  The mechanosensitive channel of large conductance, which serves as a model
system for mechanosensitive channels, has previously been crystallized in the
closed form, but not in the open form. Ensemble measurements and
electrophysiological sieving experiments show that the open-diameter of the
channel pore is >25{\AA}, but the exact size and whether the conformational
change follows a helix-tilt or barrel-stave model are unclear. Here we report
measurements of the distance changes on liposome-reconstituted MscL
transmembrane {\alpha}-helices, using a "virtual sorting" single-molecule
fluorescence energy transfer. We observed directly that the channel opens via
the helix-tilt model and the open pore reaches 2.8 nm in diameter. In addition,
based on the measurements, we developed a molecular dynamics model of the
channel structure in the open state which confirms our direct observations.
