Department of Biological Sciences, Purdue University, West Lafayette, IN 47907, USA.
Proc Natl Acad Sci U S A. 2012 Feb 7;109(6):1925-30. doi: 10.1073/pnas.1115025109. Epub 2012 Jan 23.
In eukaryotes, ubiquitination is an important posttranslational process achieved through a cascade of ubiquitin-activating (E1), conjugating (E2), and ligase (E3) enzymes. Many pathogenic bacteria deliver virulence factors into the host cell that function as E3 ligases. How these bacterial "Trojan horses" integrate into the eukaryotic ubiquitin system has remained a mystery. Here we report crystal structures of two bacterial E3s, Salmonella SopA and Escherichia coli NleL, both in complex with human E2 UbcH7. These structures represent two distinct conformational states of the bacterial E3s, supporting the necessary structural rearrangements associated with ubiquitin transfer. The E2-interacting surface of SopA and NleL has little similarity to those of eukaryotic E3s. However, both bacterial E3s bind to the canonical surface of E2 that normally interacts with eukaryotic E3s. Furthermore, we show that a glutamate residue on E3 is involved in catalyzing ubiquitin transfer from E3 to the substrate, but not from E2 to E3. Together, these results provide mechanistic insights into the ubiquitin pathway and a framework for understanding molecular mimicry in bacterial pathogenesis.
在真核生物中，泛素化是通过泛素激活酶（E1）、连接酶（E2）和连接酶（E3）酶级联反应实现的一个重要的翻译后过程。许多致病性细菌将毒力因子输送到宿主细胞中，这些因子作为 E3 连接酶发挥作用。这些细菌“特洛伊木马”如何融入真核泛素系统一直是个谜。在这里，我们报告了两种细菌 E3，沙门氏菌 SopA 和大肠杆菌 NleL 的晶体结构，它们都与人类 E2 UbcH7 复合。这些结构代表了细菌 E3 的两种不同构象状态，支持与泛素转移相关的必要结构重排。SopA 和 NleL 的 E2 相互作用表面与真核 E3 的表面几乎没有相似之处。然而，两种细菌 E3 都结合到 E2 的典型表面，该表面通常与真核 E3 相互作用。此外，我们表明，E3 上的一个谷氨酸残基参与催化泛素从 E3 转移到底物，但不参与从 E2 转移到 E3。总之，这些结果为泛素途径提供了机制上的见解，并为理解细菌发病机制中的分子模拟提供了框架。
