Structural dissection of the catalytic domain of the serine threonine kinase StkP of Streptococcus pneumoniae

Serine/threonine kinases of the Hanks family are key regulators of bacterial physiology. Among them, membrane-associated PASTA-Hanks kinases govern bacterial cytokinesis and morphogenesis, yet their activation mechanism remains unclear. Here, we report crystal structures of the catalytic domain of the PASTA-Hanks kinase StkP of the human pathogen Streptococcus pneumoniae, carrying phosphoablative or phosphomimetic mutations in its activation loop. These structures demonstrate that phosphorylation of two threonine residues modulates the activation loop's organization and dynamics and reveal an alternative mode of dimerization of the catalytic domain. Analytical ultracentrifugation, SAXS and cell imaging allow to propose a model postulating that the local concentration of StkP at the division septum promotes an inactive dimeric state in which the activation loop hampers substrate binding. The reorganization into active dimers would activate StkP and allow endogenous substrate phosphorylation. This work thus provides a mechanistic framework of the regulation of PASTA Hanks kinase for the regulation of bacterial cell division.