Unfolding barriers in bacteriorhodopsin probed from the cytoplasmic and the extracellular side by AFM.

Selecting an individual membrane protein and probing its mechanical properties has become possible by AFM-based single-molecule force spectroscopy. In contrast to earlier studies, we extracted and unfolded bacteriorhodopsin monomers from the purple membrane not only from the cytoplasmic side, but also from the extracellular side, and recorded the force extension profiles. This way different pathways through the potential landscape are explored. A map of the 21 most dominant barriers with their positions relative to the amino acid sequences is given at an accuracy of +/-3 aa. Most barriers were found to provide resistance to forced unfolding only when extracted toward one of the sides. However, certain barriers have identical positions to within a few amino acids when probed from either of the sides, which typifies them as structural traps.