A PROTEASE RESPONSIBLE FOR POSTTRANSLATIONAL CLEAVAGE OF A CONSERVED ASN-GLY LINKAGE IN GLYCININ, THE MAJOR SEED STORAGE PROTEIN OF SOYBEAN

The assembly of 11S globulin seed storage proteins in plants is regulated in part by the activity of a protease that cleaves between asparagine and glycine residues. Post-translational cleavage of subunit precursors into acidic and basic polypeptides is associated with the ability of subunits in trimers to aggregate into hexamers in vitro. An activity is present in extracts from immature soybean seeds that specifically cleaves immature 11S seed storage proteins of soybean and Vicia faba into the polypeptides of the mature proteins. Sequence microanalysis has been used to demonstrate that proglycinin and prolegumin are cut at the legitimate site when proteins synthesized in vitro are used as substrates. A single amino acid change in the cleavage site renders the substrate uncleavable. The protease responsible for this activity also hydrolyzes a synthetic octapeptide whose sequence reproduces four amino acids on either side of the glycinin subunit G4 cleavage site. This assay permitted the purification and characterization of the protease. It is a glycosylated enzyme with an acidic pH optimum and a molecular mass of about 45 kDa in solution.