Dimerization and the effectiveness of ICAM-1 in mediating LFA-1-dependent adhesion.
Dimeric intercellular adhesion molecule-1 (ICAM-1) binds more efficiently to lymphocyte function-associated antigen-1 (LFA-1) than monomeric ICAM-1. However, it is unknown whether dimerization enhances binding simply by providing two ligand-binding sites and thereby increasing avidity, or whether it serves to generate a single "fully competent" LFA-1-binding surface. Domain 1 of ICAM-1 contains both the binding site for LFA-1, centered on residue E34, and a homodimerization interface. Whether the LFA-1-binding site extends across the homodimerization interface has not been tested. To address this question, we constructed four different heterodimeric soluble forms of ICAM-1 joined at the C terminus via an alpha-helical coiled coil (ACID-BASE). These heterodimeric ICAM-1 constructs include, (i) E34/E34 (two intact LFA-1-binding sites), (ii) E34/K34 (one disrupted LFA-1-binding site), (iii) E34/DeltaD1-2 (one deleted LFA-1-binding site), and (iv) K34/K34 (two disrupted LFA-1-binding sites). Cells bearing activated LFA-1 bound similarly to surfaces coated with either E34/K34 or E34/DeltaD1-2 and with an approximately 2-fold reduction in efficiency compared with E34/E34, suggesting that D1 dimerization, which is precluded in E34/DeltaD1-D2, is not necessary for optimal LFA-1 binding. Furthermore, BIAcore (BIAcore, Piscataway, NJ) affinity measurements revealed that soluble open LFA-1 I domain bound to immobilized soluble ICAM-1, E34/E34, E34/K34, and E34/DeltaD1-D2 with nearly identical affinities. These studies demonstrate that a single ICAM-1 monomer, not dimeric ICAM-1, represents the complete, "fully competent" LFA-1-binding surface.
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Center for Blood Research and Department of Pathology, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA.
Shimaoka
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Carman
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Takagi
J
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Springer
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TA
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D016466:CHO Cells; D002448:Cell Adhesion; D006224:Cricetinae; D019281:Dimerization; D006801:Humans; D018799:Intercellular Adhesion Molecule-1 / Q000737:chemistry / Q000502:physiology*; D016169:Lymphocyte Function-Associated Antigen-1 / Q000737:chemistry / Q000502:physiology*; D008969:Molecular Sequence Data
D016169:Lymphocyte Function-Associated Antigen-1; D018799:Intercellular Adhesion Molecule-1