Ely K R, Wood M K, Rajan S S, Hodsdon J M, Abola E E, Deutsch H F, Edmundson A B
Mol Immunol. 1985 Feb;22(2):93-100. doi: 10.1016/s0161-5890(85)80002-x.
The covalently linked hybrid of two human lambda-type light chains (Mcg and Weir) crystallizes as trigonal bipyramids in ammonium sulfate [Ely et al., Molec. Immun. 22, 85-92 (1985)]. While markedly different in appearance from the barrel-shaped crystals of the parental Mcg dimer, the bipyramids of the hybrid have the same space group: trigonal P3(1)21. Moreover, the unit cell dimensions are practically identical: a = 72.3 A in both proteins; c = 188.1 A in the hybrid and 185.9 A in the Mcg dimer. These observations imply that the crystal packing and the main features of the three-dimensional structures are closely similar in the Mcg X Weir hybrid and the Mcg dimer. The "constant" domains of the Mcg and Weir proteins belong to the same genetic subclass and were expected to interact in comparable ways in hybrids and parental dimers. However, the overall similarities in the "variable" domain pairs in the hybrid and Mcg dimer were completely unpredicted, since the amino acid sequences of the heterologous variable domains differ by 36 residues. By difference Fourier analysis the Weir light chain has been tentatively identified as monomer 1 (heavy-chain analogue) and the Mcg protein as monomer 2 (light-chain analogue) in the hybrid dimer. Substitutions in key positions in the hypervariable loops explain the differences in binding activity of the Mcg and Weir dimers. In the Mcg dimer bis(dinitrophenyl)lysine spans two relatively spacious subsites (A and B), with primary contacts involving tyrosines 34 and 38 of monomer 2. The Weir dimer, which does not bind dinitrophenyl ligands, has serine and phenylalanine in homologous positions. Moreover, the bilateral replacement of valine 48 and serine 91 in Mcg by leucine and methionine in the Weir dimer should effectively block access to subsite B. In the hybrid binding activity for bis(dinitrophenyl)lysine is restored because the Mcg light chain is present as the monomer 2 subunit.
两条人λ型轻链（Mcg和Weir）的共价连接杂合体在硫酸铵中结晶为三角双锥体[Ely等人，《分子免疫学》22，85 - 92（1985）]。虽然杂合体的双锥体外观与亲本Mcg二聚体的桶状晶体明显不同，但杂合体的双锥体具有相同的空间群：三角P3(1)21。此外，晶胞尺寸几乎相同：两种蛋白质的a均为72.3 Å；杂合体的c为188.1 Å，Mcg二聚体的c为185.9 Å。这些观察结果表明，Mcg X Weir杂合体和Mcg二聚体中的晶体堆积和三维结构的主要特征非常相似。Mcg和Weir蛋白的“恒定”结构域属于同一遗传亚类，预计在杂合体和亲本二聚体中以类似方式相互作用。然而，杂合体和Mcg二聚体中“可变”结构域对的总体相似性完全出乎意料，因为异源可变结构域的氨基酸序列相差36个残基。通过差值傅里叶分析，在杂合二聚体中，Weir轻链被初步鉴定为单体1（重链类似物），Mcg蛋白为单体2（轻链类似物）。高变环关键位置的取代解释了Mcg和Weir二聚体结合活性的差异。在Mcg二聚体中，双（二硝基苯基）赖氨酸跨越两个相对宽敞的亚位点（A和B），主要接触涉及单体2的酪氨酸34和38。不结合二硝基苯基配体的Weir二聚体在同源位置具有丝氨酸和苯丙氨酸。此外，Mcg中缬氨酸48和丝氨酸91被Weir二聚体中的亮氨酸和甲硫氨酸双侧取代应有效阻断进入亚位点B。在杂合体中，对双（二硝基苯基）赖氨酸的结合活性得以恢复，因为Mcg轻链作为单体2亚基存在。
