Overview of ggFcR
Gallus gallus Fc receptor (ggFcR) was unexpectedly located to chromosome 20, a location unrelated to the LRC or FcR loci. The receptor consists of four extracellular Ig-like domains, and its binding site in IgY-Fc, mapped by mutagenesis, was found to involve residues of Cυ3 that suggest a structurally homologous location to the binding sites of FcεRI in Cε3 and Fcγ R in Cγ2. Although ggFcR bound moderately well to Fcυ3-4, residues in Cυ2 were also found to contribute to stronger binding to the whole IgY-Fc, and thus the ggFcR binding mode is slightly different. This similarity to IgE/FcεRI and IgG/Fcγ R is depicted in, although of course, the involvement of the Cυ2 domains will depend critically upon whether IgY-Fc is bent like IgE-Fc, as it is in the FcεRI complex, or in a more extended conformation.
Function of ggFcR
IgY is related through divergence of both structure and function to the other antibody isotypes (IgA, IgG, and IgE) for which receptor complexes are structurally characterized. The gene for ggFcR is the only one discovered on chromosome 20 to date that has the characteristics of an Fc receptor: extracellular Ig-like domains that bind antibodies, a transmembrane domain, and the ability to transmit an activating signal. It is thought to have translocated from the LRC, thus originating from a different cluster from mammalian Fcγ R and FcεRI. Any divergent evolutionary relationship is presumably very distant. However, to the extent that ggFcR and mammalian FcR bind at structurally homologous sites on their respective antibodies and can transmit an activating signal, both structural and functional convergence is clear, although the receptors have different tissue distributions and their functions may well differ. In the evolution of the relationship between IgY and its receptors, there is evidence of both convergence and divergence, even working independently with respect to both structure and function.
Fig.1 Comparison of putative IgY-receptor interactions with IgE-, IgG- and IgA-receptor complexes. (Zhang, 2017)
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Reference
- Zhang, X.; et al. IgY: a key isotype in antibody evolution. Biol Rev Camb Philos Soc. 2017, 92(4): 2144-2156.
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