THE DNA-DEPENDENT PROTEIN-KINASE IS INACTIVATED BY AUTOPHOSPHORYLATION OF THE CATALYTIC SUBUNIT

The DNA-dependent protein kinase (DNA-PK) requires for activity free e nds or other discontinuities in the structure of double strand DNA In vitro, DNA-PK phosphorylates several transcription factors and other D NA-binding proteins and is thought to function in DNA damage recogniti on or repair and/or transcription. Here we show that in vitro DNA-PK u ndergoes autophosphorylation of all three protein subunits (DNA-PKcs, Ku p70 and Ku p80) and that phosphorylation correlates with inactivati on of the serine/threonine kinase activity of DNA-PK Significantly, ac tivity is restored by the addition of purified native DNA-PKcs but not Ku, suggesting that inactivation is due to autophosphorylation of DNA -PKcs. Our data also suggest that autophosphorylation results in disso ciation of DNA-PKcs from the Ku-DNA complex. We suggest that autophosp horylation is an important mechanism for the regulation of DNA-PK acti vity.