Intracellular vesicular trafficking of tyrosinase gene family protein in eu- and pheomelanosome biogenesis.

The intracellular vesicular trafficking in the melanosome biogenesis (melanogenesis) is reviewed with the incorporation of our own experimental findings. The melanosome biogenesis involves four stages of melanosome maturation, which reflect the transport of structural and enzymatic proteins from Golgi (trans-Golgi network: TGN) to the melanosomal compartment and their organization therein. The major melanosomal proteins include tyrosinase gene family protein (tyrosinase and tyrosinase-related protein; TRP), lysosome-associated membrane protein (Lamp) and gp100 (pmel 17). They are glycosylated in the endoplasmic reticulum, and transported by vesicles from the TGN to the melanosomal compartment. During the formation of transport vesicles, they assemble on the cytoplasmic face of the TGN to select cargo by interacting directly or indirectly with coat proteins. Tyrosinase and TRP-1 possess the dileucine motifs at the cytoplasmic domain, to which adapter protein-3 binds to transport them from the TGN to stage I melanosomes (related to late endosomes) and then to stage II melanosomes. A number of small guanosine triphosphate-binding proteins, including rab 7, appear to be involved in this vesicular transport. Phosphatidyl inositol 3 kinase also regulates this membrane trafficking of melanosomal glycoprotein. Eumelanogenesis is controlled by melanocyte-stimulating hormone, and all three tyrosinase gene family proteins are transported from the TGN to stage II melanosomes that are elliposoidal and contain the structural matrix of filaments/lamellae. In contrast, pheomelanogenesis is primarily regulated by agouti signal protein, and only tyrosinase is transported from stage I melanosomes to stage II melanosomes that are spherical and related to lysosomes. Because of the absence of TRP-1 and TRP-2 in pheomelanogenesis, it may be suggested that tyrosinase is involved in lysosomal degradation after forming dopaquinone, to which the cysteine present in the lysosomal granule binds to form cysteinyldopas that will then be auto-oxidized to become pheomelanin.