Structure of the human antibody molecule kol (immunoglobulin G1): An electron density map at 5 Å resolution

An electron density map at 5 Å resolution of the human myeloma protein Kol (immunoglobulin G1) is reported. The density describing the Fab‡ fragments has been interpreted in terms of the known structures of a V_K fragment, Rei (Epp et al., 1974), and the mouse McPC603 Fab fragment (Segal et al., 1974). It shows 'a quaternary structure different from previously reported Fab fragments and provides evidence for flexibility in the switch peptides between variable and constant domains on heavy and light chains. No interpretable density is found C-terminal to the inter-heavy chain disulphide bonds, forcing the conclusion that the Fc fragment is disordered in the crystal lattice and implying flexibility in the Fc fragment in the residues immediately preceding the C_H2 domain. The only density not described by the Fab fragments is found linking the two Fab arms of one molecule across the 2-fold symmetry axis. This density can be interpreted as extending as far as the Cys-Pro-Pro-Cys peptide.