Phosphorylation by Aurora-B negatively regulates survivin function during mitosis

Survivin operates in a complex with aurora B kinase and is phosphorylated by it on threonine 117 in vitro. Here we ask whether phosphorylation of survivin by aurora B kinase regulates its function during mitosis in vivo. Using a phospho-specific antibody we first establish that survivin is phosphorylated at T117 during mitosis and is present at the midbody during cytokinesis. Next we use two independent RNAi complementation approaches to investigate threonine 117 mutants in survivin depleted cells. Our data suggest that while non-phosphorylatable survivin, survivin_T117A, can substitute for the wild type protein, a phosphomimic, survivin_T117E cannot restore viability, nor can it complement chromosome congression and spindle checkpoint defects that arise due to depletion of endogenous survivin. Fluorescence imaging and fluorescence recovery after photobleaching analysis suggest that the phosphomimic has reduced affinity for centromeres compared with the non-phosphorylatable form. We conclude that survivin is phosphorylated at T117 during mitosis, and once phosphorylated, dephosphorylation is crucial for chromosome congression and progression into anaphase.