DNA-activated protein kinase in Raji Burkitt's lymphoma cells

Autophosphorylation of a DNA-activated protein kinase (DNA-PK) in Raji Burkitt's lymphoma cells generated a band that corresponded to a phosphoprotein of about 300 kDa on SDS/PAGE. This band corresponds to a 300–350-kDa DNA-PK found previously in HeLa cells. In addition to the 300-kDa phosphoprotein, the band of a highly phosphorylated 58-kDa protein was detected by SDS/PAGE of partially purified DNA-PK preparations after the phosphorylation reaction in the presence of double-stranded DNA. this phosphoprotein was specifically immunoprecipitated by mAb against c-Myc. More highly purfied preparations of DNA-PK, containing neither a 58-kDa phosphoprotein nor detectable activities of other kinases, phosphorylated recombinant c-Myc proteins in the presence of DNA. the c-Myc phosphorylation by DNA-PK was markedly stimulated by relxed, double-stranded DNA, but neither by single-stranded DNA nor by RNA. Phosphopeptide mapping and phosphoamino acid analysis indicated that DNA-PK phosphorylates c-Myc in vitro at several serine residues.