Ran Ser135P shares common subcellular localization with active PAK4. Terry et al., 2007). Ran activity depends on its GTP/GDP cycle, and the subcellular localization of its regulatory Santacruzamate A enzymes. Indeed, the Ran exchange factor RCC1 is chromatin bound, whereas RanGAP1 and its accessory proteins RanBP1 and RanBP2 are essentially cytoplasmic during interphase. This partitioning restricts Ran-GTP to the nucleus and Ran-GDP to the cytoplasm (Clarke and Zhang, 2008). Nucleocytoplasmic shuttling is regulated by Ran-GTP binding to its effectors, which belong to the importin and exportin (CRM1) family. Nuclear localization sequence (NLS)Cbearing proteins bind the importins in the cytoplasm and are transported into the nucleus where the interaction of Ran-GTP with importin- releases and activates the NLS cargoes. Importin cargoes include most nuclear proteins, of which some contribute to spindle formation during mitosis (Terry et al., 2007; Clarke and Zhang, 2008). In the nucleus, Ran-GTP also promotes the CRM1 loading of nuclear export sequence (NES)Cbearing proteins and their Santacruzamate A subsequent export to the cytoplasm. When the nuclear envelope breaks down at mitosis, the Ran-GTP/GDP physical compartimentalization is abolished. At this point, Ran activity and function appears to rely on essentially two mechanisms. The first mechanism is the spatially controlled assembly of protein complexes at specific subcellular localizations. For instance, at the kinetochore region Ran-GTP/CRM1Cdependent recruitment of RanGAP1 and RanBP2 is essential for kinetochoreCmicrotubule interactions (Joseph et al., 2004; Arnaoutov et al., 2005), whereas at the centrosome the Ran-GTP/CRM1Cdependent recruitment of nucleophosmin regulates unscheduled centrosome duplication (Budhu and Wang, 2005; Wang et al., 2005). Among others, importin-, which is transported along microtubules (MTs) by dynein (Ciciarello et al., 2004), RanBP1, and centrosomal matrix A-kinase anchoring protein (AKAP450; Keryer et al., 2003) also colocalize and/or are complexed with Ran at the centrosomes. Second, a Ran-GTP diffusible gradient is established, during mitotic spindle assembly, by chromatin-bound RCC1. This gradient, first visualized by Forster resonance energy transfer (FRET) in egg extracts (Kalab et al., 2002; Caudron et al., 2005; Kalb et al., 2006), induces a spatially controlled release of spindle assembly factors (SAFs) such as TPX2, from the inhibitory NOP27 importins (Caudron et al., 2005; Bastiaens et al., 2006). In somatic cells, although the Ran-GTP gradient contributes to spindle establishment during early mitosis, it clearly becomes dispensable at Santacruzamate A metaphase (Kalb et al., 2006; Kalab and Heald, 2008). During mitosis Ran must be differentially regulated in the different complexes present at the same subcellular location. However, neither the localization nor the gradient mechanism fully explains the control of Ran activity, which argues for another level of modulation of the activity of the GTPase. We hypothesized that phosphorylation, one of the chief mechanisms regulating mitotic progression, might control Ran function, as many kinases localize to the centrosome and kinetochore regions during spindle assembly. The p21-activated kinase (PAK) family is central to many signaling pathways (Arias-Romero and Chernoff, 2008; Molli et al., 2009). This family is commonly divided into subgroups I (PAK1C3) and II (PAK 4C6). PAK4C6 are involved in controlling cross talk and reorganization of the actin and MT cytoskeletons (Cau et al., 2001; Callow et al., 2002). We previously reported that X-PAK4 (although previously called X-PAK5, it is the orthologue of hPAK4, we therefore propose to change its name to X-PAK4).
Ran Ser135P shares common subcellular localization with active PAK4
