MTA1 localizes to the mitotic spindle apparatus and interacts with TPR in spindle assembly checkpoint regulation

We previously identified a cell cycle-dependent periodic subcellular distribution of cancer metastasis-associated antigen 1 (MTA1) and unraveled a novel role of MTA1 in inhibiting spindle damage-induced spindle assembly checkpoint (SAC) activation in cancer cells. However, the more detailed subcellular localization of MTA1 in mitotic cells and its copartner in SAC regulation in cancer cells are still poorly understood. Here, through immunofluorescent colocalization analysis of MTA1 and alpha-tubulin in mitotic cancer cells, we reveal that MTA1 is dynamically localized to the spindle apparatus throughout the entire mitotic process. We also demonstrated a reversible upregulation of MTA1 expression upon spindle damage-induced SAC activation, and time-lapse imaging assays indicated that MTA1 silencing delayed the mitotic metaphase-anaphase transition in cancer cells. Further investigation revealed that MTA1 interacts and colocalizes with Translocated Promoter Region (TPR) on spindle microtubules in mitotic cells, and this interaction is attenuated on SAC activation. TPR is well-implicated in SAC regulation via binding the MAD1-MAD2 complex, however, no interactions between MTA1 and MAD1 or MAD2 were detected in our coimmunoprecipitation (co-IP) assays, suggesting that the MTA1-TPR may represent a distinct SAC-associated complex separate from the previously reported TPR-MAD1/MAD2 complex. Our data provide new insights into the subcellular localization and molecular function of MTA1 in SAC regulation in cancer, and indicate that intervention of the MTA1-TPR interaction may be effective to modulate SAC and hence chromosomal instability (CIN) in tumorigenesis.