Endogenous hSNM1B/Apollo interacts with TRF2 and stimulates ATM in response to ionizing radiation

Abstract

Human SNM1B/Apollo is involved in the cellular response to DNA-damage, however, its precise role is unknown. Recent reports have implicated hSNM1B in the protection of telomeres. We have found hSNM1B to interact with TRF2, a protein which functions in telomere protection and in an early response to ionizing radiation. Here we show that endogenous hSNM1B forms foci which colocalize at telomeres with TRF1 and TRF2. However, we observed that additional hSNM1B foci could be induced upon exposure to ionizing radiation (IR). In live-cell-imaging experiments, hSNM1B localized to photoinduced double-strand breaks (DSBs) within 10s post-induction. Further supporting a role for hSNM1B in the early stages of the cellular response to DSBs, we observed that autophosphorylation of ATM, as well as the phosphorylation of ATM target proteins in response to IR, was attenuated in cells depleted of hSNM1B. These observations suggest an important role for hSNM1B in the response to IR damage, a role that may be, in part, upstream of the central player in maintenance of genome integrity, ATM.

Evidence for hSNM1B/Apollo functioning in the HSP70 mediated DNA damage response

“…Moreover, depletion of hSNM1B (siRNA) attenuated the autophosphorylation of the ATM-kinase at serine 1981 and the phosphorylation of several ATM target proteins, such as p53 and SMC1, in response to IR. 7 These results substantiated our earlier findings on hSNM1B function in the cellular response to DNA-damage. 6 Interestingly, similar to hSNM1B, TRF2 was also shown to have a function in the early response to DNA-damage: Bradshaw et al found that transiently expressed YFP-TRF2 accumulates at sites of photoinduced DNA damage.…”

“…The C-terminally tagged hSNM1B appeared in several bands, a previously observed characteristic which is most likely due to the use of alternative start codons. 7 Following the TAP procedure, the hSNM1B fusion protein was highly enriched while no signal was detectable from the controls (Fig. 1B).…”

“…6 A series of recent publications has described the telomere component TRF2 as a direct binding partner of hSNM1B. [7][8][9][10] This interaction is illustrated by the high degree of colocalization of nuclear hSNM1B foci with TRF2 and TRF1 at telomeres as detected by indirect immunofluorescence experiments. 7 hSNM1B depletion (shRNA) resulted in the formation of telomere dysfunction-induced foci (TIFs), primarily in S-phase, and telomeric fusions when analysed in cells expressing a dominant negative mutated TRF2.…”

Augmentation of Ras-induced cell transformation : a new role for miR-200a in malignancy.

“…Interestingly, in times of telomere crisis, or during the alternative lengthening of telomeres (ALT) pathway, phosphorylated Trf2 has been shown to localize to the telomere [70]. This discovery is not unexpected because many DNA damage-related proteins and telomeric proteins have been found to function within both contexts [69,71,72]. Apollo is also recruited to DSBs and interacts with …”

“…Trf2 has also been recently implicated by various sources in the recognition and repair of DSBs in response to various forms of damaging radiation [68][69][70]. Trf2 is rapidly phosphorylated in response to DSBs.…”