Mechanical unfolding pathways of the enhanced yellow fluorescent protein revealed by single molecule force spectroscopy

We used single molecule force spectroscopy to characterize the mechanical stability of the enhanced yellow fluorescent protein (EYFP) (a mutant form of the green fluorescent protein (GFP)) and two of its circularly permutated variants. In all three constructs, we found two main unfolding peaks; the first corresponds to a transition state placed close to the termini and the second to a transition state placed halfway through the molecule. We attribute the second transition state to the shear rupture of the beta1- and beta6-strands, which we verified by introducing a point mutation in this region. Although both unfolding peaks were observed in all three EYFP variants, their relative frequency of occurrence varied. Our results demonstrated that the mechanical unfolding pathways in EYFP could be deciphered through the use of circular permutation.