Pink-eyed Dilution Protein Controls the Processing of Tyrosinase

Abstract

The processing of tyrosinase, which catalyzes the limiting reaction in melanin synthesis, was investigated in melan-p1 melanocytes, which are null at the p locus. Endoglycosidase H digestion showed that a significant fraction of tyrosinase was retained in the endoplasmic reticulum. This retention could be rescued either by transfection of melan-p1 cells with an epitope-tagged wild-type p transcript or by treatment with either bafilomycin A1 or ammonium chloride. We found that the endoplasmic reticulum contains a significant amount of p protein, thus supporting a role for p within this compartment. Using immunofluoresence, we showed that most mature full-length tyrosinase in melan-p1 cells was located in the perinuclear area near the Golgi, in contrast to its punctate melanosomal pattern in wild-type melanocytes. Expression of p in melan-p1 cells restored tyrosinase to melanosomes. Triton X-114 phase separation revealed that an increased amount of tyrosinase was proteolyzed in melan-p1 cells compared with wild-type melanocytes. The proteolyzed tyrosinase was no longer membrane bound, but remained enzymatically active and a large proportion was secreted into the culture medium of melan-p1 cells. We conclude that p regulates posttranslational processing of tyrosinase, and hypopigmentation in melan-p1 cells is the result of altered tyrosinase processing and trafficking.

Basis for Enhanced Barrier Function of Pigmented Skin

“…Though pre-melanosomes and melanosomes possess highly-acidic contents (Moellmann, et al , 1988, Ancans, et al , 2001, Chen, et al , 2002, Schallreuter, et al , 2008), their contents approach neutrality immediately prior to secretion (Ancans, et al , 2001), an antecedent that is apparently necessary for pigment transfer (Seiberg, 2001, Boissy, 2003). Melanin granules then are secreted into the extracellular spaces before being phagocytized by neighboring keratinocytes (Ando, et al , 2012).…”

“…Melanocytes could regulate epidermal function (including acidification) by paracrine influences (Slominski and Paus, 1990, Slominski, et al ., 1993), and/or by `juxtacrine' effects of melanocytes on neighboring keratinocytes. Melanosomes are acidic organelles that enclose several proteins in the eumelanin synthetic pathway (Moellmann, et al ., 1988, Schallreuter, et al ., 2008), whose activities require a reduced pH (Chen, et al ., 2002). Although the internal pH of melanosomes increases immediately prior to secretion (Ancans, et al ., 2001), our studies showed that putative melanosomes in dendrites of darkly-pigmented human melanocytes are more acidic than comparable vesicular structures in lightly-pigmented subjects (Gunathilake, et al ., 2009).…”

Recent advances in genetic analyses of oculocutaneous albinism types 2 and 4

“…The P gene encodes an integral melanosomal membrane protein composed of 833 amino acids with 12 putative transmembrane domains [23,24]. The function of the P gene product has not yet been clarified, although several potential functions have been proposed, including, membrane transport [25], sorting of tyrosinase to melanosomes [26], stabilization of a melanosomal protein complex [27], regulation of melanosomal pH [28], and control of tyrosinase processing, arsenic sensitivity and cellular glutathione metabolism [29][30][31].…”

Oculocutaneous albinism: Developing novel antibodies targeting the proteins associated with OCA2 and OCA4

“…[22], although, that interpretation was later questioned [23]. A later study suggested that the P protein localizes to and functions in the endoplasmic reticulum (ER) [24] but it was recently reported that the human P protein is rapidly processed by Golgi enzymes [25] and transits to lysosome-related organelles (LROs) via BLOC-1, a complex protein involved in protein trafficking [26,27].…”