Protease C2, a cysteine endopeptidase involved in the continuing mobilization of soybean beta-conglycinin seed proteins

Abstract

The protease that degrades the beta subunit of the soybean (Glycine max (L.) Merrill) storage protein beta -conglycinin was purified from the cotyledons of seedlings grown for 12 days. The enzyme was named protease C2 because it is the second enzyme to cleave the beta -conglycinin storage protein, the first (protease C1) being one that degrades only the alpha' and a subunits of the storage protein to products similar in size and sequence to the remaining intact beta subunit. Protease C2 activity is not evident in vivo until 4 days after imbibition of the seed. The 31 kDa enzyme is a cysteine protease with a pH optimum with beta -conglycinin as substrate of 5.5. The action of protease C2 on native beta -conglycinin produces a set of large fragments (52-46 kDa in size) and small fragments (29-25 kDa). This is consistent with cleavage of all beta -conglycinin subunits at the region linking their N- and C-domains. Protease C2 also cleaves phaseolin, the Phaseolus vulgaris is vicilin homologous to beta -conglycinin, to fragments in the 25-28 kDa range. N-Terminal sequences of isolated beta -conglycinin and phaseolin products show that protease C2 cleaves at a bond within a very mobile surface loop connecting the two compact structural domains of each subunit. The protease C2 cleavage specificity appears to be dictated by the substrate's three-dimensional structure rather than a specificity for a particular amino acid or sequence. (C) 2001 Elsevier Science B.V. All rights reserved.