Review
Lessons from constitutively active mutants of G protein-coupled receptors

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Abstract

In the past decade, the concept of constitutive activity has profoundly modified our understanding of G protein-coupled-receptors (GPCRs). Here, we review the contribution of constitutively active mutants (CAMs) to our understanding of three aspects of GPCR physiopathology: (1) GPCR activation is a complex mechanism involving both the release of inactive state conformational constraints, mimicked by most CAMs, and the creation of new interactions that stabilize the active state and are mimicked by a restricted set of CAMs; (2) GPCR phosphorylation, internalization and desensitization processes are activated by receptor conformations, which partly overlap those activating G protein; (3) natural CAMs, mostly affecting GPCRs of the endocrine system, are found in several hereditary and acquired diseases, including cancers. One major remaining question is how CAMs recapitulate the different structural modifications of the agonist-induced active conformation(s) of the wild-type receptor. This characterization is a prerequisite for further use of CAMs as ligand-free models of active GPCRs in structural, cellular and physiological studies.

Section snippets

Lessons from CAMs on the molecular mechanisms of GPCR activation

Activation of a GPCR requires a change in its conformation that triggers G protein activation, but also induces other protein interactions involved in signal transduction and receptor traffic. CAMs are thought to mimic to some extent the active conformation of the wt receptor and to adopt spontaneously a structure able to activate G proteins. The position and nature of the mutations thus provide clues to the differences between the inactive and the active conformations (Fig. 2).

The most

Lessons from CAMs on the regulation and traffic of GPCRs

There is increasing evidence that GPCRs, after agonist binding, not only activate G protein(s) but are also able to interact with a large variety of proteins, which in turn activate other signaling pathways or the desensitization of the receptor. Desensitization usually involves phosphorylation by specific and non-specific kinases and arrestin binding, which prevents interaction with G proteins and recruits the internalization and recycling machineries [37]. After internalization, the receptor

Lessons from CAMs on the role of GPCRs in pathology

The discovery that a hormone receptor can present unregulated and ligand-independent activity was rapidly correlated with autonomous hormonal secretion in several endocrine diseases. The role of CAMs in human disease was first demonstrated in 1993 for the thyrotropin (TSH) receptor in hyperfunctioning thyroid adenoma [49]. Several dozen mutations affecting the TM or extracellular domains of this GPCR activate constitutively (i.e. in absence of TSH ligand) the cAMP signaling pathway (Fig. 2).

Conclusion

CAMs have provided a useful tool to study the activation mechanisms and the pathophysiology of GPCRs, in spite of their structural differences with the agonist-linked active state(s). From these studies emerge some new concepts which might lead to very speculative hypotheses on GPCR physiology, illustrated by two examples. (1) Similar to the characterization of receptor subtypes, identification of multiple conformational active states of a given GPCR suggests the existence of different

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