New insight into the biochemical features of AtPME17, a functional Arabidopsis PME affecting plant resistance to pathogens, regulated by its pro-region

PMEs are widespread in plants and belong to a large multigene family whose members display different expression profiles, mediate different physiological responses and play critical role in the outcome of plant-pathogen interaction. Differently from fungi and bacteria, higher plants PMEs are classified in two groups on the basis of their structures: group1 contains only the catalytic PME domain; group 2 defined as ProPME, possesses, in addition to the catalytic domain, a N-terminal Pro-region. The Pro-region shows sequence similarities with characterized PME inhibitors proteins. Between Pro-region and PME domain there is a conserved Serine-like proteases (SBTs) processing site cleaved for the secretion and activation of the PME domain in the apoplast. Much remains to be discovered about the peculiar mechanism of intramolecular regulation of group 2 PMEs. The heterologous overexpression of functional ProPMEs is a major bottleneck of structural biology studies for understanding their SBT-mediated post transcriptional regulation. We have previously demonstrated that AtPME17, a ProPME isoform, from Arabidopsis highly induced in response to several pathogens strongly contributes to resistance against B. cinerea. We have expressed and purified the AtPME17 catalytic region using P. pastoris and the characterization of the biochemical features of the enzyme are in progress. In E. coli, we demonstrated that the PRO region acts as an intramolecular inhibitor of AtPME17 activity. The optimization of the bacterial expression system is in progress to obtain Pro-AtPME17 amounts useful to solve the 3D structure. These studies will give more insight into the peculiar on-off mechanism of post transcriptional regulation of this class of enzymes.