Serine/threonine-protein kinase involved in somatic mitosis and female meiosis (PubMed:16230526). Required for spindle organization in mitosis, and for the establishment or maintenance of meiosis-specific chromosomal configurations, including the prophase I karyosome and the metaphase I spindle (PubMed:15078818, PubMed:16301329). Specifically phosphorylates nucleosomal H2A on 'Thr-119' (PubMed:15078818). Required for the development and organization of indirect flight muscle sarcomeres by regulating the formation of M line and H zone and the correct assembly of thick and thin filaments in the sarcomere (PubMed:26251439). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] May interact with Unc-89 (via protein kinase domain 1) (PubMed:26251439). Interacts with L(2)gl (PubMed:31735666). Chromatin-associated during mitosis (PubMed:15078818). In indirect flight muscle, colocalizes with sls isoform A (kettin) to the sarcomere Z lines and appears to be weakly distributed across the sarcomere (PubMed:26251439). More rarely, localizes to M lines (PubMed:26251439). Expressed in ovaries (at protein level) (PubMed:16230526, PubMed:16301329). Expressed in indirect flight muscle (IFM) (at protein level) (PubMed:26251439). Present throughout development, with highest level in early embryo (at protein level). Phosphorylated during mitosis and female meiosis. RNAi-mediated knockdown in muscles causes 91 percent lethality at the early pupal stage and the few surviving animals cannot fly. Severe defects in the indirect flight muscle structure characterized by narrower and wavy myofibrils and abnormal positioning of sarcomere Z line, M line and H line. Although the spacing between Z line and M lines stays regular, in less affected myobrils Z lines and M lines appear fragmented, Z lines are diffused at the edges and sarcomere length is shorter. In the more affected myofibrils, rudimentary Z lines are shifted into the region of thick and thin filament overlap and are by-passed by abnormally long filaments and H zones fail to localize to the middle of the sarcomere. Localization of unc-89/obscurin to M lines and localization of kettin (sls isoform A) to Z line is not affected. Belongs to the protein kinase superfamily. CK1 Ser/Thr protein kinase family. VRK subfamily.