The crystal structure of an avian IgY-Fc fragment reveals conservation with both mammalian IgG and IgE.

Avian IgY is closely related to an ancestor of both mammalian IgG and IgE and thus provides insights into the evolution of antibody structure and function. A recombinant fragment of IgY-Fc consisting of a dimer of the C upsilon 3 and C upsilon 4 domains, Fc upsilon 3-4, was expressed and crystallized and its X-ray structure determined to 1.75 angstrom resolution. Fc upsilon 3-4 is the only nonmammalian Fc fragment structure determined to date and provides the first structural evidence for an ancient origin of antibody architecture. The Fc upsilon 3-4 structure reveals features common to both IgE-Fc and IgG-Fc, and the implications for IgY binding to its receptor are discussed.