Identification of Residues within the Extracellular Domain 1 of Bovine Fcγ2R Essential for Binding Bovine IgG2

Abstract

Neutrophils and monocytes in cattle express a novel class of immunoglobulin Fc receptor, specific for bovine IgG2 (bIgG2), termed bFc␥2R. In cows, the ability of neutrophils to kill immunoglobulin-opsonized microorganisms appears to depend largely on this subclass, whose interaction with bFc␥2R initiates the killing process. bFc␥2R is a transmembrane glycoprotein consisting of two extracellular immunoglobulin-like domains, followed by a 19-amino acid membrane-spanning region and a short cytoplasmic tail. Although related to other mammalian Fc␥Rs, bFc␥2R belongs to a novel gene family that includes the human killer cell inhibitory receptor and Fc␣RI (CD89) proteins. We have shown previously (Morton, H. C., van Zandbergen, G., van Kooten, C., Howard, C. J., van de Winkel, J. G., and Brandtzaeg, P. (1999) J. Exp. Med. 189, 1715-1722) that like these proteins (and unlike other Fc␥Rs), bFc␥2R binds bIgG2 via the membrane-distal extracellular domain 1 (EC1). In this present study, we introduced mutations into the predicted loop regions of the EC1 domain and assayed the resulting bFc␥2R mutants for their ability to bind bIgG2. Our results indicated that the bIgG2 binding site lies within the predicted F-G loop region of the EC1 domain. Furthermore, single amino acid mutational analysis of this region identified Phe-82 and Trp-87 as being critical for bIgG2 binding.

Chicken IgY Binds Its Receptor at the CH3/CH4 Interface Similarly as the Human IgA:FcαRI Interaction

“…Firstly, the CHIR-AB1 is located within the chicken LRC. This locus contains the Fc␣RI in those mammals where it has not been deleted (24); in addition, it harbors the bovine Fc␥2R, a receptor specific for bovine IgG2 (25). All of these receptors share the respective binding site to Ig.…”

“…The LRC on human chromosome 19 may have been the first location of FcR genes. The remains in mammals are the Fc␣RI and the bovine Fc␥R2 (25). In man, the FcR genes are located in a cluster on chromosome 1.…”

Characterization of the Ligand Binding Site of the Bovine IgA Fc Receptor (bFcαR)

“…In the case of bFc␥2R, ligand binding appears to depend on two residues in the F-G loop region and mutation of either Phe-82 or Trp-87 abolishes binding to bovine IgG2 (14). Thus, by comparison with other closely related FcRs, we identified a number of residues that we believe may be important for IgA 3.…”

“…IgA Binding-Previous mutational analysis of FcRs most closely related to bFc␣R, namely CD89 and bFc␥2R, has identified several residues within their EC1 domains important for ligand binding (11,13,14). For CD89, these studies have shown that residues Tyr-35 (in the B-C loop) and Arg-82 (in the F-G loop) are essential for IgA binding, whereas His-85 (also in the F-G loop) contributes to the binding (Fig.…”

“…Both receptors also have short cytoplasmic tails lacking other recognized signaling motifs. A further striking characteristic of CD89 (and also the closely related bFc␥2R) is that, in contrast to other FcRs, the ligand binding site is located in the membrane distal EC1 domain (11)(12)(13)(14). In this study, we have used mutational analysis to map the IgA binding site of bFc␣R.…”