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. Identified target substrates that are specifically phosphorylated by StkP in vivo, mainly on threonine residues, are DivIVA, KhpB (also called EloR/Jag) and StkP itself. 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 MreC in the elongasome (PubMed:28710862). The kinase domain is located intracellularly, while the C-terminal PASTA domain is exposed extracellularly. Localizes to the midcell division sites. Time-lapse microscopy shows that StkP displays an intermediate timing of recruitment to midcell: StkP arrives shortly after FtsA but before DivIVA. Furthermore, StkP remains at midcell longer than FtsA, until division is complete. Delocalizes from the septum in the presence of antibiotics that target the latest stages of cell-wall biosynthesis and in cells that have stopped dividing. 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 (shown in strain Rx / Cp1015), is essential for StkP activation and substrate phosphorylation, and is required for cellular targeting to midcell. Autophosphorylation occurs predominantly at the threonine residue and weakly at the serine residue. Dephosphorylated by PhpP (By similarity). According to PubMed:22211696, cells lacking this gene show round shape (the cell shape of wild-type strain is ovoid) and form long chains with unsplit cross-wall joining cells. Their septa either are not positioned at the equator, or display an aberrant ultrastructure (curly aspects). They display delocalized sites of peptidoglycan synthesis. According to PubMed:22431591, cells in which StkP is depleted clearly show an elongated phenotype, with cell-wall synthesis occurring along the peripheral side between the septal zones. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.