Johann Deisenhofer abstract: The model of human Fc fragment was refined at 2.9-Á resolution. Two different automated procedures for crystallographic refinement were used [Deisenhofer, J., & Steigemann, W.(1975) Acta Crystallogr., Sect. B B31, 238; Jack, A., & Levitt, M.(1978) Acta Crystallogr., Sect. A A34, 931], The final R value is 0.22. The dimer of CH3 domains closely resembles the CHI-CL aggregate in Fab fragments. There is no contact between CH2 domains. The contact be-tween CH2 and CH3 domains has about one-third of the size of the CH3-CH3 contact. The carbohydrate, a branched chain of nine hexose units, covers part of the C-contact face of the CH2 domain, shielding hydrophobic residues on this surface. Six atoms of the carbohydrate are within hydrogen-bonding distance of atoms in the CH2 domain. Crystallographic refinement of the complex between Fc fragment and fragment B of protein A from Staphylococcus aureus reduced the R value of the model to 0.24. A major part of the structure of fragment B consists of two a helices; the rest of the polypeptide chain is folded irregularly. In the crystal, fragment B forms two contacts with Fc fragment molecules. Contact 1 involves residues from both helices of fragment B, and residues from the CH2 and CH3 domains of Fc, and is predominantly hydrophobic. Contact 2 is smaller than contact 1. Residues from the second helix and adjacent residues of fragment B and residues only from the CH3 domain of Fc contribute to contact 2. The nature of contact 2 is mainly polar and includes a sulfate ion. There are strong arguments that contact 1 is the fragment B-Fc contact formed in solution under physiological conditions, while contact 2 is a crystal contact. e Fc fragment of an immunoglobulin molecule is the dimer of the twoC-terminal constant-homologyregions of the heavy chain. Among its physiological functions are interactions with the complement system and with specific receptors on the surface of a variety of cells. The crystal structure of an IgG Fc fragment, obtained from pooled human serum, was studied at 4-(Deisenhofer et al., 1976a) and at 3.5-Á resolution (Deisenhofer et al., 1976b) with the method of multiple isomorphous replacement. These studies revealed the quaternary structure of the molecule and the location of the carbohydrate moiety, which is covalently attached to the CH2 domain. The folding of the polypeptide chain was described on the basis of a preliminary atomic model.

Protein A is a constituent of the cell wall of Staphylococcus aureus. Part of its polypeptide chain is a series of four highly homologous regions (Sjodahl, 1977), each of which is able to bind to the Fc part of IgG from various species. Protein A is widely used as a tool in immunochemical and related studies. Its biologicalrole is unknown. The crystal structure of the complex formed by human Fc fragment and fragment B of protein A was solved by using multiple isomorphous replacement and the known model of