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 lar ge pore in response to mechanical stress. Here we demonstrate that som e mutations within this channel protein (K31D and K31E) evoke a cellul ar phenotype in which the growth rate is severely depressed. Increasin g 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 dec rease 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 th is sudden change in environmental osmolarity. Patch clamp studies in n ative bacterial membranes substantiate the hypothesis that these mutan t channels are more sensitive to mechanical stresses, especially at vo ltages approaching those estimated for bacterial membrane potentials. These data are consistent with a crucial role for MscL in the adaptati on to large osmotic downshock and suggest that if the normally tight r egulation of MscL gating is disrupted, cell growth can be severely inh ibited.