Brachet V, Raposo G, Amigorena S, Mellman I
Institut Curie, Section de Recherche Institut National de la Santé et de la Recherche Médicale CJF-95.01, Paris, France.
J Cell Biol. 1997 Apr 7;137(1):51-65. doi: 10.1083/jcb.137.1.51.
Major histocompatibility complex class II molecules are synthesized as a nonameric complex consisting of three alpha beta dimers associated with a trimer of invariant (Ii) chains. After exiting the TGN, a targeting signal in the Ii chain cytoplasmic domain directs the complex to endosomes where Ii chain is proteolytically processed and removed, allowing class II molecules to bind antigenic peptides before reaching the cell surface. Ii chain dissociation and peptide binding are thought to occur in one or more postendosomal sites related either to endosomes (designated CIIV) or to lysosomes (designated MIIC). We now find that in addition to initially targeting alpha beta dimers to endosomes, Ii chain regulates the subsequent transport of class II molecules. Under normal conditions, murine A20 B cells transport all of their newly synthesized class II I-A(b) alpha beta dimers to the plasma membrane with little if any reaching lysosomal compartments. Inhibition of Ii processing by the cysteine/serine protease inhibitor leupeptin, however, blocked transport to the cell surface and caused a dramatic but selective accumulation of I-A(b) class II molecules in lysosomes. In leupeptin, I-A(b) dimers formed stable complexes with a 10-kD NH2-terminal Ii chain fragment (Ii-p10), normally a transient intermediate in Ii chain processing. Upon removal of leupeptin, Ii-p10 was degraded and released, I-A(b) dimers bound antigenic peptides, and the peptide-loaded dimers were transported slowly from lysosomes to the plasma membrane. Our results suggest that alterations in the rate or efficiency of Ii chain processing can alter the postendosomal sorting of class II molecules, resulting in the increased accumulation of alpha beta dimers in lysosome-like MIIC. Thus, simple differences in Ii chain processing may account for the highly variable amounts of class II found in lysosomal compartments of different cell types or at different developmental stages.
主要组织相容性复合体II类分子以一种九聚体复合物的形式合成，该复合物由三个αβ二聚体与一个不变链（Ii链）三聚体相关联组成。离开反式高尔基体网络（TGN）后，Ii链胞质结构域中的一个靶向信号将该复合物导向内体，在那里Ii链被蛋白水解加工并去除，使II类分子在到达细胞表面之前能够结合抗原肽。Ii链解离和肽结合被认为发生在一个或多个与内体（称为CIIV）或溶酶体（称为MIIC）相关的内体后位点。我们现在发现，除了最初将αβ二聚体靶向内体之外，Ii链还调节II类分子的后续运输。在正常情况下，小鼠A20 B细胞将其所有新合成的II类I-A(b)αβ二聚体运输到质膜，极少有（如果有的话）到达溶酶体区室。然而，半胱氨酸/丝氨酸蛋白酶抑制剂亮肽素对Ii链加工的抑制作用阻断了向细胞表面的运输，并导致I-A(b) II类分子在溶酶体中显著但选择性地积累。在亮肽素存在的情况下，I-A(b)二聚体与一个10-kD的NH2末端Ii链片段（Ii-p10）形成稳定复合物，Ii-p10通常是Ii链加工过程中的一个瞬时中间体。去除亮肽素后，Ii-p10被降解并释放，I-A(b)二聚体结合抗原肽，并且负载肽的二聚体从溶酶体缓慢运输到质膜。我们的结果表明，Ii链加工速率或效率的改变可以改变内体后II类分子的分选，导致αβ二聚体在溶酶体样MIIC中积累增加。因此，Ii链加工的简单差异可能解释了在不同细胞类型或不同发育阶段的溶酶体区室中发现的II类分子数量的高度变异性。
