Heterogeneity of FSH receptor complexes and their interactions with selected lectins.

Abstract

Carbohydrate residues contribute to the hormone recognition site of the receptors for several hormones including insulin and TSH. The present studies were undertaken to ascertain whether glucose, mannose, or N-acetylglucosamine residues contributed to the FSH receptor recognition site. Initial studies revealed that the enriched FSH plasma membrane fraction contained less than 1% sialic acid and neutral hexoses. A partially purified enriched FSH plasma membrane binding fraction was prepared from Leydig cell-depleted calf testis by differential and sucrose density gradient centrifugation. Specific [125I]-FSH binding to the enriched FSH binding fraction was unaffected by graded concentrations (0.1 to 50 μg) of either concanavalin A or wheat germ agglutinin. In one study, the enriched FSH plasma membrane binding fraction was solubilized with 1% Triton X-100 and percolated through either a Concanavalin A-Sepharose 48 or a wheat germ agglutinin Sepharose column. When unbound and 0.2 M α-D-methylglucosylpyranoside (MEG) or N-acetylglucosamine (NAG)-eluted fractions were analyzed for specific [125I]-FSH binding, only the unbound fractions displayed specific binding. Specific FSH binding to its receptor was unaffected by 0.2 M MEG or 0.1 M NAG. When the preformed [125I]-FSH-receptor complex was solubilized with 1% Triton X-100 and percolated through a Concanavalin A-Sepharose 48 column, more than 90% of the preformed complex was retained on the column, but was eluted with 0.2 M MEG. Binding of the hormone-receptor complex was the result of binding of the lectin to the carbohydrate residues of the hormone, which do not appear to be necessary for direct hormone-receptor interaction. The results of these studies strongly suggest that neither mannose, glucose, nor N-acetylglucosamine residues constitute an intrinsic part of the FSH receptor binding site. Alternatively, if those residues are an intrinsic part of the FSH receptor, they are inaccessible to lectin binding. Although these three sugar residues may contribute to the conformation of the FSH molecule, they do not appear to be requisite for directly interacting with the receptor binding site of FSH.