SUMMARY: The fungus grown in a liquid medium containing protein secreted a protease with many similarities to one extractable from apple fruits infected with this pathogen but absent from healthy fruit. Enzymes from both sources degraded several protein substrates optimally at pH 3·4. At this pH, haemoglobin was degraded optimally at 47 °C, while at 37 °C both enzymes were most stable at pH 5. The pI values for the enzymes were close to 4·0, and their action was inhibited by pepstatin and diazoacetyl-norleucine methyl ester plus copper ions but unaffected by p-hydroxymercuribenzoate, EDTA or trypsin inhibitors. Mercaptoethanol was essential for the recovery of enzyme activity from acrylamide gels in which the and enzymes were electrophoretically homogeneous. The enzyme gave a positive reaction for glycoproteins after purification with a Sepharose-haemoglobin affinity medium. Both enzymes were single units with a molecular weight of 37000–40000 on sodium dodecyl sulphate (SDS)-poryacrylamide gels; gel filtration indicated a molecular weight of 34000–38000. The purified enzyme had no aminopeptidase or carboxypeptidase activity; the products of haemoglobin digestion were peptides with molecular weights on SDS--polyacrylamide gels of 2000-12000. Purified protease did not damage apple fruit tissue or cultured apple cells, lyse plant protoplasts or release proteins from isolated fruit cell walls. The protease had no effect on active fungal or host enzymes that might have a role in host/pathogen interactions, and did not induce phaseollin formation in bean cotyledons. The plant proteins vicilin, legumin and heat-denatured peroxidase were hydrolysed at pH 3·5 and spinach nitrate reductase was inactivated. Nońtural protease inhibitors were detected in healthy or infected apples. We suggest that the protease might be involved in the nutrition of the pathogen.


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