Sensing and responding to membrane tension: the bacterial MscL channel as a model system

Abstract

Mechanosensors are important for many life functions, including the senses of touch, balance, and proprioception; cardiovascular regulation; kidney function; and osmoregulation. Many channels from an assortment of families are now candidates for eukaryotic mechanosensors and proprioception, as well as cardiovascular regulation, kidney function, and osmoregulation. Bacteria also possess two families of mechanosensitive channels, termed MscL and MscS, that function as osmotic emergency release valves. Of the two channels, MscL is the most conserved, most streamlined in structure, and largest in conductance at 3.6 nS with a pore diameter in excess of 30 Å; hence, the structural changes required for gating are exaggerated and perhaps more easily defined. Because of these properties, as well as its tractable nature, MscL represents a excellent model for studying how a channel can sense and respond to biophysical changes of a lipid bilayer. Many of the properties of the MscL channel, such as the sensitivity to amphipaths, a helix that runs along the membrane surface and is connected to the pore via a glycine, a twisting and turning of the transmembrane domains upon gating, and the dynamic changes in membrane interactions, may be common to other candidate mechanosensors. Here we review many of these properties and discuss their structural and functional implications.

Figure 1

Structure of the homopentameric MS channel MscL from M. tuberculosis and a model for the structure of the TMDs of E. coli MscL in the open structure. The top panels show the structure of the nearly closed Tb-MscL as resolved in the crystal structure. The relatively simple topology of each MscL subunit can be observed in the side view (left), with a single subunit highlighted in red for clarity. The approximate position of the lipid membrane is indicated by the horizontal blue lines. The N-terminus forms a helix that lies along the membrane and connects with TMD1. TMD1 crosses the membrane lining the pore of the channel, as can be clearly observed in the periplasmic view (right). TMD1 is connected to TMD2 by a periplasmic loop. TMD2 surrounds the TMD1 helix and is in contact with the membrane. Finally, a cytoplasmic loop connects TMD2 with a cytoplasmic helix that forms a bundle at the C-terminal end of the channel. The bottom panels show similar side (left) and periplasmic (right) views of a theoretical model based on EPR and other experimental data (15) for what the TMDs might look like in the open structure. Note that the N- and C-terminal domains and periplasmic loop are not shown; the TMDs are connected with a straight line for orientation purposes.

Figure 2

Schematic representation of the interactions between MscL TMDs and the lipid membrane. (A) TMD1 and the N-terminal helix (S1) are represented in the closed (left) and open (right) states. The position of Gly-14 between the S1 and TMD1 domains is shown as a green sphere. The conserved phenylalanine residues at positions 7 and 10 in the S1 domain are shown as red hexagons. (B) A single MscL subunit is represented in closed (left) and open (right) states. TMD2, the cytoplasmic loop, and the C-terminal helix are highlighted in a darker color. The positions of residue N103 and the charged cluster (RKKEE) are shown as a green hexagon and a red star, respectively.