The soluble N-termini of mechanosensitive ion channels MSL8, MSL9, and MSL10 are environmentally sensitive intrinsically disordered regions with distinct biophysical characteristics

Abstract

Intrinsically disordered protein regions (IDRs) are highly dynamic sequences that rapidly sample a collection of conformations. In the past several decades, IDRs have emerged as a core component of many proteomes, comprising ~30% of all eukaryotic protein sequences. IDRs are ubiquitous throughout different biological pathways, with a notable enrichment in responses to environmental stimuli such as abiotic stress. However, the diversity of IDR-based systems that biology has evolved to respond to different stimuli is expansive, warranting the exploration of IDRs present in unique molecular contexts. Here, we identify and characterize intrinsic disorder in the soluble, cytoplasmic N-terminal domains of three members of the MscS-Like (MSL) family of mechanosensitive ion channels, MSL8, MSL9 and MSL10. In plants, MSL channels are proposed to mediate the reactions to cell swelling, pathogenic invasion, and touch. A series of bioinformatic tools unanimously predicted that the cytosolic N-termini of MSLs are intrinsically disordered. We confirmed this prediction for the N-terminus of MSL10 (MSL10N) via circular dichroism spectroscopy. MSL10N adopted a predominately helical structure when exposed to the helix-inducing compound trifluoroethanol (TFE) and underwent structural changes and alterations to homotypic interaction favorability in the presence of molecular crowding agents. Lastly, in vitro imaging of condensates indicated that MSL8N, MSL9N and MSL10N have sharply differing propensities for condensate formation both inherently and in response to salt, temperature, and molecular crowding. Altogether, these data establish the N-termini of MSL channels as intrinsically disordered regions with distinct biophysical properties and the potential to respond disparately to changes in their physiochemical environment.