Connecting the dots between mechanosensitive channel abundance, osmotic shock, and survival at single-cell resolution

TL;DR

This study quantifies the number of MscL channels needed for E. coli survival after osmotic shock, revealing that hundreds of channels are required and aligning with natural expression levels, thus clarifying their physiological role.

Contribution

It demonstrates that a high number of MscL channels are necessary for survival, providing quantitative insights into their physiological function at the single-cell level.

Findings

500-700 MscL channels per cell are needed for 80% survival

Less than 100 channels per cell result in zero survival

Wild-type cells have an average of 500-700 MscL channels

Abstract

Rapid changes in extracellular osmolarity are one of many insults microbial cells face on a daily basis. To protect against such shocks, Escherichia coli and other microbes express several types of transmembrane channels which open and close in response to changes in membrane tension. In E. coli, one of the most abundant channels is the mechanosensitive channel of large conductance (MscL). While this channel has been heavily characterized through structural methods, electrophysiology, and theoretical modeling, our understanding of its physiological role in preventing cell death by alleviating high membrane tension remains tenuous. In this work, we examine the contribution of MscL alone to cell survival after osmotic shock at single cell resolution using quantitative fluorescence microscopy. We conduct these experiments in an E. coli strain which is lacking all mechanosensitive channel genes save for MscL whose expression is tuned across three orders of magnitude through modifications of the Shine-Dalgarno sequence. While theoretical models suggest that only a few MscL channels would be needed to alleviate even large changes in osmotic pressure, we find that between 500 and 700 channels per cell are needed to convey upwards of 80% survival. This number agrees with the average MscL copy number measured in wild-type E. coli cells through proteomic studies and quantitative Western blotting. Furthermore, we observe zero survival events in cells with less than 100 channels per cell. This work opens new questions concerning the contribution of other mechanosensitive channels to survival as well as regulation of their activity.