MscS-Like10 is a stretch-activated ion channel from Arabidopsis thaliana with a preference for anions

Abstract

Like many other organisms, plants are capable of sensing and responding to mechanical stimuli such as touch, osmotic pressure, and gravity. One mechanism for the perception of force is the activation of mechanosensitive (or stretch-activated) ion channels, and a number of mechanosensitive channel activities have been described in plant membranes. Based on their homology to the bacterial mechanosensitive channel MscS, the 10 MscS-Like (MSL) proteins of Arabidopsis thaliana have been hypothesized to form mechanosensitive channels in plant cell and organelle membranes. However, definitive proof that MSLs form mechanosensitive channels has been lacking. Here we used single-channel patch clamp electrophysiology to show that MSL10 is capable of providing a MS channel activity when heterologously expressed in Xenopus laevis oocytes. This channel had a conductance of ∼100 pS, consistent with the hypothesis that it underlies an activity previously observed in the plasma membrane of plant root cells. We found that MSL10 formed a channel with a moderate preference for anions, which was modulated by strongly positive and negative membrane potentials, and was reversibly inhibited by gadolinium, a known inhibitor of mechanosensitive channels. MSL10 demonstrated asymmetric activation/ inactivation kinetics, with the channel closing at substantially lower tensions than channel opening. The electrophysiological characterization of MSL10 reported here provides insight into the evolution of structure and function of this important family of proteins.T he perception of mechanical stimuli like gravity, touch, or osmotic pressure is essential to normal plant growth and development and is further implicated in biotic and abiotic stress responses (1). One of the best-studied strategies for perceiving force involves membrane-embedded channels that are gated by tension, known as mechanosensitive (MS) channels (2). Numerous MS channel activities (>17 to date) have been described in the membranes of diverse tissues from a variety of plant species (summarized in ref. 3, also refs. 4 and 5). Many of these observed MS channel activities differ in their conductance, ion selectivity, and/or sensitivity to the direction of activation pressure, suggesting that multiple classes of mechanosensitive channels are present in plant cells.No mechanosensitive ion channel activity discovered in plant membranes has yet been definitively identified at the molecular level, but two families of proteins serve as strong candidates. The first is the Mid1-Complementing Activity (MCA) family, members of which are required for root response to touch in the model plant Arabidopsis thaliana, induce Ca 2+ uptake in rice and Arabidopsis cells (6, 7), and are associated with increased current in response to hypotonic stimulation of Xenopus oocytes (8). The second family of candidates for plant MS channels is the MscS-Like (MSL) family, first identified based on modest homology to the wellcharacterized bacterial MS channel MscS from Escherichia coli (3, 9). MscS is a lar...

Genome-Wide Association Mapping Reveals That Specific and Pleiotropic Regulatory Mechanisms Fine-Tune Central Metabolism and Growth in Arabidopsis

“…We detected and validated a QTL colocalized for starch, Suc, and Glc content at MSL10 (Figure 4; Supplemental Figure 7). MSL family members act as osmotic safety valves to release osmolytes under increased membrane tension (Maksaev and Haswell, 2012). These proteins have been implicated in pollen development (Hamilton et al, 2015), osmotic regulation in plastids , redox homeostasis in mitochondria (Lee et al, 2016), and cell death (Veley et al, 2014).…”

Arabidopsis MSL10 Has a Regulated Cell Death Signaling Activity That Is Separable from Its Mechanosensitive Ion Channel Activity

“…The function of plasma membrane (PM)-localized MSLs remains under investigation. Arabidopsis MSL10 is localized to the PM, and its MS ion channel properties have been characterized in root protoplasts (Haswell et al, 2008) and Xenopus laevis oocytes (Maksaev and Haswell, 2012). However, msl10 single mutants, msl9 msl10 double mutants, and even msl4 msl5 msl6 msl9 msl10 quintuple mutant plants responded normally to a number of mechanical and osmotic stresses (Haswell et al, 2008).…”

Recent characterizations of MscS and its homologs provide insight into the basis of ion selectivity in mechanosensitive channels

“…Recently, we used a similar approach to characterize the gating characteristics of the Arabidopsis MscS homolog MSL10 by single-channel patch-clamp electrophysiology. 25 Like EcMscS, expression of MSL10 in Xenopus oocytes produces a mechanically gated ion channel activity, and the latter closely resembles a MSL10-dependent MS channel activity previously characterized in Arabidopsis root protoplast membranes. 26 These results provide strong evidence that MscS homologs from land plants are bona fide MS channels and that mechanosensitive gating can be achieved in the MscS family without conservation of many of the specific residues previously shown to be critical for bacterial EcMscS function.…”

“…3,24,25 They share a slight conductance asymmetry under opposite membrane potentials, both exhibiting larger conductances under negative membrane potentials. Like EcMscS, MSL10 activation does not depend on the sign of membrane curvature and is easily observed both under positive and negative membrane potentials.…”

“…27 However, MSL10 differs from MscS in several key aspects. 3,24,25 Though both channels are mechanosensitive, their tension-dependent gating results in different single-channel conductances: approximately 300 pS for EcMscS and intriguing family of proteins. The wealth of information regarding the structure and function of EcMscS provides a solid basis for the interpretation of these data.…”