A 246 bp cDNA clone representing the C-terminal region of the donkey (Equus asinus) chorionic gonadotrophin (CG)-β subunit was isolated from a placental library. The transcript contained the 3′ untranslated region and 42% of the CG-β subunit coding region (amino acid residues 85–146 of the mature peptide). Comparison of the deduced donkey amino acid sequence with the published horse CG-β subunit protein sequence (where they overlapped) revealed an overall homology of 61%. However, most of the differences were in the C-terminal extension, which is thought not to be important for gonadotrophic activity, and appeared to be due to two nucleotide insertions in the donkey sequence (compared with a deduced horse nucleotide sequence) leading to a reading-frame shift. Amino acid homology in the disulphide 'core' region was 81%. Some of the differences in this region were in the 'determinant loop' (residues 93–100) and these are interpreted in relation to the observed biological activities of horse and donkey CG.
The deduced amino acid sequence of the donkey cDNA indicated that it was larger than the majority of gonadotrophin-β subunits due to a C-terminal extension. Primate and horse CG (and horse LH) β subunits have analogous C-terminal extensions. The extension in the donkey subunit is 25 amino acid residues in length, compared with 28 in the horse and 24 in man. Comparisons with other available subunit DNA sequences indicated that, like the human CG-β gene, the donkey gene probably evolved from an ancestral LH-like β gene, following nucleotide deletions that allowed readthrough into previously untranslated DNA. Furthermore, both the human and donkey CG-β genes make use of the original LH polyadenylation sequence AAUAAA for translational termination and polyadenylation. We conclude that the C-terminal extension arose independently in equids and primates but through similar mechanisms.