Melanosomal proteins--role in melanin polymerization

Melanin, the major determinant of skin colour, is a tyrosine-based heteropolymer of indeterminate molecular weight. In vivo, melanin synthesis occurs within highly specialized organelles called melanosomes. Coated vesicles encapsulating the enzyme tyrosinase and tyrosinase related proteins, fuse with premelanosomes that contain structural proteins to form mature melanosomes. Coated vesicles and premelanosomes have been shown to have only melanin monomers but not the polymer. Our earlier results have clearly shown that the presence of proteins other than tyrosinase are critical for the post-tyrosinase steps of melanin polymerization at acidic pH. Proteins in melanosomes are difficult to purify because of their firm association with melanin. Thus, with progressive melanization, melanoproteins become progressively insoluble. In this paper, we discuss the isolation and purification of melanosomal proteins and their role in melanin polymerization. We have hypothesized that the initiation of polymerization and the binding of melanin to proteins are two discrete events and we have developed assays to quantify these events. Purified melanosomal proteins differ in their ability to polymerize melanin monomers. Further, we have also shown that two polypeptides (28 and 45 kDa) purified from melanosomes inhibit melanin polymerization but can bind preformed melanin. In conclusion, melanosomal proteins regulate melanin polymerization and differ in their ability to bind melanin. Polymerization and binding abilities of melanosomal proteins are specific to each protein and melanin-protein interaction is not nonspecific.