The cDNA‐deduced primary structure of human sex hormone‐binding globulin and location of its steroid‐binding domain

Abstract

We have sequenced a cDNA for sex hormone‐binding globulin (SHBG) isolated from a phage λgt 11 human liver cDNA library. The library was screened with a radiolabeled rat androgen‐binding protein (ABP) cDNA, and the abundance of SHBG cDNAs was 1 in 750 000 plaques examined. The largest human SHBG cDNA (1194 base‐pairs) contained a reading frame for 381 amino acids. This comprised 8 amino acids of a signal peptide followed by 373 residues starting with the known NH₂‐terminal sequence of human SHBG, and ending with a termination codon. The predicted polypeptide M _r of SHBG is 40 509, and sites of attachment of one O‐linked (residue 7) and two N‐linked oligosaccharide (residues 351 and 367) chains were identified. Purified SHBG was photoaffinity‐labeled with Δ⁶‐[³H]testosterone and cleaved with trypsin. The labeled tryptic fragment was isolated by reverse‐phase HPLC, and its NH₂‐terminal sequence was determined. The results suggest that a portion of the steroid‐binding domain of SHBG is located between residue 296 and the 35 predominantly hydrophilic residues at the C‐terminus of the protein.