1brs
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PROTEIN-PROTEIN RECOGNITION: CRYSTAL STRUCTURAL ANALYSIS OF A BARNASE-BARSTAR COMPLEX AT 2.0-A RESOLUTION
OverviewOverview
We have solved, refined, and analyzed the 2.0-a resolution crystal, structure of a 1:1 complex between the bacterial ribonuclease, barnase, and a Cys-->Ala(40,82) double mutant of its intracellular polypeptide, inhibitor, barstar. Barstar inhibits barnase by sterically blocking the, active site with a helix and adjacent loop segment. Almost half of the 14, hydrogen bonds between barnase and barstar involve two charged residues, and a third involve one charged partner. The electrostatic contribution to, the overall binding energy is considerably greater than for other, protein-protein interactions. Consequently, the very high rate constant, for the barnase-barstar association (10(8) s-1 M-1) is most likely due to, electrostatic steering effects. The barnase active-site residue His102 is, located in a pocket on the surface of barstar, and its hydrogen bonds with, Asp39 and Gly31 residues of barstar are directly responsible for the pH, dependence of barnase-barstar binding. There is a high degree of, complementarity both of the shape and of the charge of the interacting, surfaces, but neither is perfect. The surface complementarity is slightly, poorer than in protease-inhibitor complexes but a little better than in, antibody-antigen interactions. However, since the burial of solvent in the, barnase-barstar interface improves the fit significantly by filling in the, majority of gaps, as well as stabilizing unfavorable electrostatic, interactions, its role seems to be more important than in other, protein-protein complexes. The electrostatic interactions between barnase, and barstar are very similar to those between barnase and the, tetranucleotide d(CGAC). In the barnase-barstar complex, the two, phosphate-binding sites in the barnase active site are occupied by Asp39, and Gly43 of barstar. However, barstar has no equivalent for a guanine, base of an RNA substrate, resulting in the occupation of the guanine, recognition site in the barnase-barstar complex by nine ordered water, molecules. Upon barnase-barstar binding, entropy losses resulting from the, immobilization of segments of the protein chain and the energetic costs of, conformational changes are minimized due to the essentially preformed, active site of barnase. However, a certain degree of flexibility within, the barnase active site is required to allow for the structural, differences between barnase-barstar binding and barnase-RNA binding. A, comparison between the bound and the free barstar structure shows that the, overall structural response to barnase-binding is significant. This, response can be best described as outwardly oriented, rigid-body movements, of the four alpha-helices of barstar, resulting in the structure of bound, barstar being somewhat expanded.
About this StructureAbout this Structure
1BRS is a Protein complex structure of sequences from Bacillus amyloliquefaciens. Full crystallographic information is available from OCA.
ReferenceReference
Protein-protein recognition: crystal structural analysis of a barnase-barstar complex at 2.0-A resolution., Buckle AM, Schreiber G, Fersht AR, Biochemistry. 1994 Aug 2;33(30):8878-89. PMID:8043575
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