Mitosis-specific MRN complex promotes a mitotic signaling cascade to regulate spindle dynamics and chromosome segregation

Abstract

The MRE11–RAD50–NBS1 (MRN) complex is well known for participating in DNA damage response pathways in all phases of cell cycle. Here, we show that MRN constitutes a mitosis-specific complex, named mMRN, with a protein, MMAP. MMAP directly interacts with MRE11 and is required for optimal stability of the MRN complex during mitosis. MMAP colocalizes with MRN in mitotic spindles, and MMAP-deficient cells display abnormal spindle dynamics and chromosome segregation similar to MRN-deficient cells. Mechanistically, both MMAP and MRE11 are hyperphosphorylated by the mitotic kinase, PLK1; and the phosphorylation is required for assembly of the mMRN complex. The assembled mMRN complex enables PLK1 to interact with and activate the microtubule depolymerase, KIF2A, leading to spindle turnover and chromosome segregation. Our study identifies a mitosis-specific version of the MRN complex that acts in the PLK1–KIF2A signaling cascade to regulate spindle dynamics and chromosome distribution.

DNA damage response proteins regulating mitotic cell division: double agents preserving genome stability

“…The evolutionary conserved MRN protein complex plays multiple roles in the DNA damage response, such as activation of the central kinase ATM and regulation of double‐strand DNA repair . Recent studies have also implicated MRN in proper spindle formation through at least two separate mechanisms .…”

“…Firstly, MRN forms a mitosis‐specific complex with the long isoform of C2orf44 protein (renamed mitosis‐specific MRN‐associated protein, MMAP) to regulate spindle dynamics . Co‐immunoprecipitation analysis showed that MMAP interacts with the C‐terminal region of MRE11 in mitotic cell extracts; however, CtIP protein that interacts with MRN to initiate DNA resection is not detected in the MMAP‐associated complexes .…”

RAD50 regulates mitotic progression independent of DNA repair functions

“…Cells that cannot satisfy the mitotic spindle assembly checkpoint, may then be stuck in division, a phenotype that has been termed “D‐mitosis” . More recently, Xu et al have described a mitotic MRN complex that does not require CtIP but associates with C2orf44/MMAP to regulate spindle turnover and chromosome segregation via the mitotic kinase PLK1 and the microtubule depolymerase KIF2A, and this function appears to be independent of a mirin‐sensitive MRE11A nuclease activity . It is thus possible that MRN acts at several distinct phases of spindle dynamics, regulating both spindle assembly and spindle turnover, and differential requirements for its nuclease activity may explain why the MRE11A nuclease inhibitors showed a more modest “slow division” phenotype compared with the absence of MRN complex in RAD50‐deficient cells.…”

“…31 More recently, Xu et al have described a mitotic MRN complex that does not require CtIP but associates with C2orf44/ MMAP to regulate spindle turnover and chromosome segregation via the mitotic kinase PLK1 and the microtubule depolymerase KIF2A, and this function appears to be independent of a mirin-sensitive MRE11A nuclease activity. 12 It is thus possible that MRN acts at several distinct phases of spindle dynamics, regulating both spindle assembly and spindle turnover, and differential requirements for its nuclease activity may explain why the MRE11A nuclease inhibitors showed a more modest "slow division" phenotype compared with the absence of MRN complex in RAD50-deficient cells. Distinct functions of MRN in different phases of mitosis may also explain why the "slow division" phenotype in RAD50-deficient cells could not be attributed to a single mitotic sub-phase but accumulated Notes: (A) No significant effect of ATM inhibition in wild-type fibroblasts ADD-T. ATM kinase activity was inhibited in ADD-T fibroblasts with KU-55933 at the indicated doses and time of mitosis was measured using time-lapse microscopy (N = 100 cells).…”

“…In previous work, we and others have reported evidence that the eukaryotic MRE11A-RAD50 complex may regulate mitotic progression. [10][11][12] MRE11A and RAD50 form heterotetramers that, together with NBN in the so-called "MRN complex," trigger the early intracellular response to DNA double-strand breaks (DSBs) and activate the ATM kinase. 13,14 MRE11A and RAD50 also mediate the signaling of ATR after replication fork arrest.…”