Characteristics of cation binding to the I domains of LFA-1 and MAC-1. The LFA-1 I domain contains a Ca2+-binding site.

Abstract The crystal structures of the I domains of integrins MAC-1 (αMβ2; CD11b/CD18) and LFA-1 (αLβ2; CD11a/CD18) show that a single conserved cation-binding site is present in each protein. Purified recombinant I domains have intrinsic ligand binding activity, and in several systems this interaction has been demonstrated to be cation-dependent. It has been proposed that the I domain cation-binding site represents a general metal ion-dependent adhesion motif utilized for binding protein ligands. Here we show that the purified recombinant I domain of LFA-1 (αLI) binds cations, but with significantly different characteristics compared with the I domain of MAC-1 (αMI). Both αLI and αMI bind54Mn2+ in a conformation-dependent manner, and in general, cations with charge and size characteristics similar to Mn2+ most effectively inhibit54Mn2+ binding. Surprisingly, however, physiological levels of Ca2+ (1–2 mm) inhibited 54Mn2+ binding to purified αLI, but not to αMI. Using45Ca2+ and 54Mn2+ in direct binding studies, the dissociation constants (K D) for the interactions between these cations and αLI were estimated to be 5–6 × 10−5and 1–2 × 10−5 m, respectively. Together with the available structural information, the data suggest differential affinities for Mn2+ and Ca2+binding to the single conserved site within αLI. Antagonism of LFA-1, but not MAC-1, -mediated cell adhesion by Ca2+ may be related to the Ca2+ binding activity of the LFA-1 I domain.