Ran at a glance

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Abstract
Ran, like other GTPases, switches between a GTP-bound and a GDP-bound form. These transitions are regulated by a guanine nucleotide exchange factor (GEF) termed RCC1 (regulator of chromosome condensation 1) and the GTPase-activating protein RanGAP1. The hydrolysis by RanGAP1 requires an additional factor, RanBP1, which possesses a RanGTP-binding domain. In interphase, where Ran's function in nucleo-cytoplasmic transport has been extensively studied, these regulators of Ran are asymmetrically distributed: RCC1 is chromatin-bound and hence nuclear, whereas RanGAP1 and RanBP1 are cytoplasmic. In metazoans, RanGAP1 is covalently modified with a small ubiquitin-like peptide, SUMO1, which targets it to the cytoplasmic face of the nuclear pore complex (NPC) through association with the nuclear pore protein RanBP2 (also called Nup358) (Mahajan et al., 1997). RanBP2 has four RanBP1-like domains and is therefore believed to function similarly to RanBP1, although in vivo evidence of this is lacking.