Degradation of tomato pathogenesis-related proteins by an endogenous 37-kDa aspartyl endoproteinase.

As a response to the stress induced by different afflicting agents, tomato plants (Lycopersicon esculentum) produce the so-called 'pathogenesis-related' proteins. Here we report the degradation of some of these proteins by a constitutive endoproteolytic activity that co-distributes with pathogenesis-related proteins in the intercellular spaces of tomato leaves infected with citrus exocortis viroid. This endoproteinase was purified, showing a pH optimum of 2.5-3.5, a Mr of 37,000 and selective inhibition by pepstatin. In crude homogenates, the enzyme does not seem to degrade other cellular proteins. This specificity indicates that the proteinase might be involved in the extracellular degradative pathway of pathogenesis-related proteins and in the regulation of their biological function.