catalytic activity:ATP + a protein = ADP + a phosphoprotein.,cofactor:Magnesium.,enzyme regulation:Activated by phosphorylation on Ser-51 and Ser-555.,function:Plays a central role in the regulatory network for splicing, controlling the intranuclear distribution of splicing factors in interphase cells and the reorganization of nuclear speckles during mitosis. Hyperphosphorylates RS domain-containing proteins such as SFRS1 and SFRS2 on serine residues during metaphase but at lower levels during interphase. Locks onto SFRS1 to form a stable complex and processively phosphorylates the RS domain. Appears to mediate HBV core protein phosphorylation which is a prerequisite for pregenomic RNA encapsidation into viral capsids.,similarity:Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family.,similarity:Contains 1 protein kinase domain.,subunit:Present in a seven component complex, the toposome, which separates entangled circular chromatin DNA during chromosome segregation. The extended N-terminal domain of isoform 1 binds to the nuclear scaffold-associated protein SAFB suggesting this isoform may phosphorylate splicing factors in close vicinity to the nuclear matrix.,tissue specificity:Isoform 2 is predominantly expressed in the testis but is also present at lower levels in heart, ovary, small intestine, liver, kidney, pancreas and skeletal muscle. Isoform 1 is only seen in the testis, at lower levels than isoform 2.,