JBC：发现决定HIV-1两种不同功能的重要保守位点

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病毒基因组在不断地遭受突变，但是有害的突变往往能够被逆转或者因为再次突变而恢复正常。HIV-1 作为逆转录病毒，其不精确性复制往往会造成大量不同的病毒基因序列。到目前为止，对于病毒蛋白比如Tat（一种在病毒基因表达以及复制中起决定作用的蛋白）是如何维持其复杂功能的机制还不清楚。

尽管HIV-1的Tat基因有着相当多的序列多样性，它如何执行复杂功能的机制还没有被完全了解。近日，美国加州大学联合我国厦门大学制药科学学院发现一项重要成果：即Tat中两个特殊位点对调节病毒两种截然不同的基因表达机制具有决定作用，并且，这对维持Tat功能以及影响病毒的潜伏性可能具有重要作用。

Tat蛋白中有几种重要的保守氨基酸位点，这此次实验中，研究人员假设这些位点中含有重要的功能性的残基，并且假设它们不是彼次孤立的保守而是作为关联对出现。为了确定这样的位点，他们使用交互信息分析和生化实验来鉴定这样的协同进化的位点，结果发现，残基35以及39是高度关联的。并且，在大量隔离的缺陷病毒中，往往会出现这对残基的突变。然而，当对异种的Tat序列引入这两种突变时，其基因表达会表现得接近于野生型。此外，对比于大多数因为协同进化而具有相似功能的蛋白残基，结构模型和生化研究表明，这两种残基造成了两种完全不同的基因表达步骤：偶联P-TEFb或者促进RNAPII的C末端结构域P-TEFb的蛋白磷酸化。而且，在模仿B型或者C型的HIV-1的Tat变异体中，其35和39位点都出现了进化优势，即P-TEFb偶联病毒RNAPII的磷酸化，这表明HIV-1的亚型已经进化出可以获得相同基因表达水平的转录策略。

转自http://www.bioon.com/biology/Class18/519515.shtmlMutual Information Analysis Reveals Coevolving Residues in Tat That Compensate for Two Distinct Functions in HIV-1 Gene

Siddharth S. Dey, Yuhua Xue, Marcin P. Joachimiak, Gregory D. Friedland, John C. Burnett, Qiang Zhou, Adam P. Arkin and David V. Schaffer.

Viral genomes are continually subjected to mutations, and functionally deleterious ones can be rescued by reversion or additional mutations that restore fitness. The error prone nature of HIV-1 replication has resulted in highly diverse viral sequences, and it is not clear how viral proteins such as Tat, which plays a critical role in viral gene expression and replication, retain their complex functions. Although several important amino acid positions in Tat are conserved, we hypothesized that it may also harbor functionally important residues that may not be individually conserved yet appear as correlated pairs, whose analysis could yield new mechanistic insights into Tat function and evolution. To identify such sites, we combined mutual information analysis and experimentation to identify coevolving positions and found that residues 35 and 39 are strongly correlated. Mutation of either residue of this pair into amino acids that appear in numerous viral isolates yields a defective virus; however, simultaneous introduction of both mutations into the heterologous Tat sequence restores gene expression close to wild-type Tat. Furthermore, in contrast to most coevolving protein residues that contribute to the same function, structural modeling and biochemical studies showed that these two residues contribute to two mechanistically distinct steps in gene expression: binding P-TEFb and promoting P-TEFb phosphorylation of the C-terminal domain in RNAPII. Moreover, Tat variants that mimic HIV-1 subtypes B or C at sites 35 and 39 have evolved orthogonal strengths of P-TEFb binding versus RNAPII phosphorylation, suggesting that subtypes have evolved alternate transcriptional strategies to achieve similar gene expression levels..

原文链接http://www.jbc.org/content/early/2012/01/17/jbc.M111.302653


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