Derrick J P, Wigley D B
Department of Biochemistry, University of Leicester, U.K.
J Mol Biol. 1994 Nov 11;243(5):906-18. doi: 10.1006/jmbi.1994.1691.
Protein G is a cell surface-associated protein from Streptococcus that binds to IgG with high affinity. We have determined the X-ray crystal structures of the third IgG-binding domain (domain III) alone to a resolution of 1.1 A (final R-factor of 19.3%), and in complex with an Fab fragment to 2.6 A (final R-factor of 16.8%). The structure of domain III is similar to the lower-resolution crystal structures of protein G domains determined previously by other investigators, but shows some minor differences when compared with the equivalent NMR structures. Domain III binds to the immunoglobulin by formation of an antiparallel interaction between the second beta-strand in domain III and the last beta-strand in the CH 1 domain. There is also a minor site of interaction between the C-terminal end of the alpha-helix in protein G and the first beta-strand in the CH 1 domain. Previous studies by NMR on the interactions between protein G and IgG have concluded that different portions of the protein G domain are involved in binding to the Fab and Fc portions. The results presented here support these findings and permit a detailed analysis of the recognition of Fab by protein G; formation of the complex buries a large water-accessible area, of a magnitude comparable with that found in antibody/antigen interactions. The majority of hydrogen bonds between the two proteins involve main-chain atoms from the CH 1 domain. The CH 1 domain residues that are in contact with protein G are shown to be highly conserved in alignments of mouse and human gamma chain amino acid sequences. We conclude that the binding site for protein G on Fab is relatively invariant across different species and gamma chain subclasses, providing an explanation for the widespread recognition of Fab fragments from mouse and human antibodies by protein G. The solution of the crystal structures of domain III alone and bound to Fab has demonstrated that there is no major structural change apparent in either protein on formation of the complex.
蛋白G是一种来自链球菌的细胞表面相关蛋白，它能与IgG高亲和力结合。我们已经确定了单独的第三个IgG结合结构域（结构域III）的X射线晶体结构，分辨率为1.1埃（最终R因子为19.3%），以及与一个Fab片段形成复合物后的结构，分辨率为2.6埃（最终R因子为16.8%）。结构域III的结构与其他研究人员先前测定的蛋白G结构域的低分辨率晶体结构相似，但与等效的核磁共振结构相比，显示出一些细微差异。结构域III通过在结构域III中的第二条β链与CH 1结构域中的最后一条β链之间形成反平行相互作用来结合免疫球蛋白。在蛋白G的α螺旋C末端与CH 1结构域中的第一条β链之间也存在一个次要的相互作用位点。先前通过核磁共振对蛋白G与IgG之间相互作用的研究得出结论，蛋白G结构域的不同部分参与与Fab和Fc部分的结合。这里给出的结果支持了这些发现，并允许对蛋白G识别Fab进行详细分析；复合物的形成掩埋了一个与抗体/抗原相互作用中发现的相当大的可及水面积。两种蛋白质之间的大多数氢键涉及来自CH 1结构域的主链原子。在小鼠和人γ链氨基酸序列比对中，与蛋白G接触的CH 1结构域残基显示高度保守。我们得出结论，蛋白G在Fab上的结合位点在不同物种和γ链亚类中相对不变，这为蛋白G广泛识别来自小鼠和人抗体的Fab片段提供了解释。单独的结构域III以及与Fab结合的晶体结构的解析表明，在形成复合物时，两种蛋白质中均未明显出现重大结构变化。
