The tension applied to the inner and outer rims of the channel by the lipid bilayer tilts the transmembrane helixes of MscL (The tilts of the M1 helices change by 35-34o during the transition), causing a gradual iris-like expansion and flattening of the MscL barrel. As a result, the transmembrane span of M2 helices is reduced, pulling the periplasmic loops into the membrane to line the extracellular entrance to the pore, establishing a pore diameter of ~3 nm. Along with this iris-like transition, the pore is now lined mostly by polar facets of M1 helices, instead of the hydrophobic constriction during closed state. Once the pore is hydrated, the MscL barrel exerts more force onto the S1-M1 linkers, pulling S1 bundle apart and completely opening the channel.