An acidic amino acid-specific protease from germinating soybeans

Abstract

The degradation of the β-conglycinin protein reserves in soybean seeds during germination and early growth begins with the proteolysis of its α and α′ 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 β-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 an α-helix. Consistent with the observation, synthetic poly- l-Glu inhibits the Protease C1-catalysed degradation of the α and α′ subunits of β-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 12.5 mM, 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 α and α′ subunits of β-conglycinin to generate proteolytic intermediates of the same size has been found in mung bean.