ABSTRACT
Dimeric intercellular adhesion molecule-1 (ICAM-1) has been known to more efficiently mediate cell adhesion than monomeric ICAM-1. Here, we found that truncation of the intracellular domain of ICAM-1 significantly enhances surface dimerization based on the two criteria: 1) the binding degree of monomer-specific antibody CA-7 and 2) the ratio of dimer/monomer when a mutation (L42âC42) was introduced in the interface of domain 1. Mutation analysis revealed that the positively charged amino acids, including very membrane-proximal 5°5R, are essential for maintaining the structural transition between the monomer and dimer. Despite a strong dimer presentation, the ICAM-1 mutants lacking an intracellular domain (IC1ΔCTD) or containing R to A substitution in position 505 (5°5R/A) supported a lower degree of cell adhesion than did wild-type ICAM-1. Collectively, these results demonstrate that the native structure of surface ICAM-1 is not a dimer, but is an intermediate monomer-dimer equilibrium structure by which the effectiveness of ICAM-1 can be fully achieved.
Subject(s)
Cell Adhesion , Intercellular Adhesion Molecule-1/chemistry , Amino Acid Substitution , Animals , Base Sequence , CHO Cells , COS Cells , Cell Adhesion/physiology , Cell Line , Cell Line, Transformed , Chlorocebus aethiops , Cricetinae , Cricetulus , DNA, Complementary , Dimerization , Endothelium, Vascular/cytology , Humans , Intercellular Adhesion Molecule-1/genetics , Intercellular Adhesion Molecule-1/metabolism , Jurkat Cells , Kidney/cytology , Macromolecular Substances/metabolism , Models, Structural , Molecular Sequence Data , Protein Structure, Tertiary/genetics , RNA, Small Interfering/metabolism , Transfection , Umbilical Veins/cytologyABSTRACT
Little is known about how adhesion molecules on APCs accumulate at immunological synapses. We show here that ICAM-1 on APCs is continuously internalized and rapidly recycled back to the interface after antigen-priming T-cell contact. The internalization rate is high in APCs, including Raji B cells and dendritic cells, but low in endothelial cells. Internalization is significantly reduced by inhibitors of Na(+)/H(+) exchangers (NHEs), suggesting that members of the NHE-family regulate this process. Once internalized, ICAM-1 is co-localized with MHC class II in the polarized recycling compartment. Surprisingly, not only ICAM-1, but also MHC class II, is targeted to the immunological synapse through LFA-1-dependent adhesion. Cytosolic ICAM-1 is highly mobile and forms a tubular structure. Inhibitors of microtubule or actin polymerization can reduce ICAM-1 mobility, and thereby block accumulation at immunological synapses. Membrane ICAM-1 also moves to the T-cell contact zone, presumably through an active, cytoskeleton-dependent mechanism. Collectively, these results demonstrate that ICAM-1 can be transported to the immunological synapse through the recycling compartment. Furthermore, the high-affinity state of LFA-1 on T cells is critical to induce targeted movements of both ICAM-1 and MHC class II to the immunological synapse on APCs.
