Characterization and determination of the biological activities of noncleavable high molecular weight forms of inhibin A and activin A.

Recombinant expression of human alpha- and beta A-inhibin subunit cDNAs in mammalian 293 cells results in the secretion of 20-53K free alpha-subunit-derived products, 30-105K alpha beta A-inhibin dimers, and 24-110K beta A-activin dimers. The present study verifies that the wide variation in the size of these products is due to incomplete cleavage of the proteolytic processing sites and the differential glycosylation of the N-linked glycosylation site at amino acid number 302 in the alpha C-subunit. The identity of each of these products was established by mutagenesis of proteolytic processing sites and N-linked glycosylation sites, combined with the analysis of transfection products by immunoprecipitation and one- and two-dimensional SDS-PAGE (SDS/SDS-beta-ME). Transient expression of processing site mutants of the alpha- and beta A-subunits in 293 cells was used to generate microgram quantities of noncleavable 55K and 65K inhibin dimers, and noncleavable 110K activin A dimers. The 55K and 65K inhibin A forms were purified and found to be fully biologically active in a rat pituitary cell bioassay. The 110K high molecular weight (HMW) form of human activin A failed to show any FSH-releasing activity in the pituitary assay. Since radioactively labeled 55K and 65K inhibin A and 110K activin A remained intact after incubation with rat pituitary cells for 72 h, there appears to be no conversion of these dimers to lower molecular weight forms by proteolytic cleavage at additional sites. These results show for the first time that 55K and 65K inhibit A are intrinsically biologically active and do not require cleavage to the 32K form for activation. In contrast, cleavage of the 110K activin A precursor to the 24K form would appear to be necessary for activity.