The Structure of the b-Propeller Domain and C-terminal Region of the Integrin aM Subunit

The aM subunit of integrin Mac-1 contains several distinct regions in its extracellular segment. The N-terminal region has been predicted to fold into a b-propeller domain composed of seven b-sheets each about 60 amino acid residues long, with the I-domain inserted between b-sheets 2 and 3. The structure of the C-terminal region is unknown. We have used monoclonal antibodies (mAbs) as probes to study the dependence of the structure of different regions of the aM subunit on association with the b2 subunit in the aM/b2 heterodimer. All of the mAbs to the I-domain immunoprecipitated the unassociated aM precursor and reacted with the aM subunit expressed alone on the surface of COS cells. By contrast, four mAbs to the b-propeller domain did not react with the unassociated aM precursor nor with the uncomplexed aM subunit expressed on COS cell surface. The four mAbs were mapped to three subregions in three different b-sheets, making it unlikely that each recognized an interface between the a and b subunits. These results suggest that folding of different b-propeller subregions is coordinate and is dependent on association with the b2 subunit. The segment C-terminal to the b-propeller domain, residues 599–1092, was studied with nine mAbs. A subset of four mAbs that reacted with the aM/b2 complex but not with the unassociated aM subunit were mapped to one subregion, residues 718– 759, and five other mAbs that recognized both the unassociated and the complexed aM subunit were localized to three other subregions, residues 599–679, 820–882, and 943–1047. This suggests that much of the region C-terminal to the b-propeller domain folds independently of association with the b2 subunit. Our data provide new insights into how different domains in the integrin a and b subunits may interact.