Functional evolution of antibody Fc domains: SAXS plots and models

Introduction: The Fc regions of antibodies contain binding sites
for receptors that facilitate effector functions after the Fab regions
engage with antigens. The IgG antibody features an extended
"hinge" region that provides flexibility between the Fab and Fc
domains. In contrast, both the more primitive IgM and the more
evolutionarily recent IgE replace this hinge with an additional pair of
domains within the homo-dimeric, six-domain Fc structure. This structural
adaptation allows for increased flexibility within the Fc region, which
nature has leveraged to regulate antibody effector functions. In the case
of pentameric or hexameric IgM, the Fc regions adopt a planar conformation
in solution until antigen binding induces a conformational change that
reveals complement binding sites. Conversely, IgE-Fc predominantly exists
in a sharply bent conformation in solution, with receptor binding
modulated by the extent of this bend and featuring allosteric interactions
between receptor binding sites. Methods: To investigate the
evolutionary trajectory of Fc conformational diversity from IgM to IgE
through the intermediary avian IgY, we employed small-angle X-ray
scattering to assess the solution conformations of their Fc regions. Our
study focused on four extant proteins: human IgM-Fc homo-dimer, chicken
IgY-Fc, platypus IgE-Fc, and human IgE-Fc. These proteins represent
evolutionary milestones: originating in jawed fish (425 million years
ago), tetrapods (310 million years ago), monotremes (166 million years
ago), and hominids (2.5 million years ago), respectively. Results and
Discussion: We analyzed the scattering curves by evaluating
contributions from a set of variously bent models identified through a
non-negative linear least-squares algorithm. Our findings reveal a
progressive increase in the proportion of acutely bent structures among
the proteins: IgM-Fc < IgY-Fc < platypus IgE-Fc <
human IgE-Fc, aligning with their evolutionary sequence. Human IgM-Fc
homo-dimer exhibits no acutely bent structures; however, a notable portion
of the protein is sufficiently bent to unveil the C1q binding site while
predominantly maintaining a fully extended conformation. In contrast,
human IgE-Fc is primarily acutely bent, consistent with previous studies.
The IgY-Fc, presented here in the first complete structural analysis of
its Fc region, displays a range of conformational states from acutely bent
to fully extended, underscoring its role as an evolutionary intermediary
between IgM and IgE.