Regulation of mitotic spindles by CDK5RAP2 and Hsc70

During development, cell division rate and cell growth rate coordinate each other to determine cell numbers and the size of multicellular organisms. The mitotic spindle is one of the key factors for cell division. It ensures the separation of the two daughter genomes and determines the position of cytokinesis furrow. Correct organization of mitotic spindles needs both proper assembly and orientation of mitotic spindle. The orientation and position of the spindles determine the plane of cell divisions and finally determines whether cell division is symmetric or asymmetric, which is essential for tissue morphogenesis and homeostasis. Many factors are involved in the assembly and orientation of mitotic spindle. Although the regulation of mitotic spindle has been studied extensively, many questions remained unsolved. In our study, we have found that loss of CDK5RAP2 function causes mitotic spindle misorientation and assembly defects. In addition, the binding of CDK5RAP2 with EB1 is essential for this regulation process. We have also identified for the first time that the heat shock protein 70 (Hsc70) localizes to mitotic spindles. Knockdown of Hsc70 causes mitotic spindle defects like decrease of microtubule intensity of mitotic spindles which suggests that Hsc70 has the ability to stabilize mitotic spindle. Suppression of Hsc70 expression causes delocalization of clathrin and TACC3 from the mitotic spindles. The TACC3/ch-TOG/clathrin complex has been proved to be important to stabilize kinetochore fibers. These indicate that perhaps Hsc70 stabilizes mitotic spindles via the TACC3/ch-TOG/clathrin complex. Our results suggested that CDK5RAP2 and Hsc70 are essential for normal formation of mitotic spindles. Identification of these two proteins can facilitate understanding the mechanisms of regulating mitotic spindle organization and further study of disorders related to mitotic spindle defects