Limited proteolysis of beta-conglycinin and glycinin, the 7S and 11S storage globulins from soybean [Glycine max (L) Merr] - Structural and evolutionary implications

The G2 (A2B1a) glycinin subunit from soybean (Glycine max L. Merr.) was purified and renatured to the homohexameric holoprotein. This protein along with purified beta-conglycinin were subjected to limited proteolysis by trypsin. The generated polypeptide fragments were separated via SDS/PAGE and the amino acid sequence of the N-terminals was determined. Four cleavage points were detected in the alpha-chain A2 of glycinin as well as in the alpha'-chain of beta-conglycinin. From the known three-dimensional structure of 7S globulin and the hypothetical model of 7S globulin-like 11S globulin structure, it was possible to draw the conclusion that two distinct types of susceptible sites for proteolytic cleavage are characteristic of the subunits of both globulins. The first includes the sequences linking N- and C-terminal domains of both globulins and the sequence of N-terminal extensions of 70-kDa subunits from the vicilin-like 7S globulins. The second type includes the loop between beta-strands E and F of the N-terminal domain of 11S globulins and of the C-terminal domain of 7S globulins. A statistically significant similarity was found between the N-terminal extension of the alpha'-chain of beta-conglycinin and the interdomain linker regions of soybean glycinin and pea legumin. It is proposed that the three sequence regions which form the first type of susceptible sites are of similar structural function and might have evolved from the N-terminal segment of a putative single-domain ancestor.