Cloning, sequencing and functional expression of zebra (Equus burchelli) LH

Although donkey luteinizing hormone exhibits a very high degree of amino acid sequence identity with horse LH, its FSH activity in non-equine species is tenfold lower. The coding regions of the common zebra (Equus burchelli) glycoprotein hormone alpha-subunit and LH beta-subunit transcripts were cloned by reverse transcription-PCR from pituitary gland RNA to investigate more precisely the structure-function relationships of this gonadotrophin family. Zebra LH was then expressed in COS-7 cells and its LH and FSH activities were assessed in a rat Leydig cell bioassay (for LH) and in a cell line stably expressing the human FSH receptor bioassay (for FSH). The recombinant zebra LH, although displaying LH activity similar to that of recombinant donkey and horse LH, had no detectable FSH activity. The LH amino acid sequences of these three species are very similar, leaving only very few amino acids as potential candidates to explain the difference in their FSH activities. Moreover, according to the difference in FSH bioactivity and to the percentage identity between the sequences, the common zebra is phylogenetically closer to the donkey than it is to the horse.