Department of Biology, Faculty of Medicine, Masaryk University, Brno, Czech Republic.
Cell Cycle. 2012 Mar 1;11(5):953-62. doi: 10.4161/cc.11.5.19445.
Mdm2 can mediate p53 ubiquitylation and degradation either in the form of the Mdm2 homodimer or Mdm2/MdmX heterodimer. The ubiquitin ligase activity of these complexes resides mainly in their respective RING finger domains and also requires adjacent C-terminal tails. So far, structural studies have failed to show significant differences between Mdm2 RING homodimers and Mdm2/MdmX RING heterodimers. Here, we report that not only the primary amino acid sequence, but also the length of the C-terminal tail of Mdm2 is highly conserved through evolution and plays an important role in Mdm2 activity toward p53. Mdm2 mutants with extended C termini do not ubiquitylate p53 despite being capable of forming Mdm2 homodimers through both RING-acidic domain and RING-RING interactions. All extended mutants also retained the ability to interact with MdmX, and this interaction led to reactivation of their E3 ubiquitin ligase activity. In contrast, only a subset of extended Mdm2 mutants was activated by the interaction with Mdm2 RING domain, suggesting that Mdm2 homodimers and Mdm2/MdmX heterodimers may not be structurally and functionally fully equivalent.
Mdm2 可以形成 Mdm2 同源二聚体或 Mdm2/MdmX 异源二聚体的形式介导 p53 的泛素化和降解。这些复合物的泛素连接酶活性主要位于它们各自的 RING 指结构域，并且还需要相邻的 C 末端尾部。到目前为止，结构研究未能显示 Mdm2 RING 同源二聚体和 Mdm2/MdmX RING 异源二聚体之间存在显著差异。在这里，我们报告不仅 Mdm2 的一级氨基酸序列，而且其 C 末端尾部的长度在进化过程中也高度保守，并在 Mdm2 对 p53 的活性中发挥重要作用。尽管能够通过 RING-酸性结构域和 RING-RING 相互作用形成 Mdm2 同源二聚体，但具有延伸 C 末端的 Mdm2 突变体不能泛素化 p53。所有延伸突变体也保留了与 MdmX 相互作用的能力，这种相互作用导致其 E3 泛素连接酶活性的重新激活。相比之下，只有一部分延伸的 Mdm2 突变体通过与 Mdm2 RING 结构域的相互作用被激活，这表明 Mdm2 同源二聚体和 Mdm2/MdmX 异源二聚体在结构和功能上可能不完全等效。
