Kahana JA , Cleveland DW .

Figure 2. Three-step mechanism by which Ran-GTP initiates centrosome-free spindle assembly. (A) The high concentration of Ran-GTP produced by chromatin-bound RCC1 in the vicinity of chromatin stimulates microtubule assembly around chromosomes (see Carazo-Salas et al. 1999). (B) These randomly-oriented microtubules are then captured at their plus ends by chromosome-bound, microtubule-binding proteins (e.g., XKLP1). (C) Finally, the minus ends of microtubules are focused into poles by cytoplasmic dynein and its microtubule-cross-linking cargo NuMA.

Figure 1. Localization of the Ran GAP and GEF leads to compartmentalization of Ran-GTP vs. Ran-GDP. (A) During interphase, nuclear localization of RCC1 (green) and cytoplasmic localization of RanGAP1 and RanBP1 (red) yield a distribution of Ran-GDP (yellow) in the cytoplasm and Ran-GTP (blue) in the nucleus. (B) During mitosis or meiosis, chromatin-associated RCC1 establishes a gradient of Ran-GTP that is concentrated at the surface of the chromosomes.

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