High-Speed AFM Observation of Antibody IGG Characteristic of Swinging Arms

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 the Fc region.High-speed AFM (HS-AFM), developed by Prof. Ando in Kanazwa University, can observe dynamic behavior of motor protein, myosin as movie without chemical fixing or stain treatment (1, 2).We observed IgG in solution using HS-AFM. “Y” shape of IgG was imaged clearly, and Fab and Fc regions were distinguished. The Fab regions moved in torsional direction like swinging arms. This behavior depends on flexible structure of hinge regions. 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 for the antibody to bind to the antigen. For the first time, we have identified the swinging nature of this soft structure, which is important for antibody function. The lacking of swing movement would lead to reduce binding between antibody and antigen (3).HS-AFM can directly observe dynamic behaviors of biomolecules as movie in solution, and reveal functions in detail.1. T. Ando et al., Proc. Natl. Acad. Sci. USA. 98, 12468- (2001).2. N. Kodera et al., Nature 468: 72- (2010)3. J. Preiner et al., Nature Communications 5: 4394- (2014).