HURP Is a Ran-Importin β-Regulated Protein that Stabilizes Kinetochore Microtubules in the Vicinity of Chromosomes

Abstract

Our study identifies HURP as a novel component of the Ran-importin beta-regulated spindle assembly pathway, supporting the conclusion that K-fiber formation and stabilization involves both the centrosome-dependent microtubule search and capture mechanism and the RanGTP pathway.

Targeted disruption of Aurora A causes abnormal mitotic spindle assembly, chromosome misalignment and embryonic lethality

“…Additionally, the presence of a relatively elevated number of cells in mitosis in the null embryos indicate that mitosis may be initiated with a delay rather than inhibited in absence of Aurora A as has been reported in case of Xenopus early embryonic cell cycles investigated with egg extracts depleted of endogenous Aurora A (Liu and Ruderman, 2006;Satinover et al, 2006). Aberrant spindle assembly is expected due to impaired formation of k fibers requiring active Aurora A complex with spindle assembly factors, such as TPX2 and HURP, among others (Gruss et al, 2001;Koffa et al, 2006;Sillje et al, 2006;Tulu et al, 2006). It has been hypothesized that microtubules can be organized at multiple, often transient, structures that can catalyze g-tubulin-dependent microtubule nucleation from their minus ends, plus ends or sides (Luders and Stearns, 2007) with the polar spindle microtubules 'collecting' preassembled chromosome-generated minispindles into a single bipolar spindle within the cells (Rieder, 2005).…”

“…Identification of microtubule-associated proteins as substrates of Aurora A (Yu et al, 2005;Koffa et al, 2006) as well as their localization on k fibers in prometaphase human cells (Sillje et al, 2006) indicates that Aurora A forms complex with mitotic spindle assembly factors in the organization of the spindle microtubules. Additionally, while silencing of Aurora A has been reported to cause delay or block in mitotic entry (Marumoto et al, 2002;Hirota et al, 2003;Du and Hannon, 2004;Satinover et al, 2006), accelerated initiation of mitosis in presence of active Aurora A has also been observed (Ma et al, 2003;Liu and Ruderman, 2006).…”

RanBP1 downregulation sensitizes cancer cells to taxol in a caspase-3-dependent manner

“…Roles of RanBP1 in control of MT function also emerge from RNA interference (RNAi) experiments: RanBP1-downregulated cells assemble mitotic spindles that are apparently normal but have altered properties, including resistance to MT depolymerization induced by chemical or physical agents, failure to associate with cyclin B1 and abnormal recruitment of the MT-stabilizing factor HURP (hepatoma up-regulated protein, itself viewed as a potential oncogene). HURP normally localizes to MT plus-ends and contributes to stabilize MT/kinetochore attachments (Koffa et al, 2006;Sillje et al, 2006;Wong and Fang, 2006), but it acquires a widespread distribution along the entire MT length on RanBP1 inactivation (Tedeschi et al, 2007). The resulting hyperstable MTs prolong or arrest prometaphase in video-recording experiments; many RanBP1-interfered cells actually undergo apoptosis after prolonged prometaphase delay, or progress further into mitosis and undergo inaccurate chromosome segregation (Li et al, 2007;Tedeschi et al, 2007).…”

Rings, bracelets, sleeves, and chevrons: new structures of kinetochore proteins

“…Another example of the ability of the MT to template novel structures is the sleeve formed by HURP (hematoma up-regulated protein), a component of the Ran-importin β-regulated spindle assembly pathway (32,33). Cryoelectron micrographs of recombinant HURP bound to MTs show a novel polymer of tubulin forming a sleeve around a normal MT (9) ( Figure 1C).…”

“…Although HURP associates with kinetochore MTs, it does not appear to be part of the kinetochore but instead forms a gradient or comet tail extending out from the kinetochore. Because HURP stabilizes kinetochore MTs (32)(33)(34) it is attractive to hypothesize that the sleeve forms on kinetochore microtubules and impedes depolymerization. HURP may use its MT stabilizing properties to promote congression to the metaphase plate and antagonize the MT destabilizing effect of kinetochore-associated kinesin-13s.…”