Activated PKR associates with ATM and inhibits ATM activation. (A) Western blotting after reciprocal co-IP of ATM and PKR from lysates of REH cells treated with either 0.5 μM PKRI for 8 hours or 5 μg/mL poly (I:C) for 6 hours demonstrates that PKR activity enhances PKR-ATM association. Input is 10% of total lysate used in the co-IP. Nonspecific IgG antibody was used as a negative co-IP control. (B) Western blotting indicates that co-IP of ATM and PKR is decreased at 30 minutes after 5 Gy IR. (C) co-IP of ATM with NBS1 or γ-H2AX is increased in REH cells by inhibition of PKR expression or activity, whereas cells treated with 10 ng/mL IFN-γ or 5 μg/mL poly(I:C) to increase PKR expression/activity have a decreased co-IP of ATM with either NBS1 or γ-H2AX.