Department of Pathology, Georgetown University Medical School, Washington, D.C., USA.
PLoS Pathog. 2010 Sep 2;6(9):e1001089. doi: 10.1371/journal.ppat.1001089.
The high-risk HPV E6 and E7 proteins cooperate to immortalize primary human cervical cells and the E7 protein can independently transform fibroblasts in vitro, primarily due to its ability to associate with and degrade the retinoblastoma tumor suppressor protein, pRb. The binding of E7 to pRb is mediated by a conserved Leu-X-Cys-X-Glu (LXCXE) motif in the conserved region 2 (CR2) of E7 and this domain is both necessary and sufficient for E7/pRb association. In the current study, we report that the E7 protein of the malignancy-associated canine papillomavirus type 2 encodes an E7 protein that has serine substituted for cysteine in the LXCXE motif. In HPV, this substitution in E7 abrogates pRb binding and degradation. However, despite variation at this critical site, the canine papillomavirus E7 protein still bound and degraded pRb. Even complete deletion of the LXSXE domain of canine E7 failed to interfere with binding to pRb in vitro and in vivo. Rather, the dominant binding site for pRb mapped to the C-terminal domain of canine E7. Finally, while the CR1 and CR2 domains of HPV E7 are sufficient for degradation of pRb, the C-terminal region of canine E7 was also required for pRb degradation. Screening of HPV genome sequences revealed that the LXSXE motif of the canine E7 protein was also present in the gamma HPVs and we demonstrate that the gamma HPV-4 E7 protein also binds pRb in a similar way. It appears, therefore, that the type 2 canine PV and gamma-type HPVs not only share similar properties with respect to tissue specificity and association with immunosuppression, but also the mechanism by which their E7 proteins interact with pRb.
高危型 HPV 的 E6 和 E7 蛋白合作使原代人宫颈细胞永生化，E7 蛋白可独立在体外转化成纤维细胞，主要是由于其与视网膜母细胞瘤肿瘤抑制蛋白 pRb 结合并使之降解的能力。E7 与 pRb 的结合由 E7 保守区 2（CR2）中的一个保守的 Leu-X-Cys-X-Glu（LXCXE）基序介导，该结构域对于 E7/pRb 结合既是必需的也是充分的。在本研究中，我们报告了与恶性相关的犬乳头瘤病毒 2 的 E7 蛋白编码一个 E7 蛋白，其 LXCXE 基序中的半胱氨酸被丝氨酸取代。在 HPV 中，E7 中的这种取代会破坏 pRb 的结合和降解。然而，尽管在这个关键位点发生了变异，犬乳头瘤病毒 E7 蛋白仍能结合并降解 pRb。即使完全删除犬 E7 的 LXSXE 结构域，也不能干扰其在体外和体内与 pRb 的结合。相反，pRb 的主要结合位点位于犬 E7 的 C 末端结构域。最后，尽管 HPV E7 的 CR1 和 CR2 结构域足以降解 pRb，但犬 E7 的 C 末端区域也需要降解 pRb。对 HPV 基因组序列的筛选显示，犬 E7 蛋白的 LXSXE 基序也存在于 γ HPV 中，我们证明 γ HPV-4 E7 蛋白也以类似的方式结合 pRb。因此，似乎 2 型犬 PV 和 γ 型 HPV 不仅在组织特异性和与免疫抑制相关的方面具有相似的特性，而且它们的 E7 蛋白与 pRb 相互作用的机制也相似。
