Function and regulation of the novel tubulin-associated protein RITA in mitosis

RITA (RBP-J interacting and tubulin associated) is a newly identified protein which acts as a negative modulator of the Notch-signaling pathway. RITA possesses a tubulin-interaction domain, a functional nuclear localization signal (NLS) and a nuclear export signal (NES). Interestingly, RITA binds to tubulin in the cytoplasm and it is to be found at the centrosomes and the mitotic spindle during mitosis. We are interested in the role and regulation of RITA in the cell cycle, in particular, in mitosis. We show that RITA is specifically phosphorylated by two important mitotic kinases cyclin-dependent kinase 1 (Cdk1) and Polo-like kinase 1 (Plk1) in vitro . Moreover, RITA directly interacts with Plk1 and cyclin B1, the regulatory subunit of Cdk1, in vivo during mitosis. Indirect immunofluorescence staining reveals that the depletion of RITA causes mitotic defects in chromosome alignment at metaphase and chromosome segregation at anaphase in different types of tumor cell lines. Time lapse imaging shows a prolonged prometaphase and metaphase. Further investigations on the precise function of RITA are ongoing. RITA may contribute to the regulation of microtubules stability.