Model of the L 2 Integrin I-Domain/ICAM-1 DI Interface Suggests That Subtle Changes in Loop Orientation Determine Ligand Specificity

The interaction of the L 2 integrin with its cellular ligand the intercellular adhesion molecule-1 (ICAM-1) is critical for the tight binding interaction between most leukocytes and the vascular endothelium before transendothelial migration to the sites of inflammation. In this article we have modeled the L subunit I-domain in its active form, which was computationally docked with the D1 domain of the ICAM-1 to probe potential protein-protein interactions. The experimentally observed key interaction between the carboxylate of Glu 34 in the ICAM-1 D1 domain and the metal ion-dependent adhesion site (MIDAS) in the open L I-domain was consistently reproduced by our calculations. The calculations reveal the nature of the L 2/ICAM-1 interaction and suggest an explanation for the increased ligand-binding affinity in the “open” versus the “closed” conformation of the L I-domain. A mechanism for substrate selectivity among L, M, and 2 I-domains is suggested whereby the orientation of the loops within the I-domain is critical in mediating the interaction of the Glu 34 carboxylate of ICAM-1 D1 with the MIDAS. Proteins 2002;48:151–160. © 2002 Wiley-Liss, Inc.