Subtypes of the Somatostatin Receptor Assemble as Functional Homo- and Heterodimers

doi:10.1074/jbc.275.11.7862

Subtypes of the Somatostatin Receptor Assemble as Functional Homo- and Heterodimers*

From the ^‡Fraser Laboratories, Departments of Medicine, Pharmacology and Therapeutics, and Neurology and Neurosurgery, McGill University and Royal Victoria Hospital, Montreal, Quebec H3A 1A1, Canada and the ^¶Department of Physics and Chemistry, Clarkson University, Potsdam, New York 13676

Abstract

The existence of receptor dimers has been proposed for several G protein-coupled receptors. However, the question of whether G protein-coupled receptor dimers are necessary for activating or modulating normal receptor function is unclear. We address this question with somatostatin receptors (SSTRs) of which there are five distinct subtypes. By using transfected mutant and wild type receptors, as well as endogenous receptors, we provide pharmacological, biochemical, and physical evidence, based on fluorescence resonance energy transfer analysis, that activation by ligand induces SSTR dimerization, both homo- and heterodimerization with other members of the SSTR family, and that dimerization alters the functional properties of the receptor such as ligand binding affinity and agonist-induced receptor internalization and up-regulation. Double label confocal fluorescence microscopy showed that when SSTR1 and SSTR5 subtypes were coexpressed in Chinese hamster ovary-K1 cells and treated with agonist they underwent internalization and were colocalized in cytoplasmic vesicles. SSTR5 formed heterodimers with SSTR1 but not with SSTR4 suggesting that heterodimerization is a specific process that is restricted to some but not all receptor subtype combinations. Direct protein interaction between different members of the SSTR subfamily defines a new level of molecular cross-talk between subtypes of the SSTR and possibly related receptor families.

Footnotes

↵* This work was supported by grants from the Canadian Medical Research Council, National Institutes of Health Grant NS32160-04, and the United States Department of Defense.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

This paper is dedicated to the memory of Monique Pellerin Rocheville.
↵§ Recipient of studentship support from the Fonds De La Recherche En Sante Du Quebec (FRSQ) and from the Royal Victoria Hospital Research Institute.
↵‖ Distinguished Scientist of the Canadian Medical Research Council. To whom correspondence should be addressed: Royal Victoria Hospital, Rm. M3-15, 687 Pine Ave. West, Montreal, Quebec H3A 1A1, Canada. Tel.: 514-842-1231 (ext. 5042); Fax: 514-849-3681; E-mail: yogesh.patel@muhc.mcgill.ca.
Abbreviations:

GPCR

G protein-coupled receptor

SST

somatostatin

SMS

octapeptide SMS-(201–995)

SCH275

des-AA^1,2,5[d-Trp⁸,IAMP⁹]SRIF

LTT-SST-28

Leu⁸-d-Trp²², Tyr²⁵, SST-28

SCH288

des-AA^1,5[Tyr²-d-Trp⁸,IAMP⁹]SRIF

SSTR

somatostatin receptor

wt hSSTR1

wild type human somatostatin receptor type 1

wt hSSTR4

wild type human somatostatin receptor type 4

HA-SSTR5

hemagglutinin-tagged somatostatin receptor type 5

ECL2

second extracellular loop segment

ECL3

third extracellular loop segment

C-tail

cytoplasmic carboxyl-terminal segment

CHO

Chinese hamster ovary

mAb

monoclonal antibody

pbFRET

photobleaching fluorescence resonance energy transfer
- Received September 20, 1999.
- Revision received December 8, 1999.
The American Society for Biochemistry and Molecular Biology, Inc.