title: Phosphorylation-dependent C-terminal binding of 14-3-3 proteins promotes cell surface expression of HIV co-receptor GPR15.
abstract: Membrane trafficking is dictated by dynamic molecular interactions involving discrete determinants in the cargo proteins and the intracellular transport machineries. We have previously reported that cell surface expression of GPR15, a G protein-coupled receptor (GPCR) that serves as a co-receptor for HIV, is correlated with the mode III binding of 14-3-3 proteins to the receptor C terminus. Here we provide a mechanistic basis for the role of 14-3-3 in promoting the cell surface expression of GPR15. The Ala mutation of penultimate phospho-Ser (S359A) that abolishes 14-3-3 binding resulted in substantially reduced O-glycosylation and the cell surface expression of GPR15. The surface membrane protein CD8 fused with the C-terminal tail of GPR15(S359A) mutant was re-localized in the endoplasmic reticulum (ER). In the context of S359A mutation, the additional mutations in the upstream stretch of basic residues (RXR motif) restored O-glycosylation and the cell surface expression. The RXR motif was responsible for the interaction with coatomer protein I (COPI), which was inversely correlated with the 14-3-3 binding and cell surface expression. These results suggest that 14-3-3 binding promotes cell surface expression of GPR15 by releasing the receptor from ER retrieval/retention pathway that is mediated by the interaction of RXR motif and COPI. Moreover, 14-3-3 binding substantially increased the stability of GPR15 protein. Thus 14-3-3 proteins play multiple roles in biogenesis and trafficking of an HIV co-receptor GPR15 to control its cell surface density in response to the phosphorylation signal.
authors.affiliation: Department of Biochemistry and Molecular Genetics, University of Illinois, Chicago, Illinois 60607, USA.
authors.forename: Yukari
authors.initials: Y
authors.lastname: Okamoto
authors.forename: Sojin
authors.initials: S
authors.lastname: Shikano
mesh_terms: D048948:14-3-3 Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D020755:Coat Protein Complex I / Q000235:genetics / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D057809:HEK293 Cells; D006367:HeLa Cells; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000737:chemistry / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism*; D016296:Mutagenesis / Q000502:physiology; D010766:Phosphorylation / Q000502:physiology; D055550:Protein Stability; D017434:Protein Structure, Tertiary; D021381:Protein Transport / Q000502:physiology; D043562:Receptors, G-Protein-Coupled / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015728:Receptors, HIV / Q000378:metabolism*; D018000:Receptors, Peptide / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology
chemical_list: D048948:14-3-3 Proteins; D020755:Coat Protein Complex I; C103539:GPR15 protein, human; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D043562:Receptors, G-Protein-Coupled; D015728:Receptors, HIV; D018000:Receptors, Peptide