Barnase

Barnase is a bacterial protein that consists of 110 amino acids and has ribonuclease activity. It is synthesized and secreted by the bacterium Bacillus amyloliquefaciens, but is lethal to the cell when expressed without its inhibitor barstar. The inhibitor binds to and occludes the ribonuclease active site, preventing barnase from damaging the cell's RNA after it has been synthesized but before it has been secreted. The barnase/barstar complex is noted for its extraordinarily tight protein-protein binding. The folding of barnase has been extensively studied in the laboratory of Alan Fersht, who used it as the test case in developing a method of characterizing protein folding transition states known as phi value analysis.