Khalil Hayssam, Brunet Alexandre, Saba Ingrid, Terra Rafik, Sékaly Rafick Pierre, Thibodeau Jacques
Laboratoire d'Immunologie Moléculaire, Département de Microbiologie et Immunologie, Faculté de Médecine, Université de Montréal, CP 6128 Succursale Centre-Ville, Montréal, Québec H3C 3J7, Canada.
Int Immunol. 2003 Oct;15(10):1249-63. doi: 10.1093/intimm/dxg124.
The human-specific p35 isoform of the invariant chain (Ii) includes an R-X-R endoplasmic reticulum (ER) retention motif that is inactivated upon HLA-DR binding. Although the masking is assumed to involve the cytoplasmic tails of class II molecules, the mechanism underlying this function remains to be investigated. Moreover, in light of the polymorphic nature of the class II cytosolic tails, little is known about the capacity of various isotypes or alleles to overcome the retention signal of Iip35. To gain further insights into these issues, we first addressed the proposed role of the HLA-DR cytoplasmic tails. As shown by flow cytometry, the presence of Iip35 in transfected HeLa cells prevented surface expression of HLA-DR molecules lacking their cytoplasmic tails (DRalphaTM/betaTM). These truncated class II molecules and Iip35 accumulated in the ER, and co-localized with calnexin, as determined by confocal microscopy. Sensitivity of DRalphaTM/betaTM to endoglycosidase H treatment confirmed that these molecules do not reach the trans-Golgi network when associated with Iip35. Further characterization revealed that the beta chain cytosolic tail is critical for efficient ER egress of class II/Iip35 complexes. Interestingly, our results clearly demonstrate for the first time that DP and DQ isotypes can also overcome the retention motif of Iip35 through a mechanism involving their very distinctive polymorphic beta chain cytoplasmic tails. Altogether, these results further dissect the masking of di-basic retention signals, and emphasize the interplay between class II molecules and Ii for the transport of the complex to the endocytic pathway.
恒定链（Ii）的人类特异性p35亚型包含一个R-X-R内质网（ER）滞留基序，该基序在与HLA-DR结合后失活。尽管推测这种掩盖作用涉及II类分子的胞质尾，但该功能的潜在机制仍有待研究。此外，鉴于II类胞质尾的多态性，对于各种同种型或等位基因克服Iip35滞留信号的能力知之甚少。为了进一步深入了解这些问题，我们首先探讨了HLA-DR胞质尾的假定作用。如流式细胞术所示，转染的HeLa细胞中Iip35的存在阻止了缺乏胞质尾的HLA-DR分子（DRalphaTM/betaTM）的表面表达。这些截短的II类分子和Iip35在内质网中积累，并通过共聚焦显微镜确定与钙连蛋白共定位。DRalphaTM/betaTM对内切糖苷酶H处理的敏感性证实，当与Iip35结合时，这些分子不会到达反式高尔基体网络。进一步的表征表明，β链胞质尾对于II类/Iip35复合物有效地从内质网输出至关重要。有趣的是，我们的结果首次清楚地表明，DP和DQ同种型也可以通过一种涉及其非常独特的多态性β链胞质尾的机制克服Iip35的滞留基序。总之，这些结果进一步剖析了双碱性滞留信号的掩盖作用，并强调了II类分子和Ii之间对于复合物向胞吞途径运输的相互作用。
