The melanosomal/lysosomal protein OA1 has properties of a G protein-coupled receptor

Università degli Studi di Genova, Genova, Liguria, Italy
Pigment Cell Research (Impact Factor: 4.29). 05/2006; 19(2):125-35. DOI: 10.1111/j.1600-0749.2006.00292.x
Source: PubMed


The protein product of the ocular albinism type 1 gene, named OA1, is a pigment cell-specific integral membrane glycoprotein, localized to melanosomes and lysosomes and possibly implicated in melanosome biogenesis. Although its function remains unknown, we previously showed that OA1 shares structural similarities with G protein-coupled receptors (GPCRs). To ascertain the molecular function of OA1 and in particular its nature as a GPCR, we adopted a heterologous expression strategy commonly exploited to demonstrate GPCR-mediated signaling in mammalian cells. Here we show that when expressed in COS7 cells OA1 displays a considerable and spontaneous capacity to activate heterotrimeric G proteins and the associated signaling cascade. In contrast, OA1 mutants carrying either a missense mutation or a small deletion in the third cytosolic loop lack this ability. Furthermore, OA1 is phosphorylated and interacts with arrestins, well-established multifunctional adaptors of conformationally active GPCRs. In fact, OA1 colocalizes and coprecipitates with arrestins, which downregulate the signaling of OA1 by specifically reducing its expression levels. These findings indicate that heterologously expressed OA1 exhibits two fundamental properties of GPCRs, being capable to activate heterotrimeric G proteins and to functionally associate with arrestins, and provide proof of principle that OA1 can actually function as a canonical GPCR in mammalian cells.

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    • "eviously described ligand for OA1 is L - DOPA ( Lopez et al . , 2008 ) , which is generated by tyrosinase , an enzyme not expressed in HeLa cells . This suggests that the specific effects of OA1 - wt in HeLa cells might depend either on the presence of alternative ligands able to activate the receptor or on its considerable constitutive activity ( Innamorati et al . , 2006 ) . Furthermore , the increase in MVB number as a result of OA1 activity occurs without expressing MART1 , which has previously been shown to stabilise OA1 ( Giordano et al . , 2009 ) . Fig . 4 . OA1 expression does not affect delivery of EGF to the lysosome . ( A ) Confocal images of HeLa cells pulsed with EGF - 488 for 10 , 30 , 60 an"
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    ABSTRACT: Multivesicular endosomes/bodies (MVBs) deliver proteins like activated EGF receptors (EGFR) to the lysosome for degradation, and, in pigmented cells, MVBs containing PMEL are an initial stage in melanosome biogenesis. The mechanisms regulating numbers and fate of different populations of MVB are unclear. Here we focus on the role of the G protein coupled receptor, OA1, which is expressed exclusively in pigmented cells and mutations in which cause the most common type of ocular albinism. By exogenously expressing PMEL HeLa cells have been shown to form MVBs resembling early stage melanosomes. To focus on the role of OA1 in the initial stages of melanosome biogenesis we take advantage of the absence of the later stages of melanosome maturation in HeLa cells to determine whether OA1 activity can regulate MVB number and fate. Expression of wild type but not OA1 mutants carrying inactivating mutations/deletions causes MVB numbers to increase. Whilst OA1 expression has no effect on delivery of EGFR-containing MVBs to the lysosome it inhibits the lysosomal delivery of PMEL and PMEL-containing MVBs accumulate. We propose that OA1 activity delays delivery of PMEL-containing MVBs to the lysosome to allow time for melanin synthesis and commitment to melanosome biogenesis.
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    • "The absence or decrease in pigmentation can occur in the skin, hair, and the eyes (oculocutaneous albinism, OCA) or only impair pigmentation in the eyes (ocular albinism, OA). Until very recent, there were four genes known to be associated with OCA, namely: TYR (OCA1), OCA2 (OCA2), TYRP1 (OCA3), and SLC45A2 (OCA4), and one single gene associated with OA: GPR143 (OA1) (Grønskov et al., 2007; Innamorati et al., 2006; King and Oetting, 2006; King et al., 2001; Schiaffino and Tacchetti, 2005; Suzuki and Tomita, 2008). Moreover, there are also syndromic forms of albinism, such as Hermansky–Pudlak Syndrome (HPS) and Chediak–Higashi Syndrome (CHS), which are characterized by more severe phenotypes affecting a range of additional cell types, beyond pigment cells, and are less common than OCA and OA types of albinism (Huizing et al., 2008; Ito et al., 2005; Wei and Li, 2013). "
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    • "Like the HPS genes, Oa1/OA1 was identified by positional cloning [32]–[34] and displays structural characteristics typical of transmembrane G-protein coupled receptors [35]. Further studies have shown that OA1 indeed functions as an intracellular G-protein coupled receptor in both Saccharomyces cerevisiae [36] and mammalian cells [37], and L-DOPA was suggested as a ligand for OA1 but the signal transduction mechanisms triggered by L-DOPA is still unclear [38]. OA1 is localized at the membrane of melanosomes at all stages of maturation but higher amounts are found in melanosomes at early stages of maturation (stage I and II) and in late endosomes [39]. "
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