STRUCTURE AND CHROMOSOMAL LOCATION OF THE GENE ENCODING MOUSE CORTICOSTEROID-BINDING GLOBULIN - STRAIN DIFFERENCES IN CODING SEQUENCE AND STEROID-BINDING ACTIVITY

Corticosteroid-binding globulin (CBG) is a member of the serine protei nase inhibitor superfamily and is responsible for the plasma transport of glucocorticoids. The mouse Cbg gene structure has been deduced fro m two non-overlapping DNA fragments of a lambda EMBL-3 genomic library , as well as PCR amplification of the approx. 2 kb of genomic DNA that lies between them. Mouse Cbg comprises five exons that span a region of approx. 10.5 kb, and has been localized in tight linkage with the A at (alpha(1)-antitrypsin) and Spi (serine proteinase inhibitor) gene c omplex on chromosome 12, in a region syntenic with this genetic locus on human chromosome 14. Intron-specific oligodeoxyribonucleotide prime rs were also used to PCR-amplify Cbg coding regions from several mouse strains. No differences were found in the Cbg coding sequences of BAL B/c and C57BL/6J-cpk/cpk mice, while two mutations were found within R IIIS/J Cbg that result in Lys(201)-->Glu and Ala(357)-->Thr substituti ons in the mature mouse CBG polypeptide. To assess what impact these s ubstitutions might have on the steroid-binding activity of RIIIS/J CBG , these mutations were introduced separately or together into a BALB/c mouse Cbg cDNA. Expression of these mutants in the MDCK cell line ind icated that the Lys(201)-->Glu substitution accounts for the abnormal steroid-binding affinity of CBG in RIIIS/J mice.