LAUSR

Dear Editor, ATM (ataxia telangiectasia-mutated) is a master regulator in response to DNA damage and activates downstream pathways involved in cell cycle checkpoints, DNA damage repair, transcription regulation, immune response, central nervous system development and metabolism. Loss of ATM activity in human results in the pleiotropic neurodegeneration disorder ataxia-telangiectasia (A-T) that is characterized by immunodeficiency, cancer predisposition, premature aging and insulin-resistant diabetes. Despite extensive studies over the past two decades, it remains controversial how ATM is activated. Particularly, whether ATM exists in a monomeric form, whether the monomer is more active than dimer, and how dimer-to-monomer transition affects the ATM kinase activity, remain controversial. To prepare ATM proteins for biochemical and structural studies, we overexpressed human ATM in HEK 293 suspension cells. In most cases, the purified ATM existed in a dimeric form. Monomeric ATM could be obtained when the cells were lysed using homogenizer in a buffer lacking DNase I. The monomeric and dimeric ATM peaks were fairly separated using glycerol gradient centrifugation and the peak fractions were used in the following biochemical and structural analyses (Fig. 1a; Supplementary information, Fig. S1). The generation of monomeric ATM is probably induced by double-strand breaks (DSBs) during cell disruption. To compare the kinase activities of dimeric and monomeric ATM, we performed an in vitro kinase assay using two representative substrates, the purified human p53 and Chk2 (kinase dead mutant, referred to as Chk2 below). The monomeric ATM shows kinase activity ~10-fold higher than the dimeric ATM (Fig. 1b; Supplementary information, Fig. S2a). No obvious difference in the level of S1981 autophosphorylation was observed between the two ATM forms (Supplementary information, Fig. S2b), in agreement with previous studies. Therefore, ATM monomer is enzymatically more active than dimer and such difference is independent of ATM autophosphorylation at S1981. We next determined the cryo-EM structures of dimeric and monomeric ATM using single particle reconstructions (Supplementary information, Figs. S3 and S4, Table 1 and movies S1 and S2). The cryo-EM maps of ATM dimer and monomer were refined to 4.3 and 7.8 Å resolution, respectively. The ATM dimer structure reveals a butterfly architecture with two monomers arranged in a two-fold symmetry, indicating a similar fold to that of previously reported ATM dimer in the closed form. Each monomer has an N-terminal superhelical α-solenoid (designated N), followed by a C-terminal compact core containing the FAT, KD, and FATC domains (designated C) (Fig. 1c, d; Supplementary information, Figs. S5 and S6). The FAT domain contains three tetratricopeptide repeat domains (TRD) followed by a short HEAT-repeats domain (HRD) (Fig. 1c, d; Supplementary information, Fig. S6f). The ATM dimerization is mediated by two C (Fig. 1d, f). A four-helix bundle (fα19–fα22) of the TRD3 makes extensive contacts with the C-lobe of ATM′ (Fig. 1f; Supplementary information, Fig. S6b). The two parallel helices kα10/10′ (k represents kinase domain) are sandwiched by helices fα21-22/ fα21′-22′. A loop connecting fα19 and fα20 packs against the FATC′ of the KD′. The helices fα21-fα22 of TRD3 and kα9a′ and kα10′ of the KD′ together buttress the kinase domain. The protruded portion of the two long α helices (fα21–fα22) (designated TRD3 dimeric helices, TRD3-DH) packs against two kinase regulatory elements of the other ATM molecule: the kα9b′-9c′ and the activation loop of the KD’ (Fig. 1f). The PIKK regulatory domain (PRD) consisting kα9b-9c and the following linker is predicted to prohibit substrate entry and inhibit kinase activity in PIKK kinases. In ATM dimer, the PRD′ is well-ordered because of stabilization by the TRD3-DH and helices kα9a’ and kα10′ of the KD′, supporting an inhibitory function. The structure of ATM monomer reveals a similar overall fold to one copy of ATM in the dimeric form (Fig. 1e; Supplementary information, Fig. S6d). Compared with ATM dimer, the monomer reveals a more open catalytic pocket due to lack of stabilization by dimer contacts (Fig. 1g, h; Supplementary information, movie S3). In particular, the height of catalytic pocket, as represented by the distance between LBE (LST8 binding element) and HRD, is 53 Å in dimer and 60 Å in monomer. The width of the catalytic pocket, as represented by the distance between kα1 and helices kα10, is 43 Å in dimer and 47 Å in monomer (Fig. 1h). Thus, the associated PRD tends to be more flexible due to less restraint by kα9a and kα10 and lack of stabilization by otherwise associated TRD3-DH’ in ATM dimer. The EM density indicates that the PRD is highly dynamic and disordered in ATM monomer, indicating a less restrained catalytic pocket that is more favorable for substrate entry. The conformational switch of PRD is probably the key for the activation of ATM kinase (Fig. 1g–i; Supplementary information, movie S4). Thus, ATM adopts an autoinhibitory conformation in dimeric form and monomerization releases such inhibition and enhances the kinase activity. The pattern of ATM dimerization is generally similar to that of the ATR–ATRIP complex. However, ATM dimerization is primarily mediated by the upper interface and has less contacts at the lower dimer interface (Supplementary information, Fig. S7a–e). Moreover, ATM monomer has a more open catalytic pocket than the previously reported ATM open dimer (PDB: 5NP1) (Supplementary information, Fig. S7f–h), suggesting that the open dimer represents a transition state from dimer to monomer. To test the effect of PRD and TRD3-DH on the dimer-tomonomer transition and on the regulation of kinase activity, we made four internal deletions of ATM: ATM (Δ2964–2998), ATM (Δ2981–2998, the invisible linker, S represents short), ATM (Δ2408–2450, TRD3-DH), and ATM (Δ2422–2435, the invisible linker) (Fig. 1i). The four mutants were purified using glycerol gradient centrifugation and the fractions at dimer position were used in the in vitro kinase assay. Interestingly, all the four ATM mutants tend to form monomer in solution compared with the wild-type ATM (Supplementary information, Fig. S8), indicating that PRD and TRD3-DH are involved in ATM dimerization. As predicted, ATM and ATM showed robust activation of kinase activities compared with the wild-type ATM,