Coexpression of Full-Length #gamma#-Aminobutyric Acid_B (GABA_B) Receptors with Truncated Receptors and Metabotropic Glutamate Receptor 4 Supports the GABA_B Heterodimer as the Functional Receptor

摘要

Direct evidence is lacking to show thether the #gamma#-aminobutyric acid (GABA)_B gb1-gb2 heterodimer is the signaling form of hte receptor. In this study, we tested whether gb1a or gb2 subunits when coexpressed with truncated receptors or metabotriopic glutamate receptor mGluR4 could form functinal GABA receptors. Coexpression of the ligand binding N-terminal domain of gb1a or the C-terminal portion of gb1a composing the seven-transmembrane segments and intracellular loops with gb2 could not reconstitute functional receptors. We next examined whether mGluRE4, which forms homodimers and is structurally related to GABA_B, could act as a surrogate coreceptor for gb1 or bg2. The coexpression of mGluR4 and gb1 a led to the expression of gb1a monomers on cell surface membranes as determined by immunoblot analysis and flow cytometry. However, mGluR4-gb1a heterodimers were not formed, and mem-brane-expressed gb1a monomers were not functionally coupled to adenylyl cyclase in human embryonic kidney 293 cells or activated inwardly rectifying potassium (Kir) channels in Xenopus oocytes. Similarly, the coexpression of mGluR4 and gb2 led to nonfunctional GABA receptors. GABA-activated distal signaling events resulted only after the coexpression and heterodimerization of bg1 and bg2. Taken together with the truncated receptor studies, the data suggest that a high degree of structural speciificity is required to form the functional GABA_B receptor that is a gb1-gb2 heterodimer.