Cloning and analysis of the cDNA encoding the horse and donkey luteinizing hormone beta-subunits

The coding regions of the horse (Equus caballus) and donkey (E. asinus) luteinizing hormone (LH) beta-subunit transcripts were cloned from pituitary gland RNA, in order to investigate their relationships to the corresponding equine chorionic gonadotropin (CG) beta-subunits and to further understand the unusual receptor-binding properties of equine LH and CG. The horse and donkey LH beta-subunit sequences were very similar (97% identity at the nucleotide (nt) level; 93% at the amino acid (aa) level), confirming their very close evolutionary linkage and also indicating that the C-terminal extension in both subunits occurred prior to the divergence of horse and donkeys. Furthermore, sequence comparisons and Southern blot analysis confirmed that in donkeys, as in horses, the pituitary (LH) and placental (CG) beta-subunits are almost certainly derived from a single gene. This contrasts with primates which have a single LH beta gene expressed in the pituitary and a family of separate CG beta genes that are expressed in the placenta. The deduced aa sequences also revealed several differences between the horse and donkey LH/CG beta-subunits that could explain their differences in biological activity. In addition, the study confirmed that the donkey subunit contains an additional Cys residue that is not present in other gonadotropin beta-subunits.