在细胞分裂过程中纺锤丝与着丝粒起初会以随机方式相连接,使得前中期存在许多错误的连接方式。比如一个着丝粒同时受到来自相反方向的纺锤丝牵引,这种现象被称作merotelic连接。如果这些错误的连接不被纠正,将会导致着丝粒间的拉力异常,引起染色体的不同步分离。因此,真核生物采用了一种监控机制来延迟染色体分离,给纠正错误连接方式留有充足时间,该机制被称作纺锤体组装监控。Bub1是一个丝氨酸/苏氨酸蛋白激酶,它是纺锤体组装监控机制中的上游蛋白,可以招集其它监控蛋白定位到着丝粒上。虽然Bub1的序列和功能在不同物种中比较保守,可在植物界中至今仍未有相关报道。
中科院遗传与发育生物学研究所程祝宽课题组利用图位克隆的方法,在水稻中克隆了植物中首个Bub1同源基因BRK1(Bub1-related kinase1)。brk1突变体营养生长正常,但在减数分裂后期I姊妹染色单体提前分离,最终导致完全不育。BRK1具有保守的TPR和激酶结构域,而缺失了GLEBS结构域。BRK1在减数分裂及有丝分裂过程中始终定位在着丝粒蛋白复合体的外层,与酵母及多细胞动物相似,BRK1对于着丝粒区域组蛋白H2A第134位苏氨酸的磷酸化起重要作用,并且调控shugoshin蛋白的着丝粒定位。在brk1突变体减数分裂中期I,错误的merotelic连接方式不能被及时修正,使得该时期同源染色体着丝粒间的拉力降低,并引起纺锤体形态异常,最终导致后期I同源染色体分离的不同步。在BRK1丧失功能的情况下,着丝粒区域组蛋白H3第10位丝氨酸在终变期不能被磷酸化。H3第10位丝氨酸是Aurora B激酶的保守底物,而Aurora B又直接参与微管和着丝粒错误连接的纠正。因此,推测BRK1通过调节中期I早期Aurora激酶的定位从而促进错误连接方式的纠正。
该研究为探索植物纺锤体组装监控蛋白的功能开创了先例,研究结果于12月15日在Plant Cell杂志上在线发表(DOI:10.1105/tpc.112.105874)。相关研究得到科技部和国家自然科学基金委项目的资助。(生物谷Bioon.com)
DOI:10.1105/tpc.112.105874
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BRK1, a Bub1-Related Kinase, Is Essential for Generating Proper Tension between Homologous Kinetochores at Metaphase I of Rice Meiosis
Mo Wanga,1, Ding Tanga,1, Qiong Luob, Yi Jina, Yi Shena, Kejian Wanga and Zhukuan Chenga,2
Bub1 (for budding uninhibited by benzimidazole 1), one of the main spindle checkpoint kinases, acts as a kinetochore scaffold for assembling other checkpoint proteins. Here, we identify a plant Bub1-related kinase 1 (BRK1) in rice (Oryza sativa). The brk1 mutants are sterile due to the precocious separation of sister chromatids at the onset of anaphase I. The centromeric recruitment of SHUGOSHIN1 and phosphorylation of histone H2A at Thr-134 (H2A-pT134) depend on BRK1. Although the homologs can faithfully separate from each other at the end of meiosis I, the uncorrected merotelic attachment of paired sister kinetochores at the early stage of metaphase I in brk1 reduces the tension across homologous kinetochores, causes the metaphase I spindle to be aberrantly shaped, and subsequently affects the synchronicity of homolog separation at the onset of anaphase I. In addition, the phosphorylation of inner centromeric histone H3 at Ser-10 (H3-pS10) during diakinesis depends on BRK1. Therefore, we speculate that BRK1 may be required for normal localization of Aurora kinase before the onset of metaphase I, which is responsible for correcting the merotelic attachment. Glossary SAC spindle assembly checkpoint APC/C anaphase-promoting complex/cyclosome CPC chromosomal passenger complex RACE rapid amplification of cDNA ends RNAi RNA interference TPR tetratrichopeptide repeat FISH fluorescence in situ hybridization DAPI 4′,6-diamidino-2-phenylindole