Structural study of the Ly49A T cell recognition domain

以前的晶体研究表明，Ly49A同源二聚体以不对称的方式结合一个MHC分子，而Ly49C同源二聚体以对称的方式结合两个MHC。此外，结合的受体采取明显不同的同源二聚体形式：Ly49A的闭合状态和Ly49C的开放状态。MHC分子之间的空间碰撞会阻止闭合的Ly49A二聚体以开放的Ly49C二聚体的方式与两个MHC结合。为了解决单个Ly49受体是否可以经历构象转换，使它们以不同的方式结合MHC，我们在偶极耦合技术的帮助下，对未结合的Ly49A进行了溶液核磁共振研究。这项研究表明，在溶液中，未连接的Ly49A采用对称的开放状态的同源二聚体构象，类似于以前看到的Ly49C。因此，Ly49A既可以处于关闭状态，也可以处于打开状态。为了研究Ly49A二聚体是否能与溶液中的两个MHC分子结合，除了观察到晶体中的一个MHC分子结合外，我们还进行了分析超速离心实验。速度沉淀表明Ly49A二聚体可以与两个MHC在溶液中结合，这与核磁共振结果表明未结合的Ly49A主要存在于开放状态相一致。

Previous crystal studies revealed that the Ly49A homodimer binds one MHC molecule in an asymmetric interaction, whereas the Ly49C homodimer binds two MHC in a symmetrical fashion. Moreover, the bound receptors adopt distinctly different homodimeric forms: a closed state for Ly49A and an open state for Ly49C. Steric clashes between MHC molecules would preclude the closed Ly49A dimer from engaging two MHC in the manner of the open Ly49C dimer. To address whether individual Ly49 receptors can undergo a conformational switch enabling them to bind MHC in different ways, we carried out a solution NMR study of unbound Ly49A, aided by dipolar coupling technology. This study reveals that in solution unligated Ly49A adopts a symmetric, open state, homodimer conformation similar to that previously seen for Ly49C. Hence, Ly49A can assume both closed and open states. To address whether the Ly49A dimer can bind two MHC molecules in solution, besides the binding of one MHC observed in the crystal, we carried out analytical ultracentrifugation experiments. Velocity sedimentation demonstrates that the Ly49A dimer can engage two MHC in solution, in agreement with NMR results showing that unbound Ly49A exists predominantly in the open state.