AN ACIDIC AMINO ACID-SPECIFIC PROTEASE FROM GERMINATING SOYBEANS

The degradation of the beta-conglycinin protein reserves in soybean se eds 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 subse quently degraded. Determination of the N-terminal sequences of these i ntermediates provides insight regarding the requirements of the cleava ge sites. The N-terminal sequence of three such proteolytic intermedia tes has been determined. The sequence has been located in the publishe d 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 whil e the P1 position always bears either a Glu or a Gin residue. In addit ion, other sites from P3 to P7' are also rich in either Glu or Asp, an d the whole region is predicted to be in an alpha-helix. Consistent wi th the observation, synthetic poly-L-Glu inhibits the Protease C1-cata lysed degradation of the alpha and alpha' subunits of beta-conglycinin . Poly-L-Glu (av. M(r) = 1000) at 12.5 mM was mon effective at inhibit ing 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 pol ypeptides 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-alu of the same size. A serine proteas e of similar molecular weight as Protease C1 and also capable of catal ysing the proteolysis of the alpha and alpha' subunits of beta-conglyc inin to generate proteolytic intermediates of the same size has been f ound in mung bean.