Structure and function of corticosteroid-binding globulin: role of carbohydrates.

To study the site-specificity of human corticosteroid-binding globulin (CBG) glycosylation and the functional significance of individual carbohydrate chains in its molecule, a panel of recombinant CBG mutants containing each of the six potential glycosylation sites alone and in various combinations has been expressed in Chinese hamster ovary (CHO) cells. Analyses of these mutant glycoproteins showed that three of the glycosylation sites are only partially utilized, and this may contribute to the production of glycoforms with distinct physiological functions. Processing of individual carbohydrate chains (branching and fucosylation) is site-specific and may, thus, account for the formation of structural determinants essential for the recognition of CBG by cell membranes. Glycosylation at the only phylogenetically conserved consensus site, Asn238-Gly239-Thr240, is essential for the biosynthesis of CBG with steroid-binding activity. Evidence has been obtained to support the hypothesis that transient carbohydrate-polypeptide interactions between Trp266 and the maturing carbohydrate chain at Asn238 occur during early stages of the CBG biosynthesis which affect protein folding and formation of the steroid-binding site. Another tryptophan residue, Trp371, has been found to be critical for CBG-steroid interactions and is likely located in the steroid-binding site.