Plays a key role in the repair of DNA damage, functioning as part of both the non-homologous end-joining (NHEJ) and base excision repair (BER) pathways. Through its two catalytic activities, PNK ensures that DNA termini are compatible with extension and ligation by either removing 3'-phosphates from, or by phosphorylating 5'-hydroxyl groups on, the ribose sugar of the DNA backbone. a 3'end (2'-deoxyribonucleotide 3'-phosphate)-DNA + H2O = a 3'-end 2'-deoxyribonucleotide-DNA + phosphate a 5'-end dephospho-2'-deoxyribonucleoside-DNA + ATP = a 5'-end 5'-monophospho-2'-deoxyribonucleoside-DNA + ADP + H(+) Monomer (PubMed:15749016). Interacts (via FHA domain) with XRCC4; mainly interacts with XRCC4 phosphorylated by CK2 but is also able to interact at much lower level with unphosphorylated PNKP (By similarity). The FHA domain binds threonine-phosphorylated peptides from XRCC1/4, and is responsible for the recruitment of PNKP to the sites of DNA repair. The affinity is ten times greater if peptides are also phosphorylated on the serine preceeding the phosphothreonine. In the N-terminal section; belongs to the DNA 3' phosphatase family.