Ran (Ras-related nuclear protein) is a small GTP binding protein belonging to the RAS superfamily and is essential for nuclear transport and microtubule polymerization during mitosis. RCC1 (guanine nucleotide exchange factor for Ran) releases Bub1, BubR1, Mad2 and CENP-E from kinetochores in the presence of excess sperm nuclei and nocodazole in Xenopus.
Here, I found that overexpression of vectors expressing Ran GTP form in HeLa cells induced the premature exit from mitosis even in the presence of nocodazole and accelerates normal mitotic progression. Ran GTP also dissociated the spindle checkpoint proteins including Bub1, BubR1, Mad2 and CENP-E away from kinetochores. Furthermore, both overexpression of Ran’s guanine nucleotide exchange factor (RCC1) and depletion of Ran’s GTPase activating protein (Ran GAP1) by small interfering RNA (siRNA) similarly accelerated normal mitotic progression of the cell cycle. Finally, downregulation of endogenous Ran GTPase by siRNA resulted in the significant delay of the mitotic progression in HeLa cells. Therefore, these results suggest that Ran GTPase regulates the spindle checkpoint and normal mitotic progression in mammalian cells.