A new software suite, called Crystallography & NMR System (CNS), has been developed for macromolecular structure determination by X-ray crystallography or solution nuclear magnetic resonance (NMR) spectroscopy. In contrast to existing structure-determination programs the architecture of CNS is highly flexible, allowing for extension to other structure-determination methods, such as electron microscopy and solid-state NMR spectroscopy. CNS has a hierarchical structure: a high-level hypertext markup language (HTML) user interface, task-oriented user input files, module files, a symbolic structure-determination language (CNS language), and low-level source code. Each layer is accessible to the user. The novice user may just use the HTML interface, while the more advanced user may use any of the other layers. The source code will be distributed, thus source-code modification is possible.The CNS language is sufficiently powerful and flexible that many new algorithms can be easily implemented in the CNS language without changes to the source code. The CNS language allows the user to perform operations on data structures, such as structure factors, electron-density maps, and atomic properties. The power of the CNS language has been demonstrated by the implementation of a comprehensive set of crystallographic procedures for phasing, density modification and refinement. User-friendly task-oriented input files are available for nearly all aspects of macromolecular (i') 1998 International Union of Crystallography Printed in Great Britain -all rights reserved structure determination by X-ray crystallography and solution NMR.
The hinge region on the Fc fragment of human immunoglobulin G interacts with at least four different natural protein scaffolds that bind at a common site between the C(H2) and C(H3) domains. This "consensus" site was also dominant for binding of random peptides selected in vitro for high affinity (dissociation constant, about 25 nanomolar) by bacteriophage display. Thus, this site appears to be preferred owing to its intrinsic physiochemical properties, and not for biological function alone. A 2.7 angstrom crystal structure of a selected 13-amino acid peptide in complex with Fc demonstrated that the peptide adopts a compact structure radically different from that of the other Fc binding proteins. Nevertheless, the specific Fc binding interactions of the peptide strongly mimic those of the other proteins. Juxtaposition of the available Fc-complex crystal structures showed that the convergent binding surface is highly accessible, adaptive, and hydrophobic and contains relatively few sites for polar interactions. These are all properties that may promote cross-reactive binding, which is common to protein-protein interactions and especially hormone-receptor complexes.
The small molecule binds to the same site that binds the IL-2 ␣ receptor and buries into a groove not seen in the free structure of IL-2. Comparison of the bound and several free structures shows this site to be composed of two subsites: one is rigid, and the other is highly adaptive. Thermodynamic data suggest the energy barriers between these conformations are low. The subsites were dissected by using a site-directed screening method called tethering, in which small fragments were captured by disulfide interchange with cysteines introduced into IL-2 around these subsites. X-ray structures with the tethered fragments show that the subsite-binding interactions are similar to those observed with the original small molecule. Moreover, the adaptive subsite tethered many more compounds than did the rigid one. Thus, the adaptive nature of a protein-protein interface provides sites for small molecules to bind and underscores the challenge of applying structure-based design strategies that cannot accurately predict a dynamic protein surface.
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