Engineering covalent oligomers of the mechanosensitive channel of large conductance from Escherichia coli with native conductance and gating characteristics

Light-activated control of protein channel assembly mediated by membrane mechanics

Photochemical processes provide versatile triggers of chemical reactions. Here, we use a photoactivated lipid switch to modulate the folding and assembly of a protein channel within a model biological membrane. In contrast to the information rich field of water-soluble protein folding, there is only a limited understanding of the assembly of proteins that are integral to biological membranes. It is however possible to exploit the foreboding hydrophobic lipid environment and control membrane protein folding via lipid bilayer mechanics. Mechanical properties such as lipid chain lateral pressure influence the insertion and folding of proteins in membranes, with different stages of folding having contrasting sensitivities to the bilayer properties. Studies to date have relied on altering bilayer properties through lipid compositional changes made at equilibrium, and thus can only be made before or after folding. We show that light-activation of photoisomerisable di-(5-[[4-(4-butylphenyl)azo]phenoxy]pentyl)phosphate (4-Azo-5P) lipids influences the folding and assembly of the pentameric bacterial mechanosensitive channel MscL. The use of a photochemical reaction enables the bilayer properties to be altered during folding, which is unprecedented. This mechanical manipulation during folding, allows for optimisation of different stages of the component insertion, folding and assembly steps within the same lipid system. The photochemical approach offers the potential to control channel assembly when generating synthetic devices that exploit the mechanosensitive protein as a nanovalve.

Osmotic Stress

Escherichia coli and Salmonella encounter osmotic pressure variations in natural environments that include host tissues, food, soil, and water. Osmotic stress causes water to flow into or out of cells, changing their structure, physics, and chemistry in ways that perturb cell functions. E. coli and Salmonella limit osmotically induced water fluxes by accumulating and releasing electrolytes and small organic solutes, some denoted compatible solutes because they accumulate to high levels without disturbing cell functions. Osmotic upshifts inhibit membrane-based energy transduction and macromolecule synthesis while activating existing osmoregulatory systems and specifically inducing osmoregulatory genes. The osmoregulatory response depends on the availability of osmoprotectants (exogenous organic compounds that can be taken up to become compatible solutes). Without osmoprotectants, K+ accumulates with counterion glutamate, and compatible solute trehalose is synthesized. Available osmoprotectants are taken up via transporters ProP, ProU, BetT, and BetU. The resulting compatible solute accumulation attenuates the K+ glutamate response and more effectively restores cell hydration and growth. Osmotic downshifts abruptly increase turgor pressure and strain the cytoplasmic membrane. Mechanosensitive channels like MscS and MscL open to allow nonspecific solute efflux and forestall cell lysis. Research frontiers include (i) the osmoadaptive remodeling of cell structure, (ii) the mechanisms by which osmotic stress alters gene expression, (iii) the mechanisms by which transporters and channels detect and respond to osmotic pressure changes, (iv) the coordination of osmoregulatory programs and selection of available osmoprotectants, and (v) the roles played by osmoregulatory mechanisms as E. coli and Salmonella survive or thrive in their natural environments.

MscL: channeling membrane tension

Mechanosensitive channels are integral components for the response of bacteria to osmotic shock. The mechanosensitive channel of large conductance (MscL) responds to extreme turgor pressure increase that would otherwise lyse the cellular membrane. MscL has been studied as a model mechanosensitive channel using both structural and functional approaches. We will summarize the structural data and discuss outstanding questions surrounding the gating mechanism of this homo-oligomeric channel that has ~3 nS conductance. Specifically, we will explore the following: (1) the variability in oligomeric state that has been observed, (2) the open pore size measurements, and (3) the role of the C-terminal coiled coil domain for channel function. The oligomeric state of MscL has been characterized using various techniques, with a pentamer being the predominant form; however, the presence of mixtures of oligomers in the membrane is still uncertain. In the absence of structural data for the open state of MscL, the diameter of the open state pore has been estimated by several different approaches, leading to a current estimate between 25 and 30 Å. While the C-terminal domain is highly conserved among MscL homologues, it is not required for activity in vivo or in vitro. This domain is likely to remain intact during the gating transition and perform a filtering function that retains valuable osmolytes in the cytosol. Overall, studies of MscL have provided significant insight to the field, and serve as a paradigm for the analysis of non-homologous, eukaryotic mechanosensitive channel proteins.

OCAM: a new tool for studying the oligomeric diversity of MscL channels

We have developed a new technique to study the oligomeric state of proteins in solution. OCAM or Oligomer Characterization by Addition of Mass counts protein subunits by selectively shaving a protein mass tag added to a protein subunit via a short peptide linker. Cleavage of each mass tag reduces the total mass of the protein complex by a fixed amount. By performing limited proteolysis and separating the reaction products by size on a blue native PAGE gel, a ladder of reaction products corresponding to the number of subunits can be resolved. The pattern of bands may be used to distinguish the presence of a single homo-oligomer from a mixture of oligomeric states. We have applied OCAM to study the mechanosensitive channel of large conductance (MscL) and find that these proteins can exist in multiple oligomeric states ranging from tetramers up to possible hexamers. Our results demonstrate the existence of oligomeric forms of MscL not yet observed by X-ray crystallography or other techniques and that in some cases a single type of MscL subunit can assemble as a mixture of oligomeric states.

Subunit dimers of alpha-hemolysin expand the engineering toolbox for protein nanopores

Staphylococcal α-hemolysin (αHL) forms a heptameric pore that features a 14-stranded transmembrane β-barrel. We attempted to force the αHL pore to adopt novel stoichiometries by oligomerizing subunit dimers generated by in vitro transcription and translation of a tandem gene. However, in vitro transcription and translation also produced truncated proteins, monomers, that were preferentially incorporated into oligomers. These oligomers were shown to be functional heptamers by single-channel recording and had a similar mobility to wild-type heptamers in SDS-polyacrylamide gels. Purified full-length subunit dimers were then prepared by using His-tagged protein. Again, single-channel recording showed that oligomers made from these dimers are functional heptamers, implying that one or more subunits are excluded from the central pore. Therefore, the αHL pore resists all structures except those that possess seven subunits immediately surrounding the central axis. Although we were not able to change the stoichiometry of the central pore of αHL by the concatenation of subunits, we extended our findings to prepare pores containing one subunit dimer and five monomers and purified them by SDS-PAGE. Two half-chelating ligands were then installed at adjacent sites, one on each subunit of the dimer. Single-channel recording showed that pores formed from this construct formed complexes with divalent metal ions in a similar fashion to pores containing two half-chelating ligands on the same subunit, confirming that the oligomers had assembled with seven subunits around the central lumen. The ability to incorporate subunit dimers into αHL pores increases the range of structures that can be obtained from engineered protein nanopores.

Dual-color fluorescence-burst analysis to probe protein efflux through the mechanosensitive channel MscL

The mechanosensitive channel protein of large conductance, MscL, from Escherichia coli has been implicated in protein efflux, but the passage of proteins through the channel has never been demonstrated. We used dual-color fluorescence-burst analysis to evaluate the efflux of fluorescent labeled compounds through MscL. The method correlates the fluctuations in intensity of fluorescent labeled membranes and encapsulated (macro)molecules (labeled with second fluorophore) for each liposome diffusing through the observation volume. The analysis provides quantitative information on the concentration of macromolecules inside the liposomes and the fraction of functional channel proteins. For MscL, reconstituted in large unilamellar vesicles, we show that insulin, bovine pancreas trypsin inhibitor, and other compounds smaller than 6.5 kDa can pass through MscL, whereas larger macromolecules cannot.