What Is the Functional Role of the Acyltransferase-like Domain in the Svx Peptidase of the Phytopathogenic Bacterium Pectobacterium atrosepticum?

The Svx protein is an established virulence factor in the phytopathogenic pectolytic bacterium Pectobacterium atrosepticum and is secreted into the host plant apoplast. However, its particular role has long remained enigmatic. In our recent studies, we showed that Svx proteins from pectolytic bacteria are metallopeptidases composed of two domains: peptidase and acyltransferase-like domains. Structural organization of the peptidase domain active site led us to hypothesize that its preferred substrates are extensins—hydroxyproline-rich glycoproteins of the plant cell wall. Nevertheless, direct experimental confirmation of extensin hydrolysis by Svx was lacking, and the precise role of the acyltransferase-like domain remained unclear. The present study aimed to address these issues. We showed that Svx indeed cleaves extensins while not degrading some other glycosylated and non-glycosylated proteins. The acyltransferase-like domain was shown to be critical for recognition of arabinan substituents in extensins, thereby providing optimal enzyme–substrate complementarity. Deletion of the acyltransferase-like domain abolished extensin hydrolysis by the truncated variant of Svx. Our study provides the first example of an apoplast-secreted protease from a phytopathogenic bacterium whose specificity toward specific target proteins (extensins) is achieved, at least in part, through structural elements that specifically recognize the distinctive glycosylation pattern of the target proteins.