Similar to other ion channels, MscLs are organized as symmetric oligomers with the permeation pathway formed by the packing of subunits around the axis of rotational symmetry. Unlike MscS, which is heptameric, MscL is likely pentameric; although the Sa-MscL appears to be a tetramer in a crystal structure, this may be an artifact. MscL contains two transmembrane helices that are packed in an up-down/nearest neighbor topology. The permeation pathway of the MscL is approximately funnel shaped, with larger opening facing the periplasmic surface of the membrane and the narrowest point near the cytoplasm. At the narrowest point, the pore is constricted by the side chains of symmetry-related residues in Ec-MscL: Leu19 and Val23. The pore diameter of MscL in the open state has been estimated to ~3 nm, which accommodates the passage of small protein up to 9 kD.