Structural studies on equine glycoprotein hormones. Amino acid sequence of equine chorionic gonadotropin beta-subunit

The complete amino acid sequence of the beta-subunit of equine chorionic gonadotropin (eCG beta) has been established by both automated Edman and manual 5-dimethylaminonaphthalene-1-sulfonyl-Edman degradations. Specific fragments were produced by cleavage with Staphylococcus aureus V8 protease, trypsin, or dilute HCl. For the sequence analyses of the heavily glycosylated COOH-terminal portion, a chemical deglycosylation procedure with trifluoromethanesulfonic acid was employed. The peptide chain of eCG beta consists of 149 amino acid residues. Five or more oligosaccharide chains are attached to the protein, 1 unit linked by an N-glycosidic bond to asparagine at residue 13 and four or more units linked by O-glycosidic bonds to serine or threonine at residues in the COOH-terminal portion. The carbohydrate-bearing hydroxy amino acids have not yet been rigorously established. As compared to the beta-subunits of the pituitary gonadotropin hormones, lutropin, follitropin, and thyrotropin, eCG beta possesses a glycosylated COOH-terminal extension of about 30 amino acid residues, as does the human chorionic gonadotropin beta-subunit (hCG beta). When the comparison is restricted inside the disulfide bond-containing core (residues 1-110), the beta-subunit of eCG is highly homologous to hCG beta (66%). On the other hand, although the overall structural features closely resemble each other, much less homology exists in the COOH-terminal extensions of eCG beta and hCG beta.