Visualization of AqpZ-mediated water permeability in Escherichia coli by cryoelectron microscopy

Transport of water across the plasma membrane is a fundamental process occu rring in all living organisms. In bacteria, osmotic movement of water acros s the cytoplasmic membrane is needed to maintain cellular turgor; however, the molecular mechanisms of this process are poorly defined. Involvement of aquaporin water channels in bacterial water permeability was suggested by the recent discovery of the aquaporin gene, aqpZ, in Escherichia coli. By e mploying cryoelectron microscopy to compare E, coli cells containing (AqpZ( +)) and lacking (AqpZ(-)) aquaporin, we show that the AqpZ water channel ra pidly mediates large water fluxes in response to sudden changes in extracel lular osmolarity, These findings (i) demonstrate for the first time functio nal expression of a prokaryotic water channel, (ii) evidence the bidirectio nal water channel feature of AqpZ, (iii) document a role for AqpZ in bacter ial osmoregulation, and (iv) define a suitable model for studying the physi ology of prokaryotic water transport.