Washington University in St. Louis
Campus Box 1037
One Brookings Drive
St. Louis, MO 63130-4899
The long-term goal of research in the Haswell lab is to reveal the molecular mechanisms that underlie the perception and transduction of mechanical signals in plants. Many organisms sense and respond to mechanical forces, and one way in which this can be accomplished is through the activation of mechanosensitive (MS) ion channels.
Land plants provide a particularly relevant model system for the study of MS channels, as numerous MS channel activities have been identified in plant membranes, and they are implicated in a wide range of physiological processes. However, we do not know the molecular identity of the MS channels involved, nor how their activities might be regulated. To begin to gain insight into the plant mechanosensory apparatus, we have undertaken the characterization of ten Arabidopsis thaliana homologs of the bacterial mechanosensor MscS. We have discovered that plant MscS homologs are not simple safety valves, but are regulated channels with distinct and diverse roles at the organellar, cellular, and organismal level.
Three main projects in the lab are:
1. Structure-Function of Eukaryotic MscS Homologs
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 discovered that some motifs conserved between plant and bacteria are critical for function, while others are not.
2. MS Channels in the Plastid Envelope
Our studies on the plastid-localized Arabidopsis MscS homologs MSL2 and MSL3 has revealed how organelles use these proteins to respond to membrane stretch, how their activity is integrated into cellular signaling networks, and how mechanoperceptive strategies have evolved in multicellular plants. These results suggest that plant MscS homologs are not merely safety valves but regulated channels with multiple roles at the organellar, cellular, and organismal level.
3. Electrophysiology of Plant and Bacterial MS Channels
We have used single channel patch-clamp electrophysiology to characterize both E. coli MscS and Arabidopsis MSL10 expressed in Xenopus laevis oocytes. These studies establish that MSL10 is indeed gated by membrane tension and its behavior in oocytes closely resembles an MSL10-dependent activity in Arabidopsis root protoplasts. Differences in the behavior of MSL10 and MscS may have implications for their respective roles in plant and bacterial physiology.
Photo caption: (Functional and structural characteristics of MscL and MscS channels from bacteria and eukaryotes. (Top panel) Expression of MscS and MscL protects a bacterial strain lacking endogenous MscS and MscL from a 0.5 M osmotic downshock while expression of MscK and Cv-bCNG does not (Levina et al., 1999; Caldwell et al., 2010). Asterisks, over-expression of YbdG and MSL3 provides protection (Haswell et al., 2008; Schumann et al., 2010). (Top middle panel) Conductance of endogenous channels in giant E. coli spheroplasts (MscL, MscS, MscK, MscM, (reviewed in (Kung et al., 2010))), channels heterologously expressed in giant E. coli spheroplasts (MSC1, (Nakayama et al., 2007)) or endogenous channels in Arabidopsis thaliana root cells (MSL10, (Haswell et al., 2008)). (Bottom middle panel) Tension to gate expressed relative to MscL channels in the same patch (Berrier et al., 1996; Edwards et al., 2005; Li et al., 2007). (Bottom panel) Channel monomer topologies as predicted by TOPCONS (http://topcons.net/). Mature versions (after processing of chloroplast targeting sequences) of MSC1 and MSL3 are shown, and sequence loops connecting transmembrane helices were omitted for clarity.)
E. S. Hamilton, A. Schlegel, and E. S. Haswell. (2015). United in Diversity: Plant Mechanosensitive Channels. Annual Review of Plant Biology 66:113-137.
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.
M. E. Wilson2, M. R. Basu1, G. B. Bhaskara, P. E. Verslues, and E. S. Haswell. (2014). Plastid Osmotic Stress Activates Cellular Osmotic Stress Responses. Plant Physiology 165:119-128.
G. Maksaev and E. S. Haswell. (2012). MscS-Like10 is a Stretch-Activated Ion Channel from Arabidopsis thaliana with a Preference for Anions. Proceedings of the National Academy of Sciences 109:19015-19020.
K. M. Veley, S. Marshburn, C. Clure1 and E. S. Haswell. (2012). Mechanosensitive Channels Protect Plastids from Hypoosmotic Shock During Normal Plant Growth. Current Biology 22:408-413.
G. Maksaev and E. S. Haswell. (2011). Expression and Characterization of the Bacterial Mechanosensitive Channel MscS in Xenopus laevis Oocytes. J. General Physiology 138: 641-9.
M. E. Wilson2, G. S. Jensen, and E. S. Haswell. (2011). Two Mechanosensitive Channel Homologs Influence FtsZ Ring Placement in Arabidopsis. Plant Cell 23: 2939-2949.
E. S. Haswell, R. Phillips, and D. R. Rees. (2011). Mechanosensitive Channels: What Do They Do and How Do They Do It? Structure 19: 1356-1369.