Protein kinase involved in signal transduction pathways that regulate various cellular processes. Likely senses intracellular peptidoglycan subunits present in the cell division septa of actively growing cells; thus, intracellular unlinked peptidoglycan may serve as the signal molecules that trigger StkP phosphorylation activity on a set of substrates. Plays a crucial role in the regulation of cell shape and cell division of S.pneumoniae through control of at least DivIVA activity. StkP also plays an important role for bacterial survival in vivo. Identified target substrates that are specifically phosphorylated by StkP in vivo, mainly on threonine residues, are DivIVA, GlmM, PpaC, MapZ (shown in the avirulent strain Rx / Cp1015), KhpB (also called EloR/Jag, shown in strains R6 and Rx1) and StkP itself. Also able to phosphorylate FtsZ in vitro, however FtsZ may not be a target of StkP in vivo. Autophosphorylated StkP is a substrate for the cotranscribed protein phosphatase PhpP (shown in the avirulent strain Rx / Cp1015); PhpP and StkP appear to constitute a functional signaling couple in vivo. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Homodimer. StkP forms dimers through its transmembrane and extracellular domains. Dimer formation likely promotes autophosphorylation activity and might be necessary for targeting StkP substrate (By similarity). Interacts with FtsZ in vitro via its kinase domain. The kinase domain is located intracellularly, while the C-terminal PASTA domain is exposed extracellularly, being surface accessible in vivo and recognized by the immune system (PubMed:20223804). However, another study in an avirulent strain (PubMed:19502404) showed that the C-terminal PASTA domain is located in the periplasmic space, beneath the peptidoglycan cell wall. Localizes to the midcell division sites in dividing cells of growing cultures but in the stationary phase, the characteristic midcell localization of StkP disappears and StkP shows a diffuse membrane localization. Displays a temporal colocalization with FtsZ. Consists of an N-terminal kinase domain, a transmembrane domain, and a C-terminal domain containing four repeats of the PASTA signature sequence (Penicillin-binding protein and Ser/Thr protein kinase associated domain). The PASTA domain binds to peptidoglycan (PGN) subunits, is essential for StkP activation and substrate phosphorylation, and is responsible for cellular targeting to midcell (By similarity). Autophosphorylation occurs predominantly at the threonine residue and weakly at the serine residue. Dephosphorylated by PhpP (By similarity). Disruption of this gene results in a strong reduction of bacterial growth, and increased antibiotic sensitivity to penicillin, cephalosporins (e.g. ceftazidime), and vancomycin in vitro. Cells lacking this gene are highly attenuated in virulence, and demonstrate an elongated shape and very few division septa separating the daughter cells. The lack of StkP does not disturb FtsZ ring formation and does not affect the expression level and the phosphorylation status of FtsZ. Was identified as an important pneumococcal antigen that provides significant cross-protection in models of pneumococcal sepsis and pneumonia. Was selected as one of the lead candidates for development of a new pneumococcus protein-based vaccine. Is a component of a trivalent recombinant protein-based vaccine candidate for protection against S.pneumoniae, whose initial formulation development evaluations are described in PubMed:22538529. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.