Hulett M D, Witort E, Brinkworth R I, McKenzie I F, Hogarth P M
Austin Research Institute, Austin Hospital, Heidelberg, Victoria, Australia.
J Biol Chem. 1995 Sep 8;270(36):21188-94. doi: 10.1074/jbc.270.36.21188.
The low affinity receptor for IgG, Fc gamma RII (CD32), has a wide distribution on hematopoietic cells where it is responsible for a diverse range of cellular responses crucial for immune regulation and resistance to infection. Fc gamma RII is a member of the immunoglobulin superfamily, containing an extracellular region of two Ig-like domains. The IgG binding site of human Fc gamma RII has been localized to an 8-amino acid segment of the second extracellular domain, Asn154-Ser161. In this study, evidence is presented to suggest that domain 1 and two additional regions of domain 2 also contribute to the binding of IgG by Fc gamma RII. Chimeric receptors generated by exchanging the extracellular domains and segments of domain 2 between Fc gamma RII and the structurally related Fc epsilon RI alpha chain were used to demonstrate that substitution of domain 1 in its entirety or the domain 2 regions encompassing residues Ser109-Val116 and Ser130-Thr135 resulted in a loss of the ability of these receptors to bind hIgG1 in dimeric form. Site-directed mutagenesis performed on individual residues within and flanking the Ser109-Val116 and Ser130-Thr135 domain 2 segments indicated that substitution of Lys113, Pro114, Leu115, Val116, Phe129, and His131 profoundly decreased the binding of hIgG1, whereas substitution of Asp133 and Pro134 increased binding. These findings suggest that not only is domain 1 contributing to the affinity of IgG binding by Fc gamma RII but, importantly, that the domain 2 regions Ser109-Val116 and Phe129-Thr135 also play key roles in the binding of hIgG1. The location of these binding regions on a molecular model of the entire extracellular region of Fc gamma RII indicates that they comprise loops that are juxtaposed in domain 2 at the interface with domain 1, with the putative crucial binding residues forming a hydrophobic pocket surrounded by a wall of predominantly aromatic and basic residues.
免疫球蛋白G的低亲和力受体FcγRII（CD32）在造血细胞上广泛分布，在这些细胞中，它负责一系列对免疫调节和抗感染至关重要的细胞反应。FcγRII是免疫球蛋白超家族的成员，包含两个Ig样结构域的细胞外区域。人FcγRII的IgG结合位点已定位到第二个细胞外结构域的一个8个氨基酸的片段，即Asn154 - Ser161。在本研究中，有证据表明结构域1和结构域2的另外两个区域也有助于FcγRII与IgG的结合。通过交换FcγRII和结构相关的FcεRIα链之间的细胞外结构域和结构域2的片段产生的嵌合受体被用于证明，完全替换结构域1或包含残基Ser109 - Val116和Ser130 - Thr135的结构域2区域会导致这些受体失去以二聚体形式结合hIgG1的能力。对结构域2片段Ser109 - Val116和Ser130 - Thr135内及其侧翼的单个残基进行定点诱变表明，替换Lys113、Pro114、Leu115、Val116、Phe129和His131会显著降低hIgG1的结合，而替换Asp133和Pro134会增加结合。这些发现表明，不仅结构域1对FcγRII与IgG结合的亲和力有贡献，而且重要的是，结构域2区域Ser109 - Val116和Phe129 - Thr135在hIgG1的结合中也起关键作用。这些结合区域在FcγRII整个细胞外区域的分子模型上的位置表明，它们包括在结构域2中与结构域1的界面处并列的环，假定的关键结合残基形成一个疏水口袋，周围主要是芳香族和碱性残基组成的壁。
