3-DIMENSIONAL STRUCTURE OF A HUMAN-IMMUNOGLOBULIN WITH A HINGE DELETION

X-ray analysis at 3.2-angstrom resolution revealed that the Mcg IgG1 ( lambda chain) immunoglobulin is a compact T-shaped molecule. Because o f the hinge deletion, the Fc fragment lobe is pulled tightly upward in to the junction of the Fab arms. Along the molecular twofold axis, the Fab arms are joined by an interchain disulfide bond between the two l ight chains. The antigen combining sites consist of large irregular ca vities at the tips of the Fab regions. Potential complement (C1q) bind ing sites on Fc are sterically shielded by the Fab arms, but putative attachment sites are accessible for docking with the FcRI receptor on human monocytes and with protein A of Staphylococcus aureus.