The present invention is a two-domain, bi-functional fusion protein that functions as a molecular switch wherein the free energy released by the folding of a first domain of the fusion protein drives an unfolding of a second domain of the fusion protein, and vice versa. The molecular structure of the fusion protein is engineered so that, at any time, the folding of the first domain necessarily unfolds the other domain, and vice versa, thereby making the folded and unfolded states of the first and second domains mutually exclusive. This is accomplished by the insertion of ubiquitin insert protein into a surface loop of barnase target protein subject to the structural design criterion that the N-C terminal length of the ubiquitin insert protein is at least two-times greater than the Cα-Cα alpha-carbon-alpha-carbon length of the surface loop of the barnase target protein.