FcRY

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. 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 Graphical illustration of the IgY transport pathway in the ovarian follicle of laying birds.¹

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.

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Reference

1. Okamoto, Mayuko, et al. "FcRY is a key molecule controlling maternal blood IgY transfer to yolks during egg development in avian species." Frontiers in Immunology 15 (2024): 1305587. Distributed under Open Access license CC BY 4.0, without modification.

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