Cloning of a Xenopus laevis muscarinic receptor encoded by an intronless gene

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Abstract
The Xenopus laevis oocyte has endogenous sites that bind muscarinic agonists, which have been pharmacologically characterized as M3 and/or M1 receptor subtypes. In order to define the molecular identity of the receptor protein we have analyzed a Xenopus oocyte cDNA library and cloned a 2.9 kb cDNA fragment encoding a muscarinic receptor (xMR). The deduced amino acid sequence reveals a protein of 484 residues with an apparent molecular weight of 54,188 Da. Amino acid comparison with previously cloned mammalian muscarinic receptors showed a 78% identity with the human m4 subtype, presenting at the same time clustered differences within the amino-terminal region and third intracellular loop. Genomic Southern analysis displayed the presence of one main gene belonging to this subtype, and the PCR analysis revealed an intronless gene.