Structural and Functional Roles of Transmembrane Domains in B-cell Receptor Signalling

B细胞受体(BCR)复合体由一个抗原结合亚单位(膜免疫球蛋白，MiG)和一个信号亚单位(CD79a-b异源二聚体)组成，是人类最重要的免疫受体之一，控制着B细胞的发育、活性、选择和死亡。然而，BCR信号传递的机制仍然是一个猜测的问题，部分原因是缺乏原子水平的结构数据，揭示了对功能受体组装、运输和信号传递至关重要的复合体区域，即跨膜结构域(TMD)。我们假设BCR复合体中三种蛋白质的TMD是强而特异(尚不清楚)相互作用的介体，并在这里概述了首次在分子水平上描述BCR TMD的结构和相互作用的工作。我们的假设是基于25年前的报告，这些报告暗示TM结构域是蛋白质-蛋白质和蛋白质-脂质相互作用的功能必需部位，但到目前为止还没有揭示出对这一作用的分子水平的理解。这项建议概述了BCR TMD的结构和相互作用的分子水平的生物物理研究，我们将使用这些研究来设计分子来调节BCR的功能。具体地说，我们将(I)表征天然膜中BCR复合体组件内部和组件之间TMD相互作用的强度和顺序依赖性，(Ii)利用一组生物物理方法来表征多种(合成)脂类环境中TMD相互作用的结构、稳定性、化学计量和原子细节，以及(Iii)使用这些新信息来设计和测试分子破坏相互作用并原位改变BCR信号和功能的能力。这项工作将产生第一张BCRTMD之间的相互作用图和不同脂质环境下BCRTMDs的第一个结构数据，从而增强我们对BCR信号转导机制的理解，并支持基础免疫学的努力。我们设计的分子将作为使用这种方法调节BCR是可能的概念的证明，并为未来更全面的药物发现计划提供平台，以期找到治疗自身免疫性疾病、B细胞白血病和淋巴瘤的新疗法。

The B-cell receptor (BCR) complex, comprised of an antigen-binding subunit (membrane immunoglobulin, mIg) and a signalling subunit (the CD79a-b heterodimer), is one of the most important immune receptors in humans and controls B-cell development, activity, selection and death. Yet the mechanism of BCR signalling remains a matter of speculation, partly because of a lack of atomic-level structural data that reveals regions of the complex critical for functional receptor assembly, transport, and signal transmission, namely the transmembrane domains (TMDs). We hypothesise that the TMDs of the three proteins in the BCR complex are mediators of strong and specific (yet uncharacterised) interactions, and outline here work that would yield a molecular-level description of the structures and interactions of the BCR TMDs for the first time. Our hypothesis is based on reports dating back over 25 years which implicate the TM domains as sites of functionally essential protein-protein and protein-lipid interactions, but have thus far not revealed a molecular level understanding of this role. This proposal outlines a molecular-level, biophysical investigation of the structures and interactions of the BCR TMDs, which we will use to design molecules to modulate BCR function. Specifically, we will (i) characterise the strength and sequence dependence of TMD interactions within and between components of the BCR complex in a natural membrane, (ii) utilise a panel of biophysical methods to characterise the structure, stability, stoichiometry and atomic details of TMD interactions in multiple (synthetic) lipid environments, and (iii) use this new information to design and test ability of molecules to disrupt interactions and modify BCR signalling and function in situ. This work will yield the first map of interactions between the BCR TMDs and the first structural data for the BCR TMDs in different lipid environments, thus enhancing our mechanistic understanding of BCR signalling and supporting efforts in basic immunology. The molecules we design would act as a proof of concept that BCR modulation is possible using this approach, and provide a platform for a more comprehensive drug discovery programme in the future towards new theraputic treatments for autoimmune diseases, B-cell leukaemias and lymphomas.