Ultrastructure and function of dimeric, soluble intercellular adhesion molecule-1 (ICAM-1)

Previous studies have demonstrated dimerization of intercellular adhesion m olecule-1 (ICAM-1) on the cell surface and suggested a role for immunoglobu lin superfamily domain 5 and/or the transmembrane domain in mediating such dimerization. Crystallization studies suggest that domain I may also mediat e dimerization. ICAM-1 binds through domain 1 to the I domain of the integr in alpha (L)beta (2) (lymphocyte function-associated antigen 1). Soluble C- terminally dimerized ICAM-1 was made by replacing the transmembrane and cyt oplasmic domains with an a-helical coiled coil. Electron microscopy reveale d C-terminal dimers that were straight, slightly bent, and sometimes U-shap ed. A small number of apparently closed ring-like dimers and W-shaped tetra mers were found. To capture ICAM-1 dimerized at the crystallographically de fined dimer interface in domain 1, cysteines were introduced into this inte rface. Several of these mutations resulted in the formation of soluble disu lfide-bonded ICAM-1 dimers (domain 1 dimers). Combining a domain I cysteine mutation with the C-terminal dimers (domain 1/C-terminal dimers) resulted in significant amounts of both closed ring-like dimers and W-shaped tetrame rs. Surface plasmon resonance studies showed that all of the dimeric forms of ICAM-1 (domain 1, C-terminal, and domain 1/C-terminal dimers) bound simi larly to the integrin alpha (L)beta (2) I domain, with affinities similar t o1.5-3-fold greater than that of monomeric ICAM-1. These studies demonstrat e that ICAM-1 can form at least three different topologies and that dimeriz ation at domain 1 does not interfere with binding in domain I to alpha (L)b eta (2).