Deep Shashank, Walker Kerfoot P, Shu Zhanyong, Hinck Andrew P
Department of Biochemistry, University of Texas Health Science Center at San Antonio, San Antonio, Texas 78229-3900, USA.
Biochemistry. 2003 Sep 2;42(34):10126-39. doi: 10.1021/bi034366a.
Isoforms of transforming growth factor beta (TGFbeta) are 25 kDa homodimeric polypeptides that signal by binding and bringing together two related, functionally distinct cell surface receptors designated as TbetaR1 and TbetaR2. Here, we report the solution structure of the 13.8 kDa extracellular domain of human TbetaR2 (ecTbetaR2) as calculated from N(N)-H(N), C(alpha)-H(alpha), and C(alpha)-C(O) residual dipolar coupling restraints in conjunction with NOE distance, dihedral angle, and scalar coupling restraints. Comparison of the free ecTbetaR2 solution structure with the TGFbeta3-bound ecTbetaR2 crystal structure reveals backbone conformations that superimpose with RMSDs of 1.0 A over the regions of regular secondary structure and 1.4 A overall. The differences in structure fall mainly in loop regions that are either poorly defined by the available NMR data or are involved in crystal contacts. The noted similarities between the NMR structure of the free form and the crystal structure of the TGFbeta-bound form are also consistent with the close correspondence, 0.16 A RMSD for regions of secondary structure and 0.51 A RMSD overall, for the crystal structure of free ecTbetaR2 as compared to the crystal structure of TGFbeta3-bound ecTbetaR2. Despite the apparent similarities between the free and the bound forms, there appears to be small but significant differences in structure involving the interfacial contact region of the receptor. Measurements of backbone (15)N relaxation times and interpretation of these by the model-free formalism with axial diffusional anisotropy further reveal significant ms to micros time scale motions centered about two of the conserved disulfide bonds and in several residues that comprise the TGFbeta binding surface. Together, these observations indicate that binding likely occurs through a mechanism with a small component of induced fit character, whereby flexibility within the receptor facilitates the transition to the TGFbeta-bound state.
转化生长因子β（TGFβ）的异构体是25 kDa的同二聚体多肽，通过结合并聚集两个相关的、功能不同的细胞表面受体（称为TβR1和TβR2）来传递信号。在此，我们报告了人TβR2（ecTβR2）13.8 kDa细胞外结构域的溶液结构，该结构是根据N(N)-H(N)、C(α)-H(α)和C(α)-C(O)剩余偶极耦合约束以及NOE距离、二面角和标量耦合约束计算得出的。将游离ecTβR2的溶液结构与结合TGFβ3的ecTβR2晶体结构进行比较，发现在规则二级结构区域主链构象的均方根偏差（RMSD）为1.0 Å，整体为1.4 Å。结构差异主要存在于环区，这些环区要么由现有核磁共振数据定义不明确，要么参与晶体接触。游离形式的核磁共振结构与结合TGFβ形式的晶体结构之间的显著相似性也与游离ecTβR2的晶体结构与结合TGFβ3的ecTβR2的晶体结构在二级结构区域的0.16 Å RMSD和整体0.51 Å RMSD的紧密对应一致。尽管游离形式和结合形式之间存在明显相似性，但受体的界面接触区域在结构上似乎存在微小但显著的差异。对主链（15）N弛豫时间的测量以及通过具有轴向扩散各向异性的无模型形式对这些测量结果的解释进一步揭示，在围绕两个保守二硫键以及构成TGFβ结合表面的几个残基处，存在从毫秒到微秒时间尺度的显著运动。这些观察结果共同表明，结合可能通过一种具有小部分诱导契合特征的机制发生，即受体内的灵活性促进了向结合TGFβ状态的转变。
