MELANOGENESIS IN TRANSFECTED FIBROBLASTS INDUCES LYSOSOMAL ACTIVATION

Normal melanosome biogenesis requires the association of structural pr oteins with tyrosinase, 3T3 Swiss fibroblasts transfected with mouse t yrosinase cDNA (line 13.4, clone c) are a unique system in which melan ogenesis takes place in the absence of melanosomal structural proteins . Our study confirmed that transfected fibroblasts displayed tyrosinas e activity and some of them produced pigment granules, In the absence of melanosomal structural proteins the granules failed both to show a typical ultrastructure and to undergo the usual melanosome ontogenesis , The differentiating agent - dimethyl sulfoxide-increased phaeomelani n production, Pigment was localized in membrane-bound vesicles which w ere identified as lysosomes by means of immunogold electron microscopy . Cell line 13.4 had higher levels of lysosomal enzymes (beta-hexosami nidase, alpha-mannosidase) than both parental 3T3 cells and done pKG4 (fibroblasts transfected with the G418 resistance plasmid), Melanosoma l proteins act as scavengers of toxic products of melanogenesis, and o ur results suggest that in their absence cells may employ an alternati ve mechanism to sequester injurious products.