Binding properties of protein Arp, a bacterial IgA-receptor

A cell surface receptor that binds to the Fc region of IgA is expressed by certain strains of group A streptococci. The physico-chemical properties and binding characteristics of this receptor, called protein Arp, were studied. Like bacterial receptors that bind IgG, protein Arp has an elongated shape and no disulfide bonds. The affinity constant of protein Arp for three different molecular forms of IgA was determined, and was found to be more than ten-fold higher for serum IgA than for two complexed forms of IgA: secretory IgA and IgA bound to alpha 1-microglobulin. Cleavage of protein Arp with CNBr resulted in a peptide corresponding to the region located outside the cell wall, except for the N-terminal 52 amino acids. This CNBr-fragment did not bind IgA, which strongly suggests that the IgA-binding region of protein Arp is located in the N-terminal part of the molecule. In addition to the binding of IgA, protein Arp also binds to IgG weakly. The pH-dependence of these two types of binding is different, with maximal binding of IgA at neutral pH (5-7) and maximal binding of IgG at acidic pH (3-5). Both for IgA and IgG, protein Arp shows strong specificity for immunoglobulins of human origin.