Mutations in a bacterial mechanosensitive channel change the cellular response to osmotic stress

MscL is a channel found in bacterial plasma membranes that opens a large pore in response to mechanical stress. Here we demonstrate that some mutations within this channel protein (K31D and K31E) evoke a cellular phenotype in which the growth rate is severely depressed. Increasing the osmolarity of the growth medium partially rescues this "slowed growth" phenotype and decreases an abnormal cytosolic potassium loss observed in cells expressing the mutants. In addition, upon sudden decrease in osmolarity (osmotic downshock) more cytoplasmic potassium is released from cells expressing the mutants than cells expressing wild-type MscL. After osmotic downshock, all cells remained viable; hence, the differences in potassium efflux observed are not due to cell lysis but instead appear to be an exaggeration of the normal response to this sudden change in environmental osmolarity. Patch clamp studies in native bacterial membranes substantiate the hypothesis that these mutant channels are more sensitive to mechanical stresses, especially at voltages approaching those estimated for bacterial membrane potentials. These data are consistent with a crucial role for MscL in the adaptation to large osmotic downshock and suggest that if the normally tight regulation of MscL gating is disrupted, cell growth can be severely inhibited.