Structure of the water channel AqpZ from Escherichia coli revealed by electron crystallography

Molecular water channels (aquaporins) allow living cells to adapt to osmoti c variations by rapid and specific diffusion of water molecules. Aquaporins are present in animals, plants, algae, fungi and bacteria. Here we present an electron microscopic analysis of the most ancient water channel describ ed so far: the aquaporin Z (AqpZ) of Escherichia coli. A recombinant AqpZ w ith a poly(histidine) tag at the N terminus has been constructed, overexpre ssed and purified to homogeneity. Solubilized with octylglucoside, the puri fied AqpZ remains associated as a homotetramer, and assembles into highly o rdered two-dimensional tetragonal crystals with unit cell dimensions a=b=95 Angstrom, gamma=90 degrees when reconstituted by dialysis in the presence of Lipids. Three-dimensional reconstruction of negatively stained lattices revealed the p42(1)2 packing arrangement that is also observed with the hum an erythrocyte water channel (AQP1). The 8 Angstrom projection map of the A qpZ tetramer in frozen hydrated samples is similar to that of AQP1, consist ent with the high sequence homology between these proteins. (C) 1999 Academ ic Press.