Rizvi Syed Monem, Del Cid Natasha, Lybarger Lonnie, Raghavan Malini
Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, MI 48109, USA.
J Immunol. 2011 Feb 15;186(4):2309-20. doi: 10.4049/jimmunol.1002959. Epub 2011 Jan 24.
Complexes of specific assembly factors and generic endoplasmic reticulum (ER) chaperones, collectively called the MHC class I peptide-loading complex (PLC), function in the folding and assembly of MHC class I molecules. The glycan-binding chaperone calreticulin (CRT) and partner oxidoreductase ERp57 are important in MHC class I assembly, but the sequence of assembly events and specific interactions involved remain incompletely understood. We show that the recruitments of CRT and ERp57 to the PLC are codependent and also dependent upon the ERp57 binding site and the glycan of the assembly factor tapasin. Furthermore, the ERp57 binding site and the glycan of tapasin enhance β(2)m and MHC class I heavy (H) chain recruitment to the PLC, with the ERp57 binding site having the dominant effect. In contrast, the conserved MHC class I H chain glycan played a minor role in CRT recruitment into the PLC, but impacted the recruitment of H chains into the PLC, and glycan-deficient H chains were impaired for tapasin-independent and tapasin-assisted assembly. The conserved MHC class I glycan and tapasin facilitated an early step in the assembly of H chain-β(2)m heterodimers, for which tapasin-ERp57 or tapasin-CRT complexes were not required. Together, these studies provide insights into how PLCs are constructed, demonstrate two distinct mechanisms by which PLCs can be stabilized, and suggest the presence of intermediate H chain-deficient PLCs.
特定组装因子与内质网（ER）通用伴侣蛋白的复合物，统称为MHC I类肽装载复合物（PLC），在MHC I类分子的折叠和组装过程中发挥作用。糖结合伴侣蛋白钙网蛋白（CRT）和伴侣氧化还原酶ERp57在MHC I类组装中很重要，但组装事件的顺序和所涉及的特定相互作用仍未完全了解。我们发现，CRT和ERp57向PLC的募集是相互依赖的，并且还依赖于ERp57结合位点和组装因子塔帕辛的聚糖。此外，塔帕辛的ERp57结合位点和聚糖增强了β2微球蛋白（β(2)m）和MHC I类重链（H链）向PLC的募集，其中ERp57结合位点起主导作用。相比之下，保守的MHC I类H链聚糖在CRT募集到PLC中起次要作用，但影响H链向PLC的募集，并且聚糖缺陷型H链在不依赖塔帕辛和塔帕辛辅助的组装中受损。保守的MHC I类聚糖和塔帕辛促进了H链-β(2)m异二聚体组装的早期步骤，而这一步骤不需要塔帕辛-ERp57或塔帕辛-CRT复合物。总之，这些研究为PLC如何构建提供了见解，证明了两种可稳定PLC的不同机制，并表明存在中间的H链缺陷型PLC。
