Dynamic Recruitment of Human CD2 into Lipid Rafts: LINKAGE TO T CELL SIGNAL TRANSDUCTION

Abstract

CD2 mediates T cell adhesion via its ectodomain and signal transduction utilizing its 117-amino acid cytoplasmic tail. Here we show that a significant fraction of human CD2 molecules is inducibly recruited into lipid rafts upon CD2 cross-linking by a specific pair of mitogenic anti-CD2 monoclonal antibodies (anti-T11(2) + anti-T11(3)) or during cellular conjugate formation by CD58, the physiologic ligand expressed on antigen-presenting cells. Translocation to lipid microdomains is independent of the T cell receptor (TCR) and, unlike inducible TCR-raft association, requires no tyrosine phosphorylation. Structural integrity of rafts is necessary for CD2-stimulated elevation of intracellular free calcium and tyrosine phosphorylation of cellular substrates. Whereas murine CD2 contains two membrane-proximal intracellular cysteines, partitioning CD2 into cholesterol-rich lipid rafts constitutively, human CD2 has no cytoplasmic cysteines. Mapping studies using CD2 point mutation, deletion, and chimeric molecules suggest that conformational change in the CD2 ectodomain participates in inducible raft association and excludes the membrane-proximal N-linked glycans, the transmembrane segment, and the CD2 cytoplasmic region (residues 8-117) as necessary for translocation. Translocation of CD2 into lipid rafts may reorganize the membrane into an activation-ready state prior to TCR engagement by a peptide associated with a major histocompatibility complex molecule, accounting for synergistic T cell stimulation by CD2 and the TCR.