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The Journal of Neuroscience, February 15, 2001, 21(4):1203-1210
The C-Terminal Domains of the GABAB Receptor Subunits
Mediate Intracellular Trafficking But Are Not Required for Receptor
Signaling
Andrew R.
Calver1,
Melanie J.
Robbins1,
Christophe
Cosio1,
Simon Q. J.
Rice2,
Adam J.
Babbs1,
Warren D.
Hirst1,
Izzy
Boyfield1,
Martyn D.
Wood1,
Robert B.
Russell3,
Gary W.
Price1,
Andrés
Couve4,
Stephen J.
Moss4, and
Menelas N.
Pangalos1
Departments of 1 Neuroscience Research and
2 Biotechnology and Genetics and
3 Bioinformatics Research Group, SmithKline Beecham
Pharmaceuticals, New Frontiers Science Park, Harlow, Essex CM19
5AW, United Kingdom, and 4 Medical Research Council
Laboratory for Molecular Cell Biology and Department of Pharmacology,
University College London, London WC1E 6BT, United Kingdom
GABAB receptors are G-protein-coupled receptors that
mediate slow synaptic inhibition in the brain and spinal cord. These receptors are heterodimers assembled from GABAB1 and
GABAB2 subunits, neither of which is capable of producing
functional GABAB receptors on homomeric expression.
GABAB1, although able to bind GABA, is retained within the
endoplasmic reticulum (ER) when expressed alone. In contrast,
GABAB2 is able to access the cell surface when expressed
alone but does not couple efficiently to the appropriate effector
systems or produce any detectable GABA-binding sites. In the present
study, we have constructed chimeric and truncated GABAB1
and GABAB2 subunits to explore further GABAB
receptor signaling and assembly. Removal of the entire C-terminal
intracellular domain of GABAB1 results in plasma membrane
expression without the production of a functional GABAB
receptor. However, coexpression of this truncated GABAB1
subunit with either GABAB2 or a truncated
GABAB2 subunit in which the C terminal has also been
removed is capable of functional signaling via G-proteins. In contrast,
transferring the entire C-terminal tail of GABAB1 to
GABAB2 leads to the ER retention of the GABAB2
subunit when expressed alone. These results indicate that the C
terminal of GABAB1 mediates the ER retention of this
protein and that neither of the C-terminal tails of GABAB1 or GABAB2 is an absolute requirement for functional
coupling of heteromeric receptors. Furthermore although
GABAB1 is capable of producing GABA-binding sites,
GABAB2 is of central importance in the functional coupling
of heteromeric GABAB receptors to G-proteins and the
subsequent activation of effector systems.
Key words:
GABAB; GPCR; trafficking; signaling; intracellular retention; G-protein coupling; chimeras; receptor
subunits
Copyright © 2001 Society for Neuroscience 0270-6474/01/2141203-08$05.00/0
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