Biological role of tyrosinase related protein and its biosynthesis and transport from TGN to stage I melanosome, late endosome, through gene transfection study.

Tyrosinase-related protein (TRP)-1 is one of the most abundant melanosomal glycoproteins involved in melanogenesis. This report summarizes our recent research efforts related to the biological role and biosynthesis of TRP-1 and its transport from TGN (trans-Golgi network) to the stage I melanosome. Our UV irradiation and tyrosinase and TRP-1 cDNA co-transfection studies indicated that human TRP-1 is involved in not only melanogenesis but also prevention of melanocyte death, which may occur during biosynthesis of melanin pigment in the presence of tyrosinase. Furthermore, a coordinated gene interaction was indicated between tyrosinase and TRP-1, resulting in upregulation of mRNA and protein expression of LAMP (lysosome-associated membrane protein)-1 that would directly prevent the tyrosinase-mediated programmed cell death of melanocytes. Similar to tyrosinase, however, TRP-1 appears to require a molecular chaperone, calnexin, which we have cloned recently. Our cDNA transfection study of tyrosinase with calnexin showed clearly the necessity of calnexin in order to have efficient, functional activity of melanosomal glycoprotein, especially tyrosinase. Once glycosylation is completed, TRP-1 will be transported from TGN to the stage I melanosome. At this stage, TRP-1 will have its own target signal, in particular, tyrosine-rich leucine residues in cytoplasmic tail. Our TRP-1 cDNA transfection and immunoelectron microscopy study shows that TRP-1 will be transported through small vesicles, probably non-clathrin-coated type, to large vacuoles, identical to the MPR (mannose-6-phosphate receptor)-positive, late endosomes. In this transport process a low molecular weight G-protein, rab-7, was isolated from the purified melanosomal protein on 2D-PAGE and identified by subsequent sequencing and PCR amplification. Confocal microscopy with double immunostaining and immunoelectron microscopy confirmed the co-localization of rab-7 and TRP-1 in the melanosomes with early stages of maturation (I-HI). Furthermore, this process will also be regulated by phosphatidylinositol 3-kinase (PI-3 kinase).