膜脂环境调控整合素构象与功能的研究

整合素是一类重要的细胞表面受体，介导细胞粘附与迁移。整合素由α和β亚基构成跨膜异源二聚体，其活性直接受控于它的跨膜区构象变化。β亚基跨膜区中保守的碱性残基与磷脂的磷酸根存在静电作用，维持整合素的抑制型构象。申请人所在研究组发现，钙离子可以直接结合磷脂的磷酸根，从而改变膜脂环境来调控膜受体的活化（Nature 2013）。于是我们推测钙离子也可以干扰β亚基与磷脂的静电结合，改变β亚基跨膜区的构象，从而导致整合素活化。本项目以淋巴细胞归巢受体整合素α4β7 为对象，研究膜脂环境对整合素结构和功能的调控。本项目将利用液相NMR解析α4β7跨膜区异源二聚体的三维结构；研究β7跨膜区与不同种类磷脂的相互作用；研究钙离子对α4β7构象的特异性调控；并用细胞粘附实验来验证体外发现。本项目的开展将使我们从一个全新的角度来理解细胞粘附和迁移的分子机制。

Integrin family is a major class of cell-surface receptors that mediate cell adhesion and migration. Integrins are heterodimeric transmembrane proteins containing α and β subunits. Integrin activity is directly regulated by its transmembrane domain topography. Recent studies find that integrin activity is regulated by the ionic interaction between the phosphate group of phospholipids and the conserved basic residue in the β transmembrane domain. This unusual ionic interaction keeps the β transmembrane domain at a tilted crossing angle and thus locks integrin on an inhibitory conformation. However, the structural details of the integrin-lipid interaction remain elusive. In T lymphocytes, Ca2+ signaling can activate integrin. Our previous study found that Ca2+ can directly bind to the phosphate group of phospholipids and thus regulate membrane receptor activity (Nature 2013). We therefore hypothesize that Ca2+ might disrupt integrin-phospholipid interaction by directly competing with the β subunit for the binding of the phosphate group of phospholipids, which should change the β subunit topology and result in integrin activation. In this project, we will focus on integrin α4β7, a lymphocyte homing receptor, to study the regulation of integrin structure and function by membrane lipids. We will use solution NMR to determine α4β7 transmemrbane heterodimer structure and study the ionic protein-lipid interaction between β7 and different phospholipid species. Moreover, we will use NMR to study how Ca2+ specifically regulate α4β7 structure and function by modulating phospholipids. Cell adhesion experiments will also be performed to verify our in vitro findings. The accomplishment of this project will greatly contribute to our further understanding of the molecular mechanism of cell adhesion and migration from a new point of view.