Pigmentation of the hair, skin, and eyes of mammals results from a number of melanocyte-specific
proteins that are required for the biosynthesis of
melanin. Those
proteins comprise the structural and enzymatic components of melanosomes, the membrane-bound organelles in which
melanin is synthesized and deposited.
Tyrosinase (TYR) is absolutely required for melanogenesis, but other melanosomal
proteins, such as TYRP1, DCT, and gp100, also play important roles in regulating mammalian pigmentation. However, pigmentation does not always correlate with the expression of TYR
mRNA/
protein, and thus its function is also regulated at the post-translational level. Thus, TYR does not necessarily exist in a catalytically active state, and its post-translational activation could be an important control point for regulating
melanin synthesis. In this study, we used a multidisciplinary approach to examine the processing and sorting of TYR through the endoplasmic reticulum (ER), Golgi apparatus, coated vesicles, endosomes and early melanosomes because those organelles hold the key to understanding the trafficking of TYR to melanosomes and thus the regulation of melanogenesis. In pigmented cells, TYR is trafficked through those organelles rapidly, but in amelanotic cells, TYR is retained within the ER and is eventually degraded by proteasomes. We now show that TYR can be released from the ER in the presence of protonophore or
proton pump inhibitors which increase the pH of intracellular organelles, after which TYR is transported correctly to the Golgi, and then to melanosomes via the endosomal sorting system. The expression of TYRP1, which facilitates TYR processing in the ER, is down-regulated in the amelanotic cells; this is analogous to a hypopigmentary disease known as
oculocutaneous albinism type 3 and further impairs
melanin production. The sum of these results shows that organellar pH,
proteasome activity, and down-regulation of TYRP1 expression all contribute to the lack of pigmentation in TYR-positive
amelanotic melanoma cells.