X-ray structural analysis of compensating mutations at the barnase-barstar interface.

The crystal structure of the barstar mutants (Y29P) and (Y29D, Y30W) as well as that of the complexes of barstar(Y29P) with wild-type barnase and barnase(H102K) have been determined. These barstar mutants compensate for the dramatic loss of barnase-barstar interaction energy caused by a single mutation of the barnase active site His-102 to a lysine. The latter introduces an uncompensated charge in the pocket at the surface of barstar where Lys-102 is located. The analysis of the structures suggests a mechanism for this compensation based on the solvation of the charge of Lys-102. Additional compensation occurs through the formation of a hydrogen bond.