*Structural, Functional, and Regulatory Diversity in the MscS Family

MscS homologs are widely dispersed among bacterial and plant lineages, are found in some fungi, but have not been identified in plants. Our investigations are beginning to reveal much how MscS-Like proteins from Arabidopsis have diverged from E. coli MscS with respect to structure, function and mechanisms of regulation. We have recently discovered that plant-specific domains confer plant-specific functions and modes of regulation to MscS-Like proteins in Arabidopsis thaliana.

Diversity of Structure is Associated with Diversity of Function.

The mechanosensitive ion channels of the MscS superfamily provide a model system for understanding the process and the physiological function of membrane-based mechanosensation. MscS family members so far characterized are thought to function as simple osmotic safety valves, opening in response to increased membrane tension and preventing cellular rupture under environmental hypoosmotic shock. However, this family of proteins exhibits an amazing diversity of size, topology, and domain organization, raising the possibility that they have evolved unique functions and regulatory mechanisms. We recently discovered that characterizing this family as just osmotic safety valves is a significant oversimplification. We employed MscS-Like10 (MSL10) from Arabidopsis thaliana in our study, as it is the only plant MSL to have its stretch-activation characterized in detail, and has a soluble N-terminal domain that is unique to land plants. We discovered that MscS-Like 10 has two functions, each attributable to a different domain of the protein.The C-terminal domain, which is conserved among all MscS-Like ion channels, mediates tension-regulated ion flux. The plant-specific N-terminal domain is capable of inducing cell death, and its activity is negatively regulated by its phosphorylation. These results support a model wherein MSL10 is a membrane tension sensor capable of at least two outputs—the release of osmolytes from the cell, and the triggering of cell death and other stress responses, as shown in the model above.

  • K. M. Veley, G. Maksaev, S. M. Kloepper, E. M. Frick, E. January and E. S. Haswell. (2014). MSL10 has a Regulated Cell Death Signaling Activity that is Separable from its Mechanosensitive Ion Channel Activity. Plant Cell 26:3115-31.

  • E. S. Hamilton, A. Schlegel, and E. S. Haswell. (2015). United in Diversity: Plant Mechanosensitive Channels. Annual Review of Plant Biology 66:113-137.

MscS family structure-function.

Homologs of MscS are found consistently in bacterial and archaeal species, occasionally in the genomes of fungal species, and in all available genomes of land plants. So far, they have not been identified in animals. The conserved ~90 amino acid MscS domain contains the channel-forming transmembrane helix and the upper portion of the soluble C-terminus. The N- and C-terminal domains of MscS family members are highly variable in size and sequence and may in some cases confer regulatory aspects to channel function. MscS itself appears to be the minimal size for this type of protein, and thus represents a simple, unregulated prototype while other family members exhibit more complex, possibly regulated functions. In certain MscS family members, N- and C-terminal domains are capable of interaction with other proteins, small molecules, and ions that may influence channel gating. For example, MscK, a MscS homolog from E. coli, does not significantly contribute to osmotic shock resistance. However, removal of the large N-terminal periplasmic domain produces a MscK derivative capable of partially rescuing a mscS- mscL- mutant from osmotic shock. Similarly, several members of the bacterial cyclic nucleotide-gated (bCNG) channel subfamily of MscS homologs do not protect E. coli from osmotic shock and do not show tension-sensitivity in electrophysiological experiments, though they are activated by cAMP. Removal of the cyclic nucleotide monophosphate-binding domain at the extreme C-terminus of bCNGs produces channels that are partially protective in the osmotic shock assay. These two examples suggest that additional protein domains found in homologs of MscS may impose regulatory control on a conserved channel pore that otherwise retains its MS character. A similar function has been ascribed to the “gatekeeper” domains that regulate a subset of voltage-activated potassium channels.

  • E. S. Haswell, R. Phillips, & D. C. Rees. (2011). Mechanosensitive Channels: What Can They Do and How Do They Do It? Structure 19: 1356-1369.
  • G. Maksaev & E. S. Haswell. (2013). Recent Characterizations of MscS and its Homologs Provide Insights into the Basis of Ion Selectivity.  Channels 7(3): 215-220.
  • M.E. Wilson, Grigory Maksaev, and E. S. Haswell. (2013). MscS-like Mechanosensitive Channels in Plants and Microbes. Biochemistry 52 (34): 5708–5722.

Two conserved motifs are critical for the function of organelle-localized MscS homologs.

Sequences conserved between MscS family members have been identified, but the functional relevance of these sequences had not been established. Our results indicate that two of the three tested motifs are essential for MSL2 function, consistent with conservation of structure and function between bacterial and plant MscS homologs.

  • G. S. Jensen and E. S. Haswell. (2012). Functional Analysis of Conserved Motifs in the Mechanosensitive Channel Homolog MscS-Like2 from Arabidopsis thalianaPLoS ONE 7(6): e40336.