The functional and structural characterization of Xanthomonas campestris pv. campestris core effector XopP revealed a new kinase activity

The exocyst complex subunit protein Exo70B1 plays a crucial role in a variety of cell mechanisms including immune responses against pathogens. The calcium dependent kinase 5 (CPK5) of Arapidopsis thaliana, phosphorylates AtExo70B1 upon functional disruption. We previously reported that, the Xanthomonas campestris pv. campestis effector XopP, compromises Exo70B1 and bypasses the hosts hypersensitive response (HR), in a way that is still unclear. Herein we designed an experimental approach based on biophysical, biochemical and molecular assays, based on structural and functional predictions, as well as, utilizing Aplhafold and DALI online servers respectively, in order to characterize the in vivo XccXopP function. The interaction between AtExo70B1 and XccXopP is very stable in high temperatures, while the AtExo70B1 appeared to be phosphorylated at XccXopP expressing transgenic Arabidopsis. XccXopP reveals similarities with known mammalian kinases, and phosphorylates AtExo70B1 at Ser107, Ser111, Ser248, Thr309 and Thr364. Furthermore, XccXopP protects AtExo70B1 from AtCPK5 phosphorylation. Together these findings show that, XccXopP is an effector, which not only functions as a novel serine/threonine kinase upon its hosts protein target AtExo70B1, but also protects the latter from the innate AtCPK5 phosphorylation, to bypass the hosts immune responses.