Cloning and sequence analysis of brain cDNA encoding a Xenopus D2 dopamine receptor

A D2 dopamine receptor pharmacologically different from the mammalian D2 receptor has previously been characterized in the amphibian Xenopus laevis. Here we report the cloning of a Xenopus D2 receptor which revealed about 75% amino acid sequence identity with its mammalian counterpart and the presence of an additional 33 amino acid sequence in the 3rd cytoplasmic loop instead of the additional 29 residues in the large form of the mammalian D2 receptor. All 7 predicted transmembrane domains are highly conserved between the Xenopus and mammalian D2 receptors, as are the 1st and 2nd intracellular loop, the 1st and 3rd extracellular loop and the carboxy-terminal portion of the receptors. The amino-terminal portion, the 2nd extracellular loop and the middle portion of the 3rd intracellular loop of these receptors, however, differ considerably. Knowledge of the locations of these regions of conservation and divergence within the D2 receptors of Xenopus and mammals will help to delineate portions of the receptor molecule that are functionally important. Interestingly, the 5'-untranslated region of the Xenopus D2 receptor mRNA contains 4 small open reading frames which may affect translational efficiency.