Cytocentrin is a Ral-binding protein involved in the assembly and functionof the mitotic apparatus

Cytocentrin is a cytosolic protein that transiently associates with the mit otic spindle poles in early prophase, and dissociates from them after compl etion of mitosis. Cloning of its cDNA demonstrated a high degree of homolog y with three proteins known to specifically interact with an activated form of Ral. Herein we demonstrate that overexpression of cytocentrin inhibits assembly of the mitotic spindle without affecting polymerization or distrib ution of interphase microtubules. Conversely, loss of cytocentrin expressio n leads to formation of monopolar spindles. These results indicate that ass ociation of cytocentrin with the centrosome may be essential for a timely s eparation of the diplosomes. They also implicate Ral GTPases and their rela ted pathways in the assembly and function of the mitotic apparatus.