Instituto Leloir and Instituto de Investigaciones Bioquímicas-Conicet, Patricias Argentinas 435, Buenos Aires, Argentina.
Biochemistry. 2011 Mar 1;50(8):1376-83. doi: 10.1021/bi101941c. Epub 2011 Feb 3.
High-risk human papillomavirus E6 participates in tumorigenic progression, mainly by its ability to promote p53 degradation. HPV transcripts show a complex splicing pattern, where E6* is the most abundant transcript in high-risk HPV types, comprising the first 50 amino acids of E6. No structural or biochemical information of this polypeptide, which contains half of the first zinc binding motif of E6, is available, due to the difficulty to acquire a compact monomeric fold in such a small polypeptide. We show that HPV16-E6* can fold into either α-helix or β-sheet large oligomers at pH 7.5 and 5.0, respectively, in the absence of zinc. The β-sheet oligomers are highly stable and unaffected by the presence of zinc, while the α-helix oligomers tend to rapidly form aggregates, prevented by the presence of the metal. Two E6* molecules bind per atom of zinc, suggesting a tetrahedral, high-affinity arrangement (K(D) < 10(-12) M), which results in a zinc-mediated E6* dimer with significant secondary structure. Endogenous E6 oligomers were previously found in the cytosol of high-risk HPV transformed cell lines, and we propose that the oligomerization determinant resides within E6*. E6* effects were reported to counteract those of E6 in cells, and the ratio between these two species modulates p53 degradation and other apoptosis-dependent signaling cascades. A residue of an evolved splicing event related to regulation of oncogene expression in HPV or a splicing event resulting from the selection of a small deleterious viral polypeptide, the abundant existence of E6* with a "chameleon" nature correlates with target plasticity, and its fate is linked to a balance between protein levels, zinc availability, redox potential, and oligomerization. In addition, the results presented here have strong implications for zinc binding sites in nascent polypeptides. This evolved promiscuous folder speaks of effect rather than function of a viral product that, when highly increased, can directly or indirectly affect various cellular processes leading to cell deregulation and tumorigenesis.
高危型人乳头瘤病毒 E6 参与肿瘤发生发展的进程，主要通过促进 p53 降解来实现。HPV 转录物表现出复杂的剪接模式，其中 E6是高危型 HPV 中最丰富的转录本，包含 E6 的前 50 个氨基酸。由于难以在如此小的多肽中获得紧凑的单体折叠，因此这种包含 E6 第一个锌结合基序一半的多肽没有结构或生化信息。我们表明，在没有锌的情况下，HPV16-E6可以在 pH7.5 和 5.0 下分别折叠成 α-螺旋或 β-折叠的大型寡聚物。β-折叠寡聚物非常稳定，不受锌的影响，而 α-螺旋寡聚物则容易迅速形成聚集体，金属的存在可以阻止这种情况。每个锌原子结合两个 E6分子，这表明存在四面体、高亲和力的排列（K(D)<10(-12) M），这导致锌介导的 E6二聚体具有显著的二级结构。以前在高危型 HPV 转化细胞系的细胞质中发现了内源性 E6 寡聚物，我们提出寡聚化决定簇位于 E6内。据报道，E6的作用可以抵消细胞中 E6 的作用，并且这两种物质的比例调节 p53 降解和其他依赖凋亡的信号级联。一个与 HPV 中致癌基因表达调控相关的进化剪接事件的残基，或者是由选择小的有害病毒多肽导致的剪接事件，大量存在的具有“变色龙”性质的 E6*与靶标可塑性相关，其命运与蛋白质水平、锌可用性、氧化还原电位和寡聚化之间的平衡有关。此外，这里呈现的结果对新生多肽中的锌结合位点具有重要意义。这种进化的杂乱无章的折叠体反映的是病毒产物的影响而不是功能，当高度增加时，该产物可以直接或间接影响各种导致细胞失调和肿瘤发生的细胞过程。
