We expressed human mi, m2 and chimeric muscarinic cholinergic receptor
s (MAChR) in tobacco plants and in cultured BY2 tobacco cells using Ag
robacterium-mediated transformation. The membranes of most transgenic
plants and calli bound muscarinic ligands with appropriate affinities,
kinetics and pharmacologic specificity, as determined by direct and c
ompetitive binding measurements using the muscarinic ligand [H-3]quinu
clidinyl benzylate (QNB). Membranes of untransformed plants and calli
or those transformed with vector alone did not bind [H-3]QNB. Prelimin
ary experiments did not suggest regulation of endogenous plant G prote
in signalling pathways by the recombinant receptors. Membranes from on
e callus clone expressed mi MAChR at the level of 2.0-2.5 pmol [H-3]QN
B bound per mg membrane protein, more than the number of mi MAChR in m
ammalian brain and comparable to that expressed in Sf9 insect cells us
ing baculovirus vectors. This work demonstrates high level expression
of active G protein-coupled receptors in plants, such that signaling m
ight be genetically reconstituted by co-expression of appropriate G pr
oteins and effecters.