Defining the Role of the Tension Sensor in the Mechanosensitive Channel of Small Conductance

Abstract

Mutations that alter the phenotypic behavior of the Escherichia coli mechanosensitive channel of small conductance (MscS) have been identified; however, most of these residues play critical roles in the transition between the closed and open states of the channel and are not directly involved in lipid interactions that transduce the tension response. In this study, we use molecular dynamic simulations to predict critical lipid interacting residues in the closed state of MscS. The physiological role of these residues was then investigated by performing osmotic downshock assays on MscS mutants where the lipid interacting residues were mutated to alanine. These experiments identified seven residues in the first and second transmembrane helices as lipid-sensing residues. The majority of these residues are hydrophobic amino acids located near the extracellular interface of the membrane. All of these residues interact strongly with the lipid bilayer in the closed state of MscS, but do not face the bilayer directly in structures associated with the open and desensitized states of the channel. Thus, the position of these residues relative to the lipid membrane appears related to the ability of the channel to sense tension in its different physiological states.

Conformational state of the MscS mechanosensitive channel in solution revealed by pulsed electron–electron double resonance (PELDOR) spectroscopy

“…Distance distributions were obtained by binning the data into 1-Å bins. The atomic coordinates of MscS used for this modeling procedure were as follows: crystal structures 2OAU (2), 2VV5 (6), the closed and open structures generated by molecular dynamics simulations (7,30) and the EPR closed (9,28) and open (8) structures.…”

The mechanosensitive channel of small conductance (MscS) functions as a Jack-In-The Box

“…Upon osmotic downshock, MscS is activated by tension in the membrane and opens to relieve excess turgor generated by the hypoosmotic shock [7, 8]. The response of MscS to membrane tension has been characterized by a variety of techniques and molecular interactions that alter channel gating have been identified through mutagenesis [2, 9–11]. …”

“…E. coli MscS is an ideal system for studying tension transduction in mechanosensitive ion channels, since structural models exist for the open, closed, and desensitized state of this channel [1, 2, 10, 12–14]. X-ray crystal structures of MscS have been resolved for the desensitized and open states of the channel [10, 13, 14].…”

“…X-ray crystal structures of MscS have been resolved for the desensitized and open states of the channel [10, 13, 14]. While there is not a closed state crystal structure of MscS, there are several models that have been developed based on experimentally obtained constraints [1, 2, 12]. Collectively, these structures make it possible to interpret functional data from mutated channels at the molecular level.…”

Mechanical properties of lipid bilayers and regulation of mechanosensitive function

“…In consequence, hydrophobic interactions with lipid tails and H-bonding with headgroups are key. 90,92,135 Likewise, as in MS channels in general, natural α-PFPs lack Trp, which are flanking residues that restrict the tilting and rotational angles of TM α-helices in membrane proteins. 75 Therefore, some relevant parallelisms appear between α-PFPs and bacterial MS channels (particularly in MscL homologs) if one considers their TM segments separately.…”