Castellino F, Han R, Germain R N
Lymphocyte Biology Section, Laboratory of Immunology, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, USA.
Eur J Immunol. 2001 Mar;31(3):841-50. doi: 10.1002/1521-4141(200103)31:3<841::aid-immu841>3.0.co;2-d.
Invariant chain (Ii) association with MHC class II molecules is strongly dependent upon interaction of CLIP (Ii exon 3, residues 81 - 104) with the peptide binding groove of the class II dimer. This dominant interaction does not adequately explain, however, the efficient association of Ii with class II molecules of diverse allelic and isotypic origin, which have markedly different affinities for synthetic peptides corresponding to CLIP. In agreement with other recent observations, we demonstrate here that class II molecules with occupied binding sites unable to engage CLIP maintain association with Ii in mild detergent. The association is direct and not mediated through unoccupied class II chains bound to properly assembled and loaded class II dimers, nor is it mediated through chaperones. The site of this CLIP-independent binding has been mapped using truncation mutants and an Ii-human transferrin receptor chimeric protein to the transmembrane segment of Ii. The existence of multiple low-affinity sites of interaction between MHC class II and Ii helps explain how effective occupancy of all newly synthesized class II molecules can occur despite substantial variations in the strength of CLIP-dependent association that arise from class II binding domain polymorphism. These data establishing a site of Ii-MHC class II association N-terminal to CLIP also provide new insight into the possible functional relationship between the sequential endocytic proteolysis of Ii from its C terminus and a series of contact sites with MHC class II molecules spread from the transmembrane region through to the tip of the lumenal segment of Ii.
恒定链（Ii）与MHC II类分子的结合强烈依赖于CLIP（Ii外显子3，第81 - 104位氨基酸残基）与II类二聚体的肽结合槽之间的相互作用。然而，这种主要的相互作用并不能充分解释Ii与不同等位基因和同种型来源的II类分子的有效结合，这些II类分子对与CLIP对应的合成肽具有明显不同的亲和力。与其他近期观察结果一致，我们在此证明，具有被占据的结合位点且无法结合CLIP的II类分子在温和去污剂中仍与Ii保持结合。这种结合是直接的，不是通过与正确组装和负载的II类二聚体结合的未占据的II类链介导的，也不是通过伴侣蛋白介导的。已使用截短突变体和Ii - 人转铁蛋白受体嵌合蛋白将这种不依赖CLIP的结合位点定位到Ii的跨膜区。MHC II类分子与Ii之间存在多个低亲和力相互作用位点，这有助于解释尽管由于II类结合域多态性导致CLIP依赖性结合强度存在很大差异，但所有新合成的II类分子如何能有效地被占据。这些确定Ii - MHC II类结合位点在CLIP N端的数据，也为Ii从其C端开始的顺序内吞蛋白水解与Ii从跨膜区到腔内段末端与MHC II类分子的一系列接触位点之间可能的功能关系提供了新的见解。
