Sperm Chromatin Decondensation by Template Activating Factor I through Direct Interaction with Basic Proteins

Template activating factor I (TAF-I) was originally identified as a host factor required for DNA replication and transcription of adenovirus genome complexed with viral basic proteins. Purified TAF-I was shown to bind to core histones and stimulate transcription from nucleosomal templates. Human TAF-I consists of two acidic proteins, TAF-Iα and TAF-Iβ, which differ from each other only in their amino-terminal regions. Here, we report that TAF-I decondenses demembraned Xenopus sperm chromatin. Human TAF-Iβ has a chromatin decondensation activity comparable to that of NAP-I, another histone binding protein, whereas TAF-Iα has only a weak activity. Analysis of molecular mechanisms underlying the chromatin decondensation by TAF-I revealed that TAF-I interacts directly with sperm basic proteins. Deletion of the TAF-I carboxyl-terminal acidic region abolishes the decondensation activity. Interestingly, the acidic region itself is not sufficient for decondensation, since an amino acid substitution mutant in the dimerization domain of TAF-I which has the intact acidic region does not support chromatin decondensation. We detected the β form of TAF-I in Xenopus oocytes and eggs by immunoblotting, and the cloning of its cDNA led us to conclude that Xenopus TAF-Iβ also decondenses sperm chromatin. These results suggest that TAF-I plays a role in remodeling higher-order chromatin structure as well as nucleosomal structure through direct interaction with chromatin basic proteins.

模板激活因子I（Template activating factor I，TAF-I）最初被鉴定为介导与病毒碱性蛋白复合的腺病毒基因组进行DNA复制与转录的宿主因子。纯化后的TAF-I可与核心组蛋白结合，并能刺激核小体模板的转录活性。人类TAF-I由两种酸性蛋白构成，即TAF-Iα与TAF-Iβ，二者仅在氨基末端区域存在差异。本研究发现，TAF-I可使去膜非洲爪蟾精子染色质发生解凝聚。人类TAF-Iβ的染色质解凝聚活性与另一种组蛋白结合蛋白NAP-I相当，而TAF-Iα仅具有微弱的解凝聚活性。对TAF-I介导的染色质解凝聚分子机制的分析显示，TAF-I可直接与精子碱性蛋白发生相互作用。敲除TAF-I的羧基末端酸性区域会使其完全丧失解凝聚活性。值得注意的是，仅酸性区域本身并不足以介导解凝聚过程：即便保留了完整酸性区域的TAF-I二聚化结构域氨基酸替换突变体，也无法支持染色质解凝聚反应。我们通过免疫印迹法在非洲爪蟾卵母细胞与卵细胞中检测到了TAF-Iβ的存在，且通过其互补DNA（complementary DNA，cDNA）的克隆实验证实，非洲爪蟾TAF-Iβ同样可使精子染色质发生解凝聚。上述结果表明，TAF-I可通过直接结合染色质碱性蛋白，参与重塑高级染色质结构与核小体结构。