title: The melanosomal/lysosomal protein OA1 has properties of a G protein-coupled receptor.
abstract: 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.
authors.affiliation: DIBIT, Scientific Institute San Raffalele, Via Olgettina 58, 20132 Milan, Italy.
authors.forename: Giulio
authors.initials: G
authors.lastname: Innamorati
authors.forename: Rosanna
authors.initials: R
authors.lastname: Piccirillo
authors.forename: Paola
authors.initials: P
authors.lastname: Bagnato
authors.forename: Ilaria
authors.initials: I
authors.lastname: Palmisano
authors.forename: Maria Vittoria
authors.initials: MV
authors.lastname: Schiaffino
mesh_terms: D000595:Amino Acid Sequence / Q000235:genetics; D000818:Animals; D019390:Arrestins / Q000378:metabolism; D019556:COS Cells; D002522:Chlorocebus aethiops; D005136:Eye Proteins / Q000235:genetics / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism; D015870:Gene Expression; D006801:Humans; D008247:Lysosomes / Q000235:genetics / Q000378:metabolism*; D020460:Melanosomes / Q000235:genetics / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D011499:Protein Processing, Post-Translational / Q000502:physiology; D043562:Receptors, G-Protein-Coupled / Q000235:genetics / Q000378:metabolism*; D017384:Sequence Deletion; D015398:Signal Transduction / Q000502:physiology*; D040681:Structural Homology, Protein
chemical_list: D019390:Arrestins; D005136:Eye Proteins; C095276:GPR143 protein, human; D008562:Membrane Glycoproteins; D043562:Receptors, G-Protein-Coupled; D019204:GTP-Binding Proteins