The mitotic gradient of RanGTP (yellow) in a mitotic spindle (red).
The fluorescence micrograph shows a mitotic spindle assembled in a Xenopus egg extract in the presence of rhodamine-tubulin, labeling the microtubules red. Depicted in yellow is the gradient of nuclear transport cargo molecules released in the vicinity of chromosomes by locally generated RanGTP, visualized using a fluorescence resonance energy transfer (FRET) biosensor probe. The RanGTP and cargo gradients promote spindle assembly. For details see: Visualization of a RanGTP gradient in interphase and mitotic Xenopus egg extracts. [P. Kalab, K. Weis and R. Heald (2002) Science <b>295</b>, 2452-2456]