Norito Kotani

Swinging Arms of Antibody IGG Make the Antigen Binding Docile

High-speed AFM (HS-AFM) had been applied to observe dynamic behavior of biomolecules as movie (1, 2). HS-AFM can image directly in nano-scale without chemical fixing or fluorescence.Antibody IgG molecule is a “Y” shape protein. It has two Fab regions and one Fc region. Fab regions bind to the antigens. Hinge region connects a Fab region to a Fc region.We have observed IgG in solution using HS-AFM. “Y” shape of IgG was imaged clearly, and the difference of Fab and Fc structures was distinguished. HS-AFM has high resolution on both time and scale.The Fab regions moved in torsional direction like swinging arms in HS-AFM movie. This behavior depends on soft structure of hinge regions. Antibodies move their arms like swiveling in solution, and/or groping for the antigens on surface.We have identified the swinging characteristic of hinge region. We analyzed the Fab swivel movements as random walks, and estimated the flexibility of the IgG hinge region. The flexible nature of hinge region contributes to binding Fab to antigen. The lacking of swing movement would lead to reduce binding between antibody and antigen (3).We imaged antibody-antigen interaction. In this condition, antibodies were obserbed flexible movement. In binding condition, Fab regions bound to antigen rigitly, Fc region swang flexibly in solution.HS-AFM can directly observe dynamic behaviors of biomolecules as movie in solution, and reveal functions in detail. Direct and dynamic imaging have great advantages to study properties of proteins.1. T. Ando et al., Proc. Natl. Acad. Sci. U.S.A. 98, 12468- (2001).2. N. Kodera et al., Nature 468: 72- (2010).3. J. Preiner et al., Nature Communications 5: 4394- (2014).