Serine/threonine protein kinase that plays important roles in several processes including nuclear viral egress, viral replication or regulation of host cell cycle progression (PubMed:25339763, PubMed:31548682, PubMed:31291580). Participates in the acquisition of tegument during virion morphogenesis in the nucleus. Phosphorylates the viral nuclear egress complex (NEC) subunits UL50 and UL53 (PubMed:25339763). Redistributes the host nuclear lamina by phosphorylating cellular Lamins-A/C. Plays a role in viral DNA synthesis by phosphorylating the DNA polymerase processivity factor UL44. Stimulates host cell cycle to support viral DNA synthesis by phosphorylating host retinoblastoma/RB1 protein. Additional substrates have been identified including host EF1D or H2B. Phosphorylates also host SAMHD1 and thereby counteracts its antiviral effect by reducing its dNTP hydrolase activity (PubMed:31548682, PubMed:31291580). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with UL83. Autophosphorylates on serine and threonine residues. Monophosphorylates and thereby activates the antiviral nucleoside analog ganciclovir (GCV) used to treat CMV infections. Subsequent di- and triphosphorylation are carried out by cellular enzymes. The viral DNA polymerase incorporates GCV triphosphate into the viral genome, leading to marked attenuation of viral DNA replication and successful suppression of the infection, while the uninfected cell does not have this ability because it lacks the viral kinase. Mutations in phosphotransferase may induce CMV resistance to antiviral therapies in immunocompromised patients. Belongs to the protein kinase superfamily. Tyr protein kinase family. HCMV ganciclovir subfamily.