Generation of GTP-bound Ran by RCC1 is required for chromatin-induced mitotic spindle formation

Chromosomes are segregated by two antiparallel arrays of microtubules arran ged to form the spindle apparatus. During cell division, the nucleation of cytosolic microtubules is prevented and spindle microtubules nucleate from centrosomes (in mitotic animal cells) or around chromosomes (in plants and some meiotic cells)(1,2). The molecular mechanism by which chromosomes indu ce local microtubule nucleation in the absence of centrosomes is unknown(3- 5), but it can be studied by adding chromatin beads to Xenopus egg extracts (6). The beads nucleate microtubules that eventually reorganize into a bipo lar spindle. RCC1, the guanine-nucleotide-exchange factor for the GTPase pr otein Ran, is a component of chromatin. Using the chromatin bead assay, we show here that the activity of chromosome-associated RCC1 protein is requir ed for spindle formation. Ran itself, when in the GTP-bound state (Ran-GTP) , induces microtubule nucleation and spindle-like structures in M-phase ext ract. We propose that RCC1 generates a high local concentration of Ran-GTP around chromatin which in turn induces the local nucleation of microtubules .