Institut de Recherches Cliniques de Montréal, Montreal, Quebec H2W 1R7, Canada.
J Virol. 2010 Nov;84(22):11747-60. doi: 10.1128/JVI.01445-10. Epub 2010 Sep 15.
The initiator protein E1 from human papillomavirus (HPV) is a helicase essential for replication of the viral genome. E1 contains three functional domains: a C-terminal enzymatic domain that has ATPase/helicase activity, a central DNA-binding domain that recognizes specific sequences in the origin of replication, and a N-terminal region necessary for viral DNA replication in vivo but dispensable in vitro. This N-terminal portion of E1 contains a conserved nuclear export signal (NES) whose function in the viral life cycle remains unclear. In this study, we provide evidence that nuclear export of HPV31 E1 is inhibited by cyclin E/A-Cdk2 phosphorylation of two serines residues, S92 and S106, located near and within the E1 NES, respectively. Using E1 mutant proteins that are confined to the nucleus, we determined that nuclear export of E1 is not essential for transient viral DNA replication but is important for the long-term maintenance of the HPV episome in undifferentiated keratinocytes. The findings that E1 nuclear export is not required for viral DNA replication but needed for genome maintenance over multiple cell divisions raised the possibility that continuous nuclear accumulation of E1 is detrimental to cellular growth. In support of this possibility, we observed that nuclear accumulation of E1 dramatically reduces cellular proliferation by delaying cell cycle progression in S phase. On the basis of these results, we propose that nuclear export of E1 is required, at least in part, to limit accumulation of this viral helicase in the nucleus in order to prevent its detrimental effect on cellular proliferation.
人乳头瘤病毒（HPV）的起始蛋白 E1 是复制病毒基因组所必需的解旋酶。E1 包含三个功能域：C 端具有 ATP 酶/解旋酶活性的酶结构域、识别复制起点特定序列的中央 DNA 结合结构域，以及体内病毒 DNA 复制所必需但在体外非必需的 N 端区域。E1 的这个 N 端部分包含一个保守的核输出信号（NES），其在病毒生命周期中的功能尚不清楚。在这项研究中，我们提供的证据表明，位于 HPV31 E1 NES 附近和内部的两个丝氨酸残基 S92 和 S106 的 cyclin E/A-Cdk2 磷酸化抑制了 HPV31 E1 的核输出。使用局限于核内的 E1 突变蛋白，我们确定 E1 的核输出对于瞬时病毒 DNA 复制不是必需的，但对于未分化角质细胞中 HPV 染色体外体的长期维持是重要的。E1 核输出对于病毒 DNA 复制不是必需的，但对于多个细胞分裂的基因组维持是必需的这一发现提出了这样一种可能性，即 E1 的持续核积累对细胞生长有害。为了支持这种可能性，我们观察到 E1 的核积累通过延迟 S 期的细胞周期进程，极大地降低了细胞增殖。基于这些结果，我们提出 E1 的核输出是必需的，至少部分必需的，以限制这种病毒解旋酶在核内的积累，以防止其对细胞增殖产生不利影响。
