Maresca Thomas J, Niederstrasser Hanspeter, Weis Karsten, Heald Rebecca
Department of Molecular and Cell Biology, University of California, Berkeley 94720-3200, USA.
Curr Biol. 2005 Oct 11;15(19):1755-61. doi: 10.1016/j.cub.2005.08.049.
Regulation of microtubule dynamics and organization in mitosis by a number of microtubule-associated proteins (MAPs) is required for proper bipolar spindle assembly, yet the precise mechanisms by which many MAPs function are poorly understood. One interesting class of MAPs is known to localize to the nucleus during interphase yet fulfill important spindle functions during mitosis. We have identified Xenopus nuclear factor 7 (Xnf7), a developmental regulator of dorsal-ventral patterning, as a microtubule-binding protein that also associates with the nuclear import receptor importin alpha/beta. Xnf7 localized to interphase nuclei and metaphase spindles both in Xenopus egg extracts and cultured cells. Xnf7-depleted spindles were hypersensitive to microtubule-depolymerizing agents. Functional characterization of Xnf7 revealed that it binds directly to microtubules, exhibits RING-finger-dependent E3-ubiquitin-ligase activity, and has C-terminal-dependent microtubule-bundling activity. The minimal microtubule-bundling domain of Xnf7 was sufficient to rescue the spindle-hypersensitivity phenotype. Thus, we have identified Xnf7 as a nuclear MAP whose microtubule-bundling activity, but not E3-ligase activity, contributes to microtubule organization and spindle integrity. Characterization of the multiple activities of Xnf7 may have implications for understanding human diseases caused by mutations in related proteins.
许多微管相关蛋白（MAPs）对有丝分裂过程中微管动力学和组织的调节是正确的双极纺锤体组装所必需的，然而许多MAPs发挥作用的精确机制仍知之甚少。已知一类有趣的MAPs在间期定位于细胞核，但在有丝分裂期间发挥重要的纺锤体功能。我们已经鉴定出非洲爪蟾核因子7（Xnf7），一种背腹模式的发育调节因子，作为一种微管结合蛋白，它也与核输入受体输入蛋白α/β相关联。Xnf7在非洲爪蟾卵提取物和培养细胞中均定位于间期细胞核和中期纺锤体。Xnf7缺失的纺锤体对微管解聚剂高度敏感。Xnf7的功能特性表明，它直接与微管结合，表现出依赖于RING指的E3泛素连接酶活性，并具有依赖于C末端的微管成束活性。Xnf7的最小微管成束结构域足以挽救纺锤体超敏表型。因此，我们已经鉴定出Xnf7作为一种核MAP，其微管成束活性而非E3连接酶活性有助于微管组织和纺锤体完整性。对Xnf7多种活性的表征可能对理解由相关蛋白突变引起的人类疾病具有启示意义。
