Cambridge Institute for Medical Research, University of Cambridge, Cambridge CB2 0XY, United Kingdom.
Proc Natl Acad Sci U S A. 2011 Feb 1;108(5):2034-9. doi: 10.1073/pnas.1016229108. Epub 2011 Jan 18.
The assembly of MHC class I molecules is governed by stringent endoplasmic reticulum (ER) quality control mechanisms. MHC class I heavy chains that fail to achieve their native conformation in complex with β2-microglobulin (β2m) and peptide are targeted for ER-associated degradation. This requires ubiquitination of the MHC class I heavy chain and its dislocation from the ER to the cytosol for proteasome-mediated degradation, although the cellular machinery involved in this process is unknown. Using an siRNA functional screen in β2m-depleted cells, we identify an essential role for the E3 ligase HRD1 (Synoviolin) together with the E2 ubiquitin-conjugating enzyme UBE2J1 in the ubiquitination and dislocation of misfolded MHC class I heavy chains. HRD1 is also required for the ubiquitination and degradation of the naturally occurring hemochromatosis-associated HFE-C282Y mutant, which is unable to bind β2m. In the absence of HRD1, misfolded HLA-B27 accumulated in cells with a normal MHC class I assembly pathway, and HRD1 depletion prevented the appearance of low levels of cytosolic unfolded MHC I heavy chains. HRD1 and UBE2J1 associate in a complex together with non-β2m bound MHC class I heavy chains, Derlin 1, and p97 and discriminate misfolded MHC class I from conformational MHC I-β2m-peptide heterotrimers. Together these data support a physiological role for HRD1 and UBE2J1 in the homeostatic regulation of MHC class I assembly and expression.
MHC I 类分子的组装受到严格的内质网 (ER) 质量控制机制的控制。未能与 β2-微球蛋白 (β2m) 和肽形成天然构象的 MHC I 重链被靶向 ER 相关降解。这需要 MHC I 重链的泛素化及其从 ER 易位到细胞质进行蛋白酶体介导的降解，尽管涉及该过程的细胞机制尚不清楚。在β2m 耗尽的细胞中使用 siRNA 功能筛选，我们确定了 E3 连接酶 HRD1（Synoviolin）与 E2 泛素缀合酶 UBE2J1 在错误折叠的 MHC I 重链的泛素化和易位中的重要作用。HRD1 还需要对自然发生的血色素沉着症相关 HFE-C282Y 突变体进行泛素化和降解，该突变体无法结合β2m。在没有 HRD1 的情况下，错误折叠的 HLA-B27 在具有正常 MHC I 组装途径的细胞中积累，并且 HRD1 耗尽阻止了低水平未折叠的细胞质 MHC I 重链的出现。HRD1 和 UBE2J1 与非β2m 结合的 MHC I 重链、Derlin 1 和 p97 一起在复合物中相互作用，并区分错误折叠的 MHC I 与构象 MHC I-β2m-肽异三聚体。这些数据共同支持 HRD1 和 UBE2J1 在 MHC I 组装和表达的动态平衡调节中的生理作用。
