FIGURE 5.

ATM Ser²⁹⁹⁶ autophosphorylation is dependent on ATM protein kinase. A, the ATM-specific inhibitor KU55933 prevents ATM autophosphorylation on Ser²⁹⁹⁶ and Ser¹⁹⁸¹ sites after exposure to 10 Gy. B, ATM autophosphorylation on Ser²⁹⁹⁶⁷ is not detected in AT1ABR cells, expressing a mutant, less stable, and kinase-dead form of ATM. C, C3ABR is a control cell line. ATM was immunoprecipitated from extracts prepared 1 h after exposure of cells to 6 Gy, and immunoblotting was carried out as described in Fig. 2. IR, irradiation; IP, immunoprecipitation.