Origin of the heavy and light protomers of androgen-binding protein from the rat testis

Androgen-binding protein (ABP) isolated from rat epididymides by androgen affinity chromatography is a dimer with a native molecular weight of 85,000. When fractionated on sodium dodecyl sulfate gels two components were identified which were present in a 3:1 ratio; these were designated heavy (H) and light (L) (Mr of 45,000 and 41,000), respectively. The fact that H and L both have steroid binding sites and that they can be cross-linked suggests that both are protomers of native ABP. NH2-terminal analysis, amino acid analysis, and peptide maps suggest an extensive homology between H and L. In addition, peptide maps of H and L photolabeled with delta 6-[3H]testosterone suggest that the binding sites on these protomers are identical. ABP was also purified 33,800-fold from testes. The H and L protomers from this preparation were identical with those from the epididymides as judged by peptide maps. In addition, [35S]methionine was incorporated into the H and L synthesized by Sertoli cells in the same 3:1 ratio as in highly purified ABP from testis and epididymis. These observations suggest that L is not derived from H following secretion. Selective incorporation of [3H] fucose into the H protomer suggests that differences in carbohydrate composition account for at least part of the difference between H and L. These results support the conclusion that native ABP is composed of two kinds of protomers which exist in a ratio of 3:1. These protomers are similar polypeptides that differ partly in carbohydrate composition.