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Curr Biol
2004 Oct 26;1420:1801-11. doi: 10.1016/j.cub.2004.10.002.
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XRHAMM functions in ran-dependent microtubule nucleation and pole formation during anastral spindle assembly.
Groen AC
,
Cameron LA
,
Coughlin M
,
Miyamoto DT
,
Mitchison TJ
,
Ohi R
.
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BACKGROUND: The regulated assembly of microtubules is essential for bipolar spindle formation. Depending on cell type, microtubules nucleate through two different pathways: centrosome-driven or chromatin-driven. The chromatin-driven pathway dominates in cells lacking centrosomes.
RESULTS: Human RHAMM (receptor for hyaluronic-acid-mediated motility) was originally implicated in hyaluronic-acid-induced motility but has since been shown to associate with centrosomes and play a role in astral spindle pole integrity in mitotic systems. We have identified the Xenopus ortholog of human RHAMM as a microtubule-associated protein that plays a role in focusing spindle poles and is essential for efficient microtubule nucleation during spindle assembly without centrosomes. XRHAMM associates both with gamma-TuRC, a complex required for microtubule nucleation and with TPX2, a protein required for microtubule nucleation and spindle pole organization.
CONCLUSIONS: XRHAMM facilitates Ran-dependent, chromatin-driven nucleation in a process that may require coordinate activation of TPX2 and gamma-TuRC.
Figure 2.
XRHAMM Concentrates at Spindle Poles
(A) The localizations of XRHAMM (green), tubulin (red), and DNA (blue) throughout the cell cycle in Xenopus XTC tissue cells. XRHAMM associates with microtubules during prophase and metaphase, with higher concentrations near the spindle pole. During anaphase, XRHAMM is undetectable in the spindle midzone, but during telophase, it is not on the mitotic spindle. During interphase, XRHAMM concentrates as puncta that colocalize with the centriole marker centrin, indicating that it localizes to the centrosome.
(B) The localization of XRHAMM in Xenopus egg extract spindles. XRHAMM localizes to the microtubules of the spindle and focuses at the spindle pole. The scale bar represents 3 μm.
Figure 3.
XRHAMM Depends on Dynein/Dynactin Complex to Concentrate on Spindle Poles
(A) Unlike in control spindle reactions, XRHAMM is not concentrated on the unfocused spindle pole when dynein/dynactin function has been perturbed with 1 mg/ml of p50.
(B) Linescans of p50 treated (left) and control spindles (right) stained for XRHAMM (green) and tubulin (red). Notice that the fluorescence intensity of XRHAMM at the pole is higher relative to the midzone only when the dynein/dynactin complex is active. The scale bar represents 2 μm.
Figure 4.
XRHAMM Is Essential for Efficient Anastral Spindle Assembly
(A) Anastral DNA bead spindles formed in extracts mock depleted, XRHAMM depleted, and XRHAMM depleted with recombinant XRHAMM added back to endogenous levels (70 nM). Note that without XRHAMM, no nucleation occurs around the DNA beads. The scale bar represents 3 μm.
(B) Quantification of phenotypes associated with XRHAMM depletions (number of experiments: 3; number of counted structures: 900; error bars = standard error of the mean). DNA bead spindles efficiently assembled around groups of approximately 15â40 beads, and thus any bead aggregates of this size that did not assemble spindles or microtubules were counted as no nucleation. Note that the dominant phenotype of XRHAMM-depleted extracts is DNA bead aggregates that do not nucleate microtubules, whereas spindles are the dominant phenotypes in mock depletions or depleted extracts rescued with purified recombinant XRHAMM.
(C) (Left) XRHAMM immunoblot of extracts mock depleted, depleted with XRHAMM addback to endogenous levels (70 nM), and depleted for XRHAMM. (Right) Coomassie-stained SDS-PAGE gel of purified recombinant XRHAMM.
(D) Quantification of the number of Ran(Q69L)GTP-anastral poles (added at 0.3 mg/ml) formed in mock-depleted extracts, XRHAMM-depleted extracts, and XRHAMM-added-back-to-XRHAMM-depleted extracts. Note that without XRHAMM, fewer Ran dependent spindle poles assemble (number of experiments = 3; error bars = standard error of the mean).
Figure 5.
XRHAMM Interacts with TPX2 and γ-tubulin
(A) Coomassie-stained SDS-PAGE of lysates of XRHAMM and IgG immunoprecipitations. Note the lack of specific stoichiometric XRHAMM-interacting proteins.
(B) Immunoblots showing XRHAMM immunoprecipitates with TPX2 and γ-tubulin from meiotic Xenopus extracts.
(C) Immunoblots showing that GST-TPX2 specifically interacts with XRHAMM.
(D) The localizations of XRHAMM and TPX2 on unfixed Xenopus egg extract spindles. Note that XRHAMM and TPX2 colocalize predominantly at the spindle pole.
(E) The localizations of XRHAMM and γ-tubulin on unfixed Xenopus egg extract spindles. Note that XRHAMM and γ-tubulin only colocalize at the spindle pole. The scale bar represents 2 μm.
Figure 6.
XRHAMM Is Essential for Concentrating TPX2 on Spindle Poles
(A) DMSO-induced microtubule spindle poles concentrate XRHAMM at the pole. The scale bar represents 3 μm.
(B and C) DMSO-induced microtubule poles assembled in XRHAMM-depleted extracts are not focused and do not concentrate TPX2 at the pole, whereas NuMA and γ-tubulin remain faintly localized to the unfocused center of the pole. NuMA and γ-tubulin were sometimes difficult to image on the unfocused spindle poles because the distribution was dispersed and faint. The images shown in this figure are representative for the bright distributions that were observed in many of the poles examined (>60%). Addition of purified recombinant XRHAMM rescues the unfocused pole defect and the mislocalization of TPX2. The scale bar represents 3 μm.
Figure 7.
XRHAMM-Associated Proteins Nucleate Microtubules in Pure Tubulin Solutions
(A) Protein A beads bound to XRHAMM antibodies and treated with Ran(Q69L)GTP-activated extracts can assemble microtubules (iii). Beads retrieved from Ran(Q69L)GTP-treated extract in the presence of nocodazole (noc; iv), washed on ice with CSF-XB, and challenged with pure tubulin at 1 mg/ml at 37°C can nucleate microtubules (v). Ran(Q69L)GTP is required to induce microtubule nucleation in vitro (ii), showing that XRHAMM-associated proteins do not merely stabilize microtubules.
(B) Depletion of either TPX2 (ii) or γ-tubulin (iii) inhibits nucleation from XRHAMM beads in pure tubulin solutions (1 mg/ml).
(C) Immunoblots showing depletions of TPX2 and γ-tubulin in extracts used in (B). Notice that XRHAMM does not significantly codeplete with TPX2 or γ-tubulin.
