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. Is involved in competence triggering, via the transduction of signals culminating directly or indirectly in ComD activation. Is important for the resistance of S.pneumoniae to various environmental stress conditions. Appears to be a global regulator that positively controls the transcription of a set of genes encoding functions involved in cell wall metabolism, pyrimidine biosynthesis, DNA repair, iron uptake, and oxidative stress response, and that seems to down-regulate genes employed in competence. Since StkP is unlikely to directly regulate transcription, the input signal must be transmitted through an effector molecule. Identified target substrates that are specifically phosphorylated by StkP in vivo, mainly on threonine residues, are DivIVA, GlmM, PpaC, MapZ, KhpB (also called EloR/Jag) and StkP itself. Autophosphorylated StkP is a substrate for the cotranscribed protein phosphatase PhpP; 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] StkP is activated continuously during growth and its activity is inhibited upon growth arrest. Inhibited by staurosporine, a known protein kinase inhibitor. Active over the wide range of pH from 3 to 9. Homodimer. StkP forms dimers through its transmembrane and extracellular domains. Dimer formation likely promotes autophosphorylation activity and might be necessary for targeting StkP substrate. Interacts with PhpP via its kinase domain. The C-terminal PASTA domain is located in the periplasmic space, beneath the peptidoglycan cell wall, and the kinase domain is located in the cytoplasm. However, another study in the virulent strain TIGR4 (PubMed:20223804) showed that the C-terminal PASTA domain is exposed extracellularly. Localizes to the midcell division sites. The phpP and stkP genes form an operon. 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 C-terminal domain binds to synthetic and native peptidoglycan (PGN) subunits and to beta-lactam antibiotics, which mimic the terminal portions of the PGN stem peptide. Deletion of both the transmembrane domain and the PASTA domain negatively affects the stability of the core kinase domain. In contrast, the membrane anchored kinase domain and the full-length form of StkP are stable and capable of autophosphorylation. However, the membrane anchored kinase domain (i.e. StkP lacking the C-terminal PASTA domain) is unable to phosphorylate its substrates, which means that the PASTA domain is essential for StkP activation and substrate phosphorylation. The PASTA domain is also responsible for cellular targeting to midcell (shown in the virulent strain D39). Both the transmembrane and extracellular domains promote dimerization of StkP. Autophosphorylation occurs predominantly on threonine residue(s) and weakly on serine residue(s). Dephosphorylated by PhpP. Cells lacking this gene have a significantly reduced growth rate in the early exponential phase, exhibit an increased rate of autolysis at the stationary phase of growth and do not achieve the same final optical density as the wild-type strain. Their growth is almost completely impaired at 40 degrees Celsius, in contrast to that of the wild-type strain which is unaffected at this temperature. The mutant strain is also more susceptible to oxidative stress, to osmotic stress, and is less tolerant to acidic pH. Moreover, inactivation of stkP leads to morphological changes: a significant number of cells do not have the typical ovoid shape but appear to be longer with rather oval poles, and they exhibit an apparent cell division defect. Complete loss of Thr-phosphorylation, competence is decreased (PubMed:27776484). Strain Rx1 is unencapsulated. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.