THE BETA-CONGLYCININS OF SOYBEAN REMAIN ASSEMBLED IN A 7S CONFORMATION WHILE UNDERGOING PROTEOLYTIC CLEAVAGE DURING GERMINATION AND EARLY SEEDLING GROWTH

The degradation of the beta-conglycinins, the second most abundant seed storage protein complex of Glycine max, that occurs as a result of proteolysis during seed germination and early seedling growth, was investigated. The beta-conglycinins of soybean are composed of a semi-random association of three different subunits, alpha' alpha, and beta, in a trimeric complex with a sedimentation coefficient of 7S. Western immunoblot analysis of the degradation products showed that proteolytic cleavage yields specific fragments as has been shown in other legumes. The proteolytic fragments produced in G. max, cv. Provar are designated here as FPI (62 kDa), FPII (57 kDa), FPIII (52 kDa), FPIV (31 kDa), and FPV (27 kDa). Comparison of the fragments from G. max cv. Keburi, a variant lacking the alpha' subunit, with those from G. max cv. Provar showed that the FPIV fragment is derived from the alpha' subunit. All fragments stained with periodic acid-Schiffs reagent, indicating that exhaustive deglycosylation of these subunits is not a prerequisite for cleavage. All of the fragments detected in these experiments sediment in linear sucrose density gradients with sedimentation coefficients of about 7S, identical to that of intact beta-conglycinin. These results suggest that as proteolysis of the beta-conglycinins occur during germination and early growth, the cleavage products are retained within the holoprotein structure. Further proteolysis cleaves the polypeptides into smaller fragments leading to the disintegration of the 7S storage protein structure.