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

Authors

  • Giulio Innamorati,

    1. DIBIT, Scientific Institute San Raffalele, Via Olgettina 58, 20132 Milan, Italy
    Search for more papers by this author
  • Rosanna Piccirillo,

    1. DIBIT, Scientific Institute San Raffalele, Via Olgettina 58, 20132 Milan, Italy
    Search for more papers by this author
  • Paola Bagnato,

    1. DIBIT, Scientific Institute San Raffalele, Via Olgettina 58, 20132 Milan, Italy
    2. Department of Experimental Medicine, University of Genova Medical School, Via deToni 14, 16132 Genoa, Italy
    Search for more papers by this author
  • Ilaria Palmisano,

    1. DIBIT, Scientific Institute San Raffalele, Via Olgettina 58, 20132 Milan, Italy
    2. Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, University of Lecce, Via Provinciale Lecce-Monteroni, 73100 Lecce, Italy
    Search for more papers by this author
  • Maria Vittoria Schiaffino

    Corresponding author
    1. DIBIT, Scientific Institute San Raffalele, Via Olgettina 58, 20132 Milan, Italy
      *Address correspondence to M. Vittoria Schiaffino,
      e-mail: schiaffino.mariavittoria@hsr.it
    Search for more papers by this author

*Address correspondence to M. Vittoria Schiaffino,
e-mail: schiaffino.mariavittoria@hsr.it

Summary

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.

Ancillary