The Etiology of Oculocutaneous Albinism (OCA) Type II: The Pink Protein Modulates the Processing and Transport of Tyrosinase

Abstract

Oculocutaneous albinism (OCA) is caused by reduced or deficient melanin pigmentation in the skin, hair, and eyes. OCA has different phenotypes resulting from mutations in distinct pigmentation genes involved in melanogenesis. OCA type 2 (OCA2), the most common form of OCA, is an autosomal recessive disorder caused by mutations in the P gene, the function(s) of which is controversial. In order to elucidate the mechanism(s) involved in OCA2, our group used several antibodies specific for various melanosomal proteins (tyrosinase, Tyrp1, Dct, Pmel17 and HMB45), including a specific set of polyclonal antibodies against the p protein. We used confocal immunohistochemistry to compare the processing and distribution of those melanosomal proteins in wild type (melan-a) and in p mutant (melan-p1) melanocytes. Our results indicate that the melanin content of melan-p1 melanocytes was less than 50% that of wild type melan-a melanocytes. In contrast, the tyrosinase activities were similar in extracts of wild type and p mutant melanocytes. Confocal microscopy studies and pulse-chase analyses showed altered processing and sorting of tyrosinase, which is released from melan-p1 cells to the medium. Processing and sorting of Tyrp1 was also altered to some extent. However, Dct and Pmel17 expression and subcellular localization were similar in melan-a and in melan-p1 melanocytes. In melan-a cells, the p protein showed mainly a perinuclear pattern with some staining in the cytoplasm where some co-localization with HMB45 antibody was observed. These findings suggest that the p protein plays a major role in modulating the intracellular transport of tyrosinase and a minor role for Tyrp1, but is not critically involved in the transport of Dct and Pmel17. This study provides a basis to understand the relationship of the p protein with tyrosinase function and melanin synthesis, and also provides a rational approach to unveil the consequences of P gene mutations in the pathogenesis of OCA2.

Inheritance of a novel mutated allele of the OCA2 gene associated with high incidence of oculocutaneous albinism in a Polynesian community

“…It has been proposed to regulate melanosomal pH, 15 with an additional possible role in the processing and trafficking of melanosomal proteins. 17,18 It has recently been confirmed as localizing with mature melanosomes. 19 There have been numerous mutations and polymorphisms found in the OCA2 locus, which are listed on the International Albinism Center Database (http://albinismdb.med.umn.edu).…”

Localization to Mature Melanosomes by Virtue of Cytoplasmic Dileucine Motifs Is Required for Human OCA2 Function

“…Alterations in substrate levels could in turn have downstream effects on ion transport between the cytosol and other compartments, such as the ER. Such effects might alter folding or disulfide bond formation of proteins such as tyrosinase in the ER, perhaps explaining the reduced rate of processing and ER exit of tyrosinase in OCA2-deficient melanocytes (Toyofuku et al, 2002). Tyrosinase contains 15 lumenal Cys residues and many intramolecular disulfide bonds (Wang and Hebert, 2006), so changes in the disulfide bonding capacity of the ER could have a significant effect on tyrosinase maturation.…”

“…OCA2-immunoreactive subcellular fractions from melanocytes contain ER markers as well as melanin, and OCA2 colocalized extensively with ER markers by indirect IFM (Chen et al, 2002). Moreover, cells that lack OCA2 show a reduction in terminal glycosylation of Tyr (Chen et al, 2002;Toyofuku et al, 2002), suggesting that OCA2 activity might provide an optimal environment within the ER to facilitate Tyr folding. Thus far, this controversy has not been resolved.…”

“…OCA2 was originally thought to localize predominantly to mature melanized melanosomes based on subcellular fractionation of melanocytes (Rosemblat et al, 1994), poor extraction by detergent from melanized melanocytes relative to nonmelanized melanocytes (Donatien and Orlow, 1995), and interpretation of results from confocal immunofluorescence microscopy (IFM) analyses (Toyofuku et al, 2002). Tyrp1-containing compartments in melanocytes from OCA2-deficient mice are less acidic than in wild-type melanocytes, suggesting that OCA2 not only localizes to melanosomes but also modulates their pH (Puri et al, 2000), although this interpretation is disputed because Tyrp1 does not localize to melanosomes properly in OCA2-deficient melanocytes (Manga et al, 2001).…”

Mouse coat color mutations: From fancy mice to functional genomics

“…The P protein is a membrane protein with similarity to the Escherichia coli Na ϩ /H ϩ anti-porter protein. The exact function of the P protein is enigmatic, and the idea that it regulates melanosomal pH is inconsistent with the results of Toyofuku et al (2002) and Chen et al (2002), who showed that P protein is involved in processing and sorting of tyrosinase, which is retained in the endoplasmic reticulum in mutant melanocytes. It is also inconsistent with the observation of Staleva et al (2002), who showed that the P protein may be involved in glutathione metabolism.…”