Differential sensitivity of purified isoforms of cucumber anionic virus-inducible peroxidase to exogenous proteases

Three different molecular forms, isoforms, of the major virus-inducible ani onic peroxidase (PRX) of cucumber (Cucumis sativus L.) were purified to hom ogeneity from crude extracts of hypersensitively reacting cotyledons and su bjected to proteolysis with five exogenous endoproteinases. The PRX isoform s were fully resistant to degradation by trypsin and chymotrypsin even thou gh at a prolonged incubation. Partial proteolysis with pepsin yielded pepti des which were similar in size and serological properties. When papain was used, the peptides released from PRX1 isoform differed both in size and num ber but not serologically from the peptides released from isoforms PRX2 and PRX3 confirming similar primary structure of polypeptide chains. PRX3 was the only substrate giving a peptide map after incubation with protease It. Under experimental conditions used in this work, PRX1 and PRX2 were degrade d completely with protease It. These results indicate that PRX1, PRX2, and PRX3 contain similar antigenic determinants and indicate very similar but n ot identical primary structures. Several practical implications of the pres ent study are also mentioned.