Tampé R, Tyvoll D, McConnell H M
Stauffer Laboratory for Physical Chemistry, Stanford University, CA 94305.
Proc Natl Acad Sci U S A. 1991 Dec 1;88(23):10667-70. doi: 10.1073/pnas.88.23.10667.
Major histocompatibility complex (MHC) class II molecules are heterodimers formed by noncovalent linkage of alpha and beta chains. It has been shown that the subunits of the MHC class II molecules IAd and IEk bind antigenic peptides as well as antigenic peptides labeled with fluorescent probes. Laser scanning fluorescence microscopy on SDS/polyacrylamide gels demonstrates that the subunit-peptide complexes of IAd are stable over a wide pH range. Below pH 5.3 the heterodimer of IAd dissociates into the free chains, which still bind antigenic peptides such as the 18-amino acid peptide obtained by a tyrosine addition to a chicken ovalbumin peptide, Ova-(323-339)Y. The stability of preformed subunit complexes with fluorescein-labeled Ova-(323-339)Y was investigated by using high-performance size exclusion chromatography and epifluorescence microscopy. Each subunit forms a long-lived complex, both in detergent solutions and in reconstituted lipid bilayers. At 37 degrees C and pH 7.0 the dissociation half-time of the beta-subunit-peptide complex was determined to be 28 hr and that of the alpha-subunit-peptide complex was 10 hr. In contrast to the dissociation of the peptide from the IAd heterodimer, the half-times for dissociation of the peptide from the separate chains are not decreased at pH 5.0.
主要组织相容性复合体（MHC）II类分子是由α链和β链通过非共价连接形成的异二聚体。已表明，MHC II类分子IAd和IEk的亚基能结合抗原肽以及用荧光探针标记的抗原肽。对SDS/聚丙烯酰胺凝胶进行激光扫描荧光显微镜观察表明，IAd的亚基 - 肽复合物在很宽的pH范围内是稳定的。在pH 5.3以下，IAd的异二聚体解离成游离链，这些游离链仍能结合抗原肽，如通过在鸡卵清蛋白肽Ova-(323 - 339)Y上添加酪氨酸得到的18个氨基酸的肽。通过使用高效尺寸排阻色谱和落射荧光显微镜研究了与荧光素标记的Ova-(323 - 339)Y预先形成的亚基复合物的稳定性。每个亚基在去污剂溶液和重组脂质双层中都形成长寿命复合物。在37℃和pH 7.0时，β亚基 - 肽复合物的解离半衰期测定为28小时，α亚基 - 肽复合物的解离半衰期为10小时。与肽从IAd异二聚体上解离不同，在pH 5.0时，肽从单独链上解离的半衰期并未缩短。
