The molecular biology of avian gonadotropin

Complimentary DNA for precursor molecules of chicken and quail luteinizing hormone (LH) beta subunits have cloned. From nucleotide sequences of the cDNA, the primary structures of the LH beta subunit molecules of these avian species were deduced. The primary structures were similar, the amino acid sequence homology being 91.6%. For the alpha subunit, cloning of cDNA has been performed also in these two avian species. Predicted primary structures of the alpha subunit molecules of these birds were completely identical, although nucleotide sequences of their cDNA were slightly different. The LH beta subunit molecules of both birds had 15 Pro residues at the same positions. Ten of them shared the same positions with Pro residues in the mammalian LH beta subunits, in which about 20 Pro residues exist. Prediction of the secondary structure and exposure of each amino acid residue in the chicken LH beta subunit molecule were performed with the aid of a computer program for protein engineering. It was revealed that most of 15 Pro residues did not exist in regions where the beta structure was predicted but were distributed in regions where a turn or loop was predicted. In addition, three of four Tyr residues were predicted to be located inside the molecule. These results suggest that the predicted presence of a number of Pro residues in the loop of the secondary structure is a cause of high animal group and hormone specificities in the LH-LH receptor interaction. The predicted internal localization of Tyr residues is considered to cause loss of receptor binding activity by conventional radioiodination procedures.