Tyrosinase and tyrosinase-related protein 1 require Rab7 for their intracellular transport

We have recently identified the association of Rab7 in melanosome biogenesis and proposed that Rab7 is involved in the transport of tyrosinase-related protein 1 from the trans-Golgi network to melanosomes, possibly passing through late-endosome-delineated compartments. In order to further investigate the requirement of Rab7-containing compartments for vesicular transport of tyrosinase family proteins, we expressed tyrosinase and tyrosinase-related protein by recombinant adenovirus and analyzed their localization in human amelanotic melanoma cells (SK-mel-24) in the presence or absence of a dominant-negative mutant of Rab7 (Rab7N125I). Co-infection of the recombinant adenoviruses carrying tyrosinase (Ad-HT) and TRP-1 (Ad-TRP-1) resulted in the enhancement of tyrosinase activity and melanin production compared to a single infection of Ad-HT. In the Ad-HT-infected SK-mel-24 cells many of the newly synthesized tyrosinase proteins were colocalized in lysosomal lgp85-positive granules of the entire cytoplasm, whereas in the presence of Rab7N125I the colocalization of tyrosinase and lgp85 proteins was decreased markedly in the distal area of the cytoplasm. In the Ad-TRP-1-infected SK-mel-24 cells, TRP-1, which is reported to be present exclusively in melanosomes, was detected throughout the cytoplasm, but not colocalized in prelysosomal (early endosomal) EEA-1 granules. In the presence of Rab7N125I, however, TRP-1 was retained in the EEA-1-positive granules. Our findings indicate that the dominant-negative mutant of Rab7 impairs vesicular transport of tyrosinase and TRP-1, suggesting that the transport of these melanogenic proteins from the trans-Golgi network to maturing melanosomes requires passage through endosome-delineated compartments.