Location, location, location...site-specific GPCR phosphorylation offers a mechanism for cell-type-specific signalling

Abstract

It is now established that most of the approximately 800 G-protein-coupled receptors (GPCRs) are regulated by phosphorylation in a process that results in the recruitment of arrestins, leading to receptor desensitization and the activation of arrestin-dependent processes. This generalized view of GPCR regulation, however, does not provide an adequate mechanism for the control of tissue-specific GPCR signalling. Here, we review the evidence that GPCR phosphorylation is, in fact, a flexible and dynamic regulatory process in which GPCRs are phosphorylated in a unique manner that is associated with the cell type in which the receptor is expressed. In this scenario, phosphorylation offers a mechanism of regulating the signalling outcome of GPCRs that can be tailored to meet a specific physiological role.

Figure 1

Differential phosphorylation of the M₃ muscarinic receptor in CHO cells and cerebellar granule neurones. (a) Schematic representation of the generation of a receptor phospho-peptide map that reveals a specific pattern of phospho-peptides referred to here as a phosphorylation signature. The receptor (e.g. M₃ muscarinic) is immunoprecipitated from cells labelled with [³²P]-orthophosphate using a receptor-specific antibody. (1) The radiolabelled receptor is excised from the SDS–PAGE gel and the gel slice is subjected to tryptic digest. (2) The resulting tryptic peptides are spotted onto a cellulose plate and the plate is subjected to electrophoresis in one dimension and to ascending chromatography in the other dimension (for details, see Ref. [37]). (b) Shown are two phospho-peptide maps of the M₃ muscarinic receptor. (i) The map from the receptor isolated from CHO cells transfected with the mouse M₃ muscarinic receptor. (ii) The map generated from the M₃ muscarinic receptor endogenously expressed in cerebellar granule neurones (CGNs). It can be seen that there are phospho-peptides that are common to the two maps (labelled with numbers) in addition to phospho-peptides that are specific either to receptors isolated from CHO cells (open arrows) or from receptors isolated from CGNs (closed arrow). This experiment demonstrates that the M₃ muscarinic receptor does have a cell–type-specific phosphorylation signature. Figure reproduced, with permission, from Ref. .

Figure I

Cell-type-specific phosphorylation of a GPCR by three different protein kinases.