, and that its characteristics lend support to a bipartite pathway f or melanosomal biogenesis.