The structure of the β-propeller domain and C-terminal region of the integrin αM subunit -: Dependence on β subunit association and prediction of domains

The alpha M subunit of integrin Mac-1 contains several distinct regions in its extracellular segment. The N-terminal region has been predicted to fold into a beta-propeller domain composed of seven beta-sheets each about 60 amino acid residues long, with the I-domain inserted between beta-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 alpha M subunit on association with the beta 2 subunit in the alpha M/beta 2 heterodimer. All of the mAbs to the I-domain immunoprecipitated the unassociated alpha M precursor and reacted with the alpha M subunit expressed alone on the surface of COS cells. By contrast, four mAbs to the beta-propeller domain did not react with the unassociated alpha M precursor nor with the uncomplexed alpha M subunit expressed on COS cell surface. The four mAbs were mapped to three subregions in three different beta-sheets, making it unlikely that each recognized an interface between the alpha and beta subunits. These results suggest that folding of different beta-propeller subregions is coordinate and is dependent on association with the beta 2 subunit. The segment C-terminal to the beta-propeller domain, residues 599-1092, was studied with nine mAbs. A subset of four mAbs that reacted with the alpha M/beta 2 complex but not with the unassociated alpha M subunit were mapped to one subregion, residues 718-759, and five other mAbs that recognized both the unassociated and the complexed alpha M 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 beta-propeller domain folds independently of association with the beta 2 subunit. Our data provide new insights irate how different domains in the integrin alpha and beta subunits may interact.