FIBROBLASTS CO-EXPRESSING TYROSINASE AND THE B-PROTEIN SYNTHESIZE BOTH EUMELANIN AND PHEOMELANIN

Melanin synthesis in the mouse involves the interaction of many pigmen tation loci. Tyrosinase, the product of the albino (c) locus, catalyse s the first step of the pathway. The brown (b) locus protein has signi ficant homology to tyrosinase and controls black/brown coat coloration , but its function is controversial. To investigate the function of th e b-protein and its interaction with tyrosinase, we established cell l ines expressing both tyrosinase and the b-protein by transfecting tyro sinase-expressing fibroblasts with a b-protein expression vector. The tyrosinase-expressing parent line does not have L-dopachrome tautomera se activity, but this enzyme is detectable in double transfectants as well as in fibroblasts expressing the b-protein alone. Cells expressin g both proteins have a higher steady-state level of tyrosinase than fi broblasts expressing tyrosinase alone, and contain elevated levels of melanin intermediates. This is thought to result from interaction of t yrosinase with the b-protein. Only phaeomelanin is detectable in fibro blasts expressing tyrosinase alone, whereas double transfectants synth esise significantly more phaeomelanin and detectable eumelanin.