Managing hypoosmotic stress: aquaporins and mechanosensitive channels in Escherichia coli

When Escherichia coli cells are Subject to hypoosmotic shock they are subje ct to substantial Rows of water that can be equivalent to a 4-5-fold increa se in the pressure exerted from the cytoplasm on the membrane and peptidogl ycan wall. The recently described aquaporin that facilitates rapid water mo vement across the cytoplasmic membrane is repressed during growth at high o smolarity. This may enable the cell to reduce the rate of pressure build up during transitions from high to low osmolarity. The presence of multiple m echanosensitive channels in the E. coli cell membrane is well documented. T he recent identification of genes that inactivate the MscL and MscS channel s has established their role in releasing the pressure built up by hypoosmo tic shock. The isolation of specific mutations and the structural studies o n MscL now pave the way to a molecular understanding of the mechanism of ac tivation of mechanosensitive channels.