On the Cover: Model of the extracellular domains of Fc&agr;RI showing IgA binding residues. The ectodomains of the Fc&agr;R model are shown with the &agr; carbon trace as a blue ribbon. The membrane-proximal, transmembrane, and cytoplasmic regions have not been modeled. The side chains of residues mutated in this study that are important in IgA binding are shown as solid symbols. The residues marked on the ribbon in gray produced no significant change in IgA binding activity when mutated to alanine. The His residues marked on the ribbon in magenta produced no change in IgA binding activity when mutated to alanine or glutamate in our previous study. The labeled residues colored red were essential for binding (>100-fold decrease in affinity). Orange and yellow residues were of lesser importance in IgA binding when tested by mutation to alanine. All these residues lie on a single face of the receoptor, with the F-G loop forming a prominent bulge. The proposed orientation allows the binding surface to be displayed away from the cell surface. B is related to A by 90° rotation about the y-axis. (Figure 7 from Wines, B. D., C. T. Sardjono, H. M. Trist, C. S. Lay, and P. M. Hogarth. The interaction of Fc&agr;RI with IgA and its implications for ligand binding by immunoreceptors of the leukocyte receptor cluster. 1781.)