Abstract
Energy- and nutrient-sensing proteins such as AMPK, mTOR and S6K1 are now recognized as novel regulators of mitotic completion in proliferating cells. We investigated the cellular distribution of the Ser2481 autophosphorylation of mTOR, which directly monitors mTORC-specific catalytic activity, during mammalian cell mitosis and cytokinesis. Automated immunofluorescence experiments in human carcinoma cell lines revealed that phospho-mTOR (Ser2481) exhibited profound spatial and temporal dynamics during cell division. Phospho-mTOR (Ser2481) was strikingly enriched in mitotic cells, and in prophase, bright phospho-mTOR (Ser2481) staining could be clearly observed among condensed chromosomes. Phospho-mTOR (Ser2481) then redistributes from diffuse cytosolic staining that partially colocalizes with the mitotic spindle during the early phases of mitosis to the furrow at the onset of cytokinesis. Like the bona fide chromosomal passenger proteins (CPPs) INCENP and Aurora B, phospho-mTOR (Ser2481) displayed noteworthy accumulation in the central spindle midzone and the midbody regions, which persisted during the furrowing process. Accordingly, double-staining experiments confirmed that phospho-mTOR (Ser2481) largely colocalized with CCPs in the midbodies. The CPP-like mitotic localization of phospho-mTOR (Ser2481) was fully prevented by the microtubule-depolymerizing drug nocodazole; mitotic traveling of phospho-mTOR (Ser2481) to the midbody during telophase and cytokinesis, where it appears to be integrated into the CPP-driven cytokinetic machinery, may therefore require dynamic microtubules. Although the Ser2448-phosphorylated form of mTOR was also found at high levels during M-phase in human cancer cells, we failed to observe a significant association of phospho-mTOR (Ser2448) with CCP-positive mitotic and cytokinetic structures. Our findings add phospho-mTOR (Ser2481) to the growing list of phospho-active forms of proteins belonging to the AMPK/mTOR/S6K1 signaling axis that reside at the mitotic and cytokinetic apparatus. Future studies should elucidate how the specific ability of phospho-mTOR (Ser2481) to spatially and temporally couple to the cleavage furrow and midbody region as a CPP-like protein can signal to or from adjacent signaling complexes and/or with the basic machinery of cell abscission.