Successful and timely repair of DNA DSBs is critically important for the maintenance of genomic stability and cell survival. Defective DSB repair, often as a consequence of mutations in DSB repair genes, is closely associated with genomic and chromosomal abnormalities and is a high risk factor for cancer development[
1‐
3]. The molecular mechanisms that are involved with DSB repair have become increasingly complex – in particular this process can be regulated at various levels by many different protein factors[
4]. The regulation, in most of cases, is achieved through protein interactions, subcellular localizations, and post-translational modifications.
It is evident that mammalian cells could resolve DSBs in a number of ways, in which HR and NHEJ represent the two predominant DSB repair pathways[
1,
2]. Generally, the HR pathway carries out error-free DSB repair in either S or G2 phases of the cell cycle, in which a homologous repair template (
i.
e. donor sequence) on a homologous chromosome or a sister chromatid is utilized to restore the integrity of the broken chromosome[
1]. It is presently conceived that HR-mediated DSB repair can be accomplished by at least two main mechanisms – the classic double-strand break repair (DSBR or double-Holliday junction) and the synthesis-dependent strand annealing (SDSA) pathways. DSBR will give rise to unique sequence configurations known as crossover and non-crossover, whereas SDSA-mediated DSB repair will not be expected to alter HR donor sequences. The HR pathway is purportedly critical for the repair of one-ended DSBs that may arise when replication forks encounter single-strand breaks[
4], in which the error-prone NHEJ is presumably less favorable. In addition, DSB repair mediated by the error-prone NHEJ pathway is frequently associated with deletions at the repair joints due to the processing of DNA ends before rejoining[
2]. It is commonly accepted that NHEJ can occur throughout all cell cycle phases; however, very little is known about how NHEJ or HR is selected at S and G2 phases when both pathways are operational.
Besides functioning in DNA mismatch repair (MMR), members of MMR family also play important roles in modulating DSB repair[
5‐
10]. Evidently, the MutS homologue proteins hMSH2 and hMSH6 (together with hMLH1) coexist with proteins involved in DSB repair in a mega protein complex[
11], and the involvement of these MMR proteins in the process of DSB repair has been suggested by many studies[
5‐
10]. Interestingly, two of the MutS homologue proteins, hMSH4 and hMSH5, do not appear to function in MMR but rather participate in the process of HR[
12‐
16], and a role of hMSH5 in mitochondria DNA repair has been recently suggested[
17]. Although Msh4 and Msh5 null mutations share similar meiotic HR defects in mice, evidence suggests that hMSH4 and hMSH5 can exert distinct functions in mitotic cells[
12]. In particular, hMSH4 interacts with an array of proteins—including hMSH5, VBP1, hMLH1, hMLH3, hRad51, DMC1 and GPS2—known to function in several aspects of the DNA damage response[
13,
15,
18‐
23]. However, the expression levels of hMSH4 vary dramatically in different cell types – with a moderate expression in the testis and relatively low levels in other tissues including ovary, thymus, colon, pancreas, brain, liver, and placenta[
20,
24]. These observations raised the possibility that, by interacting with different binding partners or through varying the levels of protein expression, hMSH4 may exert diverse cellular functions. For example, hMSH5-binding stabilizes hMSH4 in the nucleus, whereas VBP1 competes with hMSH5 to bind hMSH4 and assists in its localization to the cytoplasm[
20,
25]. Here, we demonstrate that hMSH4 interacts with eIF3f – a regulatory subunit of the eIF3 complex that has also been implicated in the regulation of apoptosis and tumorigenesis in humans[
26‐
32]. This interaction stabilizes hMSH4 in both the cytoplasm and the nucleus. Our study indicates that, through interacting with eIF3f, hMSH4 inhibits NHEJ-mediated DSB repair, therefore sensitizing cells to IR-induced DNA damage.