Introduction of FcRY
Numerous structurally diverse cell surface receptors have the ability to bind the constant regions of Ig, such as the classical Fc receptor family (FcR). FcRs are involved in various immune reactions, including phagocytosis, antibody-dependent cellular cytotoxicity, and immediate hypersensitivity. In mammalian FcRs, FcγR and FcRn can specifically bind to IgG where they can act as humoral immunity regulators and maternal IgG transporters, respectively. FcRn is one of a few transcytotic receptors known to carry out physiologically relevant bidirectional transcytosis. The mammalian FcRn receptors bind the IgG of various mammals and the IgG subclasses with very different affinities, but chicken IgY is not bound.
Despite similarities in functions and binding properties, FcRY and mammalian FcRn are not related in sequence or domain organization. Instead, FcRY is the chicken counterpart of the mammalian M (muscle)-type secretory phospholipase A2 receptor (PLA2 R), which is a member of the mannose receptor (MR) family of proteins. Mammalian MR family members mediate a variety of functions, including roles in the innate and adaptive immune system (MR), internalization of soluble PLA2 enzymes, presentation of antigens to T cells, and remodeling of the extracellular matrix. From Genbank, it can be further shown that FcRY and chicken PLA2R-1 could be the same substance just in different names since they have the same gene sequences.
Fig 1. Comparison of the binding of IgY to FcRY and the binding of IgG to FcRn. (Zhang, 2017)
The Architecture of FcRY
FcRY is a type I transmembrane protein that shares 55% amino acid sequence identity with the human PLA2 R. Same to all mammalian PLA2 Rs, FcRY has a common domain architecture with other members of the MR family: an N-terminal signal sequence followed by a cysteine-rich domain, a single fibronectin type II domain, eight to ten C-type lectin-like domains (CTLDs), a transmembrane domain, and a cytoplasmic tail containing about 40 residues. The CTLDs in PLA2 R mediate a protein-protein interaction independently of carbohydrates, and FcRY is thought to recognize IgY through protein-protein interaction. The avian FcRY cytoplasmic domain includes two potential endocytosis motifs: an acid-based dihydrophobic sequence and a low-density lipoprotein receptor-like tyrosine-based internalization motif. Besides the yolk sac membrane, FcRY expression was also observed in liver, ovary, oviduct, and spleen and lower levels of expression were observed in most tissues, but no expression in brain and cecum. This may reflect a role protecting IgY from catabolism which is similar to FcRn’s function to IgG in mammals.
Fig 2. FcRY structure. (He, 2011)
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References
- Zhang, X., et al. IgY: a key isotype in antibody evolution. Biological reviews of the Cambridge Philosophical Society. 2017, 92(4): 2144–2156.
- He, Y., Bjorkman, P. J. Structure of FcRY, an avian immunoglobulin receptor related to mammalian mannose receptors, and its complex with IgY. Proceedings of the National Academy of Sciences of the United States of America. 2011, 108(30): 12431–12436.
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