Dual energy landscape: the functional state of the beta-barrel outer membrane protein G molds its unfolding energy landscape.

摘要

We applied dynamic single-molecule force spectroscopy to quantify the parameters (free energy of activation and distance of the transition state from the folded state) characterizing the energy barriers in the unfolding energy landscape of the outer membrane protein G (OmpG) from Escherichia coli. The pH-dependent functional switching of OmpG directs the protein along different regions on the unfolding energy landscape. The two functional states of OmpG take the same unfolding pathway during the sequential unfolding of beta-hairpins I-IV. After the initial unfolding events, the unfolding pathways diverge. In the open state, the unfolding of beta-hairpin V in one step precedes the unfolding of beta-hairpin VI. In the closed state, beta-hairpin V and beta-strand S11 with a part of extracellular loop L6 unfold cooperatively, and subsequently beta-strand S12 unfolds with the remaining loop L6. These two unfolding pathways in the open and closed states join again in the last unfolding step of beta-hairpin VII. Also, the conformational change from the open to the closed state witnesses a rigidified extracellular gating loop L6. Thus, a change in the conformational state of OmpG not only bifurcates its unfolding pathways but also tunes its mechanical properties for optimum function.