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

Exo70B1 is a protein subunit of the exocyst complex with a crucial role in a variety of cell mechanisms, including immune responses against pathogens. The calcium dependent kinase 5 (CPK5) of Arabidopsis thaliana (hereafter Arabidopsis), phosphorylates AtExo70B1 upon functional disruption. We previously reported that, the Xanthomonas campestris pv. campestris effector XopP compromises AtExo70B1, while bypassing the host's hypersensitive response (HR), in a way that is still unclear. Herein we designed an experimental approach, which includes biophysical, biochemical and molecular assays and is based on structural and functional predictions, utilizing AplhaFold and DALI online servers respectively, in order to characterize the in-vivo XccXopP function. The interaction between AtExo70B1 and XccXopP was found very stable in high temperatures, while AtExo70B1 appeared to be phosphorylated at XccXopP-expressing transgenic Arabidopsis. XccXopP revealed similarities with known mammalian kinases, and phosphorylated AtExo70B1 at Ser107, Ser111, Ser248, Thr309 and Thr364. Moreover, XccXopP protected 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 host target AtExo70B1, but also protects the latter from the innate AtCPK5 phosphorylation, in order to bypass the host's immune responses. Data are available via ProteomeXchange with identifier PXD041405.