RACK1 And Ajuba Enhance The Aurora-A Kinase On Cell Mitotic Regulation

Aurora kinases representing a novel family of serine/threonine kinases have been identified as key regulators of the mitotic and meiotic cell division process. The three members of this kinase family, identified so far, referred to as Aurora-A, Aurora-B and Aurora-C which are known to be involved in the regulation of centrosome function, bipolar spindle assembly and chromosome segregation processes. Significant interest in the subject was generated since all three Aurora kinases family members were reported to be overexpressed in many human cancers. Ectopic overexpression of one member of the family, Aurora-A, was shown to induce oncogenic transformation in cells. All three members of the mammalian kinase family have a catalytic domain that is highly conserved and an N-terminal domain of varying sizes.With further study, we found the (1-128aa) can inhibit the autophosphorylation of Thr288 in the active loop. So we suggest the active loop may play an important role in the regulation of Aurora-A. The LIM protein Ajuba is another important activator of Aurora-A which is essential in mitotic commitment. In our previous study, we showed that Ajuba plays an important role in regulation of the kinase activity of Aurora-A. One possibility for the mechanism by which Ajuba mediates the activation of Aurora-A is that the Lim domain of Ajuba can bind the N-terminal of Aurora-A, and prevent this inhibitory interaction with the catalytic domain. The Prelim of Ajuba can be phosphorylated by catalytic domain of Aurora-A and induces an activating conformational change of Aurora-A.A two-hybrid screen identified RACK1, the receptor for activated C-kinasel as an Aurora binding protein. We found that RACK1 can also interact with Ajuba. In vitro and vivo analyses revealed that RACK1, just like ajuba, induces the autophosphorylation and consequent activation of Aurora-A. Depletion of RACK1 can also prevented activation of Aurora-A in late G2 phase and inhibited mitotic entry.With further study, we found that RACK1 may prevented degradation of Aurora-A through deregulating of Cdhl, the activator of anaphase promoting complex/cyclosome(APC/C)-ubiquitin-proteasome pathway.Furthermore, Our findings also found that Aurora-A, Ajuba and RACK1 showed correlation of protein levels in HCC. We indicate a pivotal role for coodination of Aurora-A,Ajuba and RACK1 in tumorigenesis.In conclusion, we first proposed that RACK1 is an essential activator of Aurora-A in mitotic commitment and presume a possible mechanism by which RACK1 activates Aurora-A. Our work shed light on the regulation of this critical mitotic kinase and helps us to well learn about the mechanism of oncogenesis and inhibitory reagent.