Three-dimensional structure of the Mcg IgG1 immunoglobulin

The three-dimensional structure of an IgG1(lambda) immunoglobulin from a patient (Mcg) with amyloidosis was determined at 6.5-A resolution with X-ray diffraction techniques. The protein crystallized from water in the space group C2221, with a = 87.8, b = 111.3 and c = 186.3 A; the crystallographic asymmetric unit was a half-molecule consisting of one light and one heavy chain. The structure was solved by the multiple isomorphous replacement method with five heavy-atom derivatives. Electron density maps were interpreted with the aid of a protein modeling system used in conjunction with an Evans and Sutherland Picture System II graphics station. IgG1 molecules were tightly packed in the crystal lattice, with numerous intermolecular contacts. The two-fold axis relating identical halves of each molecule was found to be parallel to the y crystallographic axis. Electron density modules collectively representing one molecule were identified as three lobes representing the two antigen-binding (Fab) arms and the Fc region. An interchain disulfide bond connecting the two CL domains was located on the molecular diad and used as a landmark in the interpretation of the electron density map. A computer graphics method was developed to produce a solid image model of the IgG1 molecule in any prescribed orientation.