Expression of the H52 epitope on the β2 subunit is dependent on its interaction with the α subunits of the leukocyte integrins LFA-1, Mac-1 and p150,95 and the presence of Ca2+

Integrin-mediated adhesion is a divalent cation-dependent process. Whether divalent cations directly participate in ligand binding or exert their effects indirectly by affecting the overall structure of the integrin heterodimers is not known. In this study we describe the epitope of the mAb H52 which has been mapped to a predicted disulfide-bonded loop (C386 and C400) in the beta2 integrin subunit. In the presence of Ca2+ and Mg2+, the H52 epitope is expressed on the monomeric beta2 subunit, the LFA-1 and Mac-1 heterodimers but not on p150,95, thus implying that this epitope is masked in p150,95. However, expression of the H52 epitope on Mac-1, but not on LFA-1, or the monomeric beta2 subunit, is dependent on the presence of Ca2+, thus suggesting that the chelation of Ca2+ causes a conformational change in Mac-1 which results in the loss of the epitope. These results suggest that expression of the H52 epitope on the beta2 subunit is dependent on its interaction with the different alpha subunits. Since the epitope itself is not required for heterodimer formation nor for ligand binding, occupancy of a Ca2+ binding site(s) must therefore affect the alphabeta subunit interactions, and thus the overall conformation of Mac-1.