Gatekeepers Controlling GPCR Export and Function

doi:10.1016/j.tips.2015.06.007

Trends in Pharmacological Sciences

Volume 36, Issue 10, October 2015, Pages 636-644

https://doi.org/10.1016/j.tips.2015.06.007 Get rights and content

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Functional GABA_B receptors are obligate heterodimers of the GB1 and GB2 subunits.

The agonist binding GB1 is retained inside the cells in the absence of association with GB2.

GB2 competitively releases GB1 from PRAF2, a specific ER-resident gatekeeper.

The stoichiometry of PRAF2 relative to GB1 and GB2 is a key parameter of GABA_B function in vivo.

Regulated export of G protein-coupled receptors (GPCRs) from intracellular stores involves chaperones and escort proteins, which promote their progression to the cell surface, and gatekeepers, which retain them in intracellular compartments. Functional γ-aminobutyric acid (GABA)_B receptors, the paradigm of this phenomenon, comprise GB1 and GB2 subunits forming a heterodimer. GB1 is retained in the endoplasmic reticulum (ER) in the absence of GB2. A specific ER-resident gatekeeper, prenylated Rab acceptor family 2 (PRAF2), is involved in GB1 retention and prevents its progression into the biosynthetic pathway. GB1 can be released from PRAF2 only on competitive interaction with GB2. PRAF2 is ubiquitous and belongs to a subgroup of the mammalian Ypt-interacting protein (Yip) family. Several other GPCRs are likely to be regulated by Yip proteins, which might be involved in the pathophysiology of human diseases that are associated with impaired receptor targeting to the cell surface.

Section snippets

Molecular Mechanisms Regulating Receptor Cell Surface Density

The number of receptors present at the cell surface that can be reached and activated by cognate ligands is evidently essential in terms of downstream signaling outputs. Most studies addressing regulation of GPCRs have focused on desensitization, the termination of activated receptor signaling, GPCR endocytosis and recycling, and the transcriptional control of GPCR-coding genes. For a long time it has been assumed that, apart from translation, the only level of regulation of native receptor

The GABA_B Receptor Paradigm

The metabotropic GABA_B receptor has been one of the most intensively investigated GPCRs over the past three decades, because of the very unusual regulation of its cell surface export. It belongs to the class-C family of GPCRs, which also comprises the metabotropic glutamate receptor, the calcium-sensing receptor, and the taste T1R, which is characterized by a large extracellular Venus flytrap (VFT) domain involved in ligand recognition. Together with the ionotropic GABA_A receptor, GABA_B

A Missing Mechanistic Aspect in the Regulated Delivery of GABA_B to the Cell Surface

The cell surface density of functional heterodimeric GABA_B receptors is controlled by a unique mechanism of delivery from the biosynthetic compartments to the plasma membrane. Early studies reported that recombinant GB1 subunits fail to reach the cell surface when expressed in heterologous systems or overexpressed in ganglion neurons [23]. The explanation of this phenomenon is that GB1 is retained in the ER and does not reach the cell surface in the absence of GB2 12, 13, 14. Subsequent

PRAF2, the Missing ER Gatekeeper Controlling GABA_B Delivery to the Cell Surface

The coiled-coil domain of the carboxy-terminal tail of the GB1, containing both the RXR and LL motifs, is presumably involved in the interaction with a putative ER-resident ‘gatekeeper’ that regulates the delivery of the GABA_B heterodimer to the cell surface.

During the past 10 years, multiple GB1-interacting proteins have been reported that regulate GABA_B expression in some way, often interacting with the GB1 coiled-coil domain. However, under physiological conditions none of these proteins

PRAF Gatekeepers Might Regulate Other GPCRs in Various Tissues

Although the mechanistic aspects of PRAF2-dependent regulation have been elucidated only for the GABA_B receptor, it is plausible that PRAFs might collectively regulate several other GPCRs. As indicated above, PRAF2 was originally identified in two-hybrid screens as an interaction partner of the chemokine CCR5 receptor, regulating in some way its membrane localization [33]. PRAF3, by contrast, was reported to restrict the subcellular localization of the β₂-adrenoceptor and the D2 dopaminergic

Potential Involvement of PRAFs in Human Disease

Similar or even larger changes of PRAF2 than those we observed inducing phenotypic effects in animals can be detected either physiologically – during neuronal differentiation, for example – or in pathological samples such as human cancer tissues [36] or areas of the brain of people who have died after excessive alcohol intake [55]. These findings indicate a potential causative connection between the concentration of PRAF gatekeepers and pathological changes in humans. In addition, although not

Concluding Remarks

The identification of the resident gatekeeper that retains the GB1 subunit of the GABA_B receptor in the ER on interaction with both the LL and RXR motifs of the carboxy-terminal tail of GB1 until it is displaced by the GB2 subunit completes the picture of the complex mechanism regulating the delivery of functional GABA_B receptors to the cell surface. PRAFs were already reported to interact with and/or regulate several other GPCRs and plasma membrane transporters. In addition, among mammalian

Acknowledgments

The authors thank Dr Hervé Enslen and Dr Mark G.H. Scott of their laboratory for reviewing the manuscript. The research of their laboratory is supported by grants of the Fondation pour la Recherche Médicale and Ligue Contre le Cancer, Comité de l’Oise.

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Trends in Pharmacological Sciences

OpinionGatekeepers Controlling GPCR Export and Function

Trends

Section snippets

Molecular Mechanisms Regulating Receptor Cell Surface Density

The GABAB Receptor Paradigm

A Missing Mechanistic Aspect in the Regulated Delivery of GABAB to the Cell Surface

PRAF2, the Missing ER Gatekeeper Controlling GABAB Delivery to the Cell Surface

PRAF Gatekeepers Might Regulate Other GPCRs in Various Tissues

Potential Involvement of PRAFs in Human Disease

Concluding Remarks

Acknowledgments

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Opinion
Gatekeepers Controlling GPCR Export and Function

The GABA_B Receptor Paradigm

A Missing Mechanistic Aspect in the Regulated Delivery of GABA_B to the Cell Surface

PRAF2, the Missing ER Gatekeeper Controlling GABA_B Delivery to the Cell Surface