I87E Mutation Prevents Barstar Dimerization

The C40,82A;I87E mutant of barstar, an intracellular inhibitor of the ribonuclease barnase from Bacillus amyloliquefaciens, was obtained, and its physicochemical properties were studied. It was produced as a fusion protein with thioredoxin and then cleaved from this by EKmax enterokinase. The mutant was shown by NMR to retain the spatial structure of the wild-type protein but, in contrast to barstar, does not form the homodimers characteristic of barstar in aqueous solution. The mutant protein binds barnase with the dissociation constant (6.6 ± 1.1) × 10–11 M and exhibits other physicochemical properties similar to those of the wild-type barstar. This allows the use of C40,82A;I87E mutant instead of wild-type barstar in the investigations where the protein dimerization is undesirable.