An acidic amino acid-specific protease from germinating soybeans

The degradation of the beta-conglycinin protein reserves in soybean seeds during germination and early growth begins with the proteolysis of its alpha and alpha' subunits by an enzyme called Protease C1. In the pathway, a number of proteolytic intermediates are produced and subsequently degraded. Determination of the N-terminal sequences of these intermediates provides insight regarding the requirements of the cleavage sites. The N-terminal sequence of three such proteolytic intermediates has been determined. The sequence has been located in the published sequences of the beta-conglycinin subunits. Comparing these cleavage sites, plus those of two others previously delineated, shows that the P1' and P4' positions always bear either a Glu or an Asp residue while the P1 position always bears either a Glu or a Gln residue. In addition, other sites from P3 to P7' are also rich in either Glu or Asp, and the whole region is predicted to be in a alpha-helix. Consistent with the observation, synthetic poly-L-Glu inhibits the Protease C1-catalysed degradation of the alpha and alpha' subunits of beta-conglycinin. Poly-L-Glu (av. M(r) = 1000) at 12.5 mM was more effective at inhibiting the reaction than poly-L-Glu (av. M(r) = 600) or poly-L-Glu (av. M(r) = 14,300) at the same concentration. Comparing large synthetic polypeptides at 12.5mM, inhibition by poly-L-Asp (av. M(r) = 15,000) is as effective as poly-L-Glu (av. M(r) = 14,300), while poly-L-Ser (av. M(r) = 15,000) had no effect at all. Poly-D-Glu (av. M(r) = 15,000) is a better inhibitor than poly-L-Glu of the same size. A serine protease of similar molecular weight as Protease C1 and also capable of catalysing the proteolysis of the alpha and alpha' subunits of beta-conglycinin to generate proteolytic intermediates of the same size has been found in mung bean.