SUBSTITUTIONS OF TRYPTOPHAN RESIDUES IN HUMAN CORTICOSTEROID-BINDING GLOBULIN - IMPACT ON STEROID-BINDING AND GLYCOSYLATION

Human corticosteroid-binding globulin (CBG) contains four tryptophan residues at positions 141, 185, 266 and 371; one of which is thought to be located in the steroid-binding site. These residues were substituted by site-directed mutagenesis and expression of mutant CBG cDNAs in Chinese hamster ovary cells. Analyses of the resulting mutants indicate that Trp(371) is most likely located in the steroid-binding site, and that hydrophobic interactions between Trp(141) and the steroid molecule or other amino-acids in the CBG polypeptide may also contribute to high-affinity interactions between CBG and its steroid ligands. In addition, substitution of Trp(266) resulted in altered glycosylation of CBG, and this supports the concept that it participates in intra-molecular carbohydrate-polypeptide interactions which may influence the conformation and secretion of this glycoprotein.