DNA-Dependent Protein Kinase (DNA-PK) phosphorylates several DNA-binding substrates in vitro, including the tumor suppressor protein p53, the SV40 large T antigen and several transcription factors. DNA-PK is thought to play a role in controlling gene regulation and cell growth. DNA-PK is isolated from HeLa nuclear extracts as a complex consisting of a 400kDa catalytic subunit and a 155kDa heterodimeric DNA-binding component named Ku, which itself consists of subunits of approximately 85kDa and 70kDa.|