Ligand-induced Conformational Change Within the CD2 Ectodomain Accompanies Receptor Clustering: Implication for Molecular Lattice Formation

Abstract

CD2 mediates interaction between T cells and their cognate partners through its CD58-binding membrane-distal adhesion domain (D1) facilitat ing T cell receptor (TCR) triggering. A neoepitope defined by anti-CD2R monoclonal antibodies (mAbs) has suggested structural alteration within the CD2 ectodomain during T cell activation. Here, we map CD2R to the flexible CD2 linker region between D1 and the membrane-proximal extracellular domain (D2) and show that exposure of this conformational site is independent of temperature and metabolic energy. Co-ligation of CD2 and CD58 molecules on opposing cells within a conjugate pair induces CD2R and redistributes CD2 to the region of cell-cell contact. These CD2R +molecules, in contrast to the CD2R −molecules, are tightly clustered on the T cell surface. Hence, a ligand-mediated increase in the D1-D2 interdomain angle apparently exposes CD2R, facilitates packing of CD2 molecules in a clustered array and is linked to CD2-mediated adhesion and activation events. Conformational alteration of this type may be generally important in ordered lattice formation involving surface receptors.