RECOGNITION BETWEEN A BACTERIAL RIBONUCLEASE, BARNASE, AND ITS NATURAL INHIBITOR, BARSTAR

Background: Protein-protein recognition is fundamental to most biologi cal processes. The information we have so far on the interfaces betwee n proteins comes largely from several protease-inhibitor and antigen-a ntibody complexes. Barnase, a bacterial ribonuclease, and barstar, its natural inhibitor, form a tight complex which provides a good model f or the study and design of protein-protein non-covalent interactions. Results: Here we report the structure of a complex between barnase and a fully functional mutant of barstar determined by X-ray analysis. Ba rstar is composed of three parallel alpha-helices stacked against a th ree-stranded parallel beta-sheet, and sterically blocks the active sit e of the enzyme with an alpha-helix and adjacent loop. The buried surf ace in the interface between the two molecules totals 1630 Angstrom(2) . The barnase-barstar complex is predominantly stabilized by charge in teractions involving positive charges in the active site of the enzyme . Asp39 of barstar binds to the phosphate-binding site of barnase, mim icking enzyme-substrate interactions. Conclusion: The phosphate-bindin g site of the enzyme is the anchor point for inhibitor binding. We pro pose that this is also likely to be the case for other ribonuclease in hibitors.