MscL channel

Figure 1 Physical and chemical stimuli affecting the gating of bacterial MS channels. (A) The structure of the pentameric MscL channel (left) and a channel monomer (right) from Mycobacterium tuberculosis according to the 3-D structural model of a closed channel (7). MscL is activated by membrane stretch, amphipaths (e.g., lysophopholipids, chlorpromazine, and trinitrophenol) and parabens. The channel activity is inhibited by Gd + and static magnetic fields (SMF) and is modulated by temperature and intracellular pH (3). (B) The structure of the MscS heptamer (left) and the channel monomer (right) from E. coli based on the 3-D structural model of MscS (8) most likely depicting an inactive or desensitized functional state of the channel (3). MscS is activated by membrane stretch, amphipaths, and parabens and is modulated by voltage. The activity of the channel is inhibited by Gd + and high hydrostatic pressure (HHP) (3). The arrows point at membrane structures (i.e., channel protein and/or lipid bilayer) affected by the specific stimuli.

Corry B, Rigby P, Martinac B. Conformational changes involved in MscL channel gating measured using ERET spectroscopy. Biophys. J. 2005 91 1032-1045. 

MscL is a nonselective ion channel of -2.5 nS conductance activated in vitro by the application of membrane tension with a pc ential difference of 100 mV, this conductance is equivalent to the flow of -10 ions/second across the membrane. The functional properties of MscL suggest a physiological role in the regulation of osmotic pressure in the cell. Sequence analysis and biochemical studies indicated that the MscL channel consists of a single type of subunit of -16 kD with two transmembrane helices, and that this channel is localized... 

Mechanosensitive ion channels can be looked at as membrane-embedded mechano-electrical switches. They play a critical role in transducing physical stresses at the cell membrane (e.g. lipid bilayer deformations) into an electrochemical response. Two types of stretch-activated channels have been reported the mechanosensitive channels of large conductance (MscL) and mechanosensitive channels of small conductance (MscS). 

Mechanosensitive channels respond to changes in membrane tension. A prokaryotic large-conductance mechanosensitive channel, MscL, opens in response to osmotic stress to form a water filled channel between 3 and 4 nm across [18]. The change in pressure on the bilayer imparts a small movement in a transmembrane helix that is then followed by a dramatic rearrangement of the transmembrane domain to a fully open state. 

Perozo E et al (2002) Open channel structure of MscL and the gating mechanism of mechanosensitive channels. Nature 418 942-948... 

Fig. 1. Predicted membrane-spanning topology for mechanosensitive channels found in eukaryotes (TRPV, K2P, and DEG/ENaC channels) and bacteria (MscL and MscS). In addition to the transmembrane helices (represented as cylinders), other motifs present in these channels are designated as follows. The TRPV channels contain several cytoplasmic ankyrin domains (A) at the N terminus, and one pore-forming loop (P). K2P channels have two pore-forming loops and a self-interaction domain (SID) through which dimers are generated. DEG/ENaC sodium channels have a single pore-forming loop and three cysteine-rich domains (CRDs).

The steepness of the response of the channel to applied tension is determined by the magnitude of the change in cross-sectional area for example, in terms of units typically employed in these calculations, if AA= 100 A2, then an increase of 1 dyn cm 1 in membrane tension corresponds to a free energy change of —0.602 kj/mol, and AG will scale proportionally to changes in A A. Consequently, for a channel with A G° = 40 kj/mol and crj/2 = 10 dyn cm-1 (typical for mechanosensitive channels such as MscL), AA 660 A2. 

Owing to the pioneering efforts of C. Kung, the best characterized mechanosensitive channel is the prokaryotic MscL, the mechanosensitive channel of large conductance. MscL was originally identified, isolated, and characterized by Kung and co-workers using a biochemical approach in combination with a patch-clamp assay to isolate an intrinsically stretch activated channel from E. coli membranes (Sukharev et al.,... 

The initial characterizations of the dependence of channel conductance on membrane tension for MscL were interpreted in terms of a two-state model analogous to Eq. (1) (Sukharev etal., 1997). In a more detailed analysis, Sukharev, Sachs, and co-workers (Sukharev etal., 1999c) established the energetic parameters for the gating transition in the E. coli MscL (Ec MscL) and obtained evidence for three subconductance states (S2, S3, and S4) between the fully closed (Cl) and fully open (05) states ... 

The crystal structure of Tb MscL established that this protein assembles as a homopentamer that is organized into two domains, the transmembrane domain and the cytoplasmic domain. The transmembrane domain consists of 10 helices (2 per subunit) connected by an extracellular loop, while the cytoplasmic domain contains 5 helices that form a left-handed pentameric bundle. The sequence of the Tb.MscL subunit has 151 amino acids and can be further subdivided into five segments the N terminus, the first transmembrane helix (TM1), an extracellular loop, the second transmembrane helix (TM2), and a cytoplasmic domain (Fig. 4, see color insert). Each of the segments is discussed in more detail below. The pore is aligned along the fivefold symmetry axis and is formed by the first transmembrane helix (TM1) and an extracellular loop from each subunit. The channel has overall dimensions of approximately 85 x 50 x 50 A and both the N and C termini reside on the... 

Fig. 5. A helical net representation for the inner TM1 helix of Tb MscL illustrating the positions of helix interface residues, pore residues, and gain-of-function mutations. The side chain of Val-21 serves to constrict the channel at the narrowest position. The correspondence between residues at the interface between the inner helices and the gain-of-function mutations is evident (Ou et al., 1998), suggesting this interface is critical for the gating process. Residues 17, 20, 24, and 28 participate in the helix interface with one neighboring TM1 helix, while 18, 22, and 26 participate in the interface with the other neighboring TM1 helix if these residues are found in subunit A, then the former group interacts with subunit E, and the latter with subunit B.

In view of the substantial conductance and solute nonselectivity of MscL, the general picture that emerges for the open state of this channel is of a large, water-filled pore with a likely diameter of up to 30 A. Assuming that the fivefold symmetry of MscL is maintained in both states,... 

In addition to experimental studies, the combination of relative simplicity and structural characterization have made MscL an attractive target for computational molecular dynamic simulations of channel gating (Elmore and Dougherty, 2001 Gullingsrud et at, 2001 Bilston and... 

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