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PNAS 98 (25): 14649-14654

Copyright © 2001 by the National Academy of Sciences.


Function of GB1 and GB2 subunits in G protein coupling of GABAB receptors

Marta Margeta-Mitrovic, Yuh Nung Jan, and Lily Yeh Jan*

The Howard Hughes Medical Institute, Departments of Physiology and Biochemistry, University of California, San Francisco, CA 94143

Contributed by Lily Yeh Jan

Accepted for publication October 17, 2001.

Abstract: Many G protein-coupled receptors (GPCRs) have recently been shown to dimerize, and it was suggested that dimerization may be a prerequisite for G protein coupling. {gamma}-aminobutyric acid type B (GABAB) receptors (GPCRs for GABA, a major inhibitory neurotransmitter in the brain) are obligate heterodimers of homologous GB1 and GB2 subunits, neither of which is functional on its own. This feature of GABAB receptors allowed us to examine which of the eight intracellular segments of the heterodimeric receptor were important for G protein activation. Replacing any of the three intracellular loops of GB2 with their GB1 counterparts resulted in nonfunctional receptors. The deletion of the complete GB2 C terminus significantly attenuated the receptor function; however, the proximal 36 residues were sufficient for reconstitution of wild type-like receptor activity. In contrast, the GB1 C terminus could be deleted and GB1 intracellular loops replaced with their GB2 or mGluR1 equivalents without affecting the receptor function. In addition, a large portion of the GB1 i2 loop could be replaced with a random coil peptide without any functional consequences. Thus, GB2 intracellular segments are solely responsible for specific coupling of GABAB receptors to their physiologic effectors, Gi and G protein-activated K+ channels. These findings strongly support a model in which a single GPCR monomer is sufficient for all of the specific G protein contacts.

* To whom reprint requests should be addressed. E-mail: gkw{at}

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