Managing hypoosmotic stress: aquaporins and mechanosensitive channels in Escherichia coli

When Escherichia coli cells are subject to hypoosmotic shock they are subject to substantial flows of water that can be equivalent to a 4-5-fold increase in the pressure exerted from the cytoplasm on the membrane and peptidoglycan wall. The recently described aquaporin that facilitates rapid water movement across the cytoplasmic membrane is repressed during growth at high osmolarity. This may enable the cell to reduce the rate of pressure build up during transitions from high to low osmolarity. The presence of multiple mechanosensitive channels in the E. coli cell membrane is well documented. The recent identification of genes that inactivate the MscL and MscS channels has established their role in releasing the pressure built up by hypoosmotic shock. The isolation of specific mutations and the structural studies on MscL now pave the way to a molecular understanding of the mechanism of activation of mechanosensitive channels.