The three-dimensional structure of human erythrocyte aquaporin CHIP

Water-permeable membranes of several plant and mammalian tissues contain specific water channel proteins, the 'aquaporins'. The best characterized aquaporin is CHIP, a 28 kDa red blood cell channel-forming integral protein. Isolated CHIP and Escherichia coli lipids may be assembled into 2-D crystals for structural analyses. Here we present (i) a structural characterization of the solubilized CHIP oligomers, (ii) projections of CHIP arrays after negative staining or metal-shadowing, and (iii) the 3-D structure at 1.6 nm resolution. Negatively stained CHIP oligomers exhibited a side length of 6.9 nm with four-fold symmetry, and a mass of 202 +/- 3 kDa determined by scanning transmission electron microscopy. Reconstituted into lipid bilayers, CHIP formed 2-D square lattices with unit cell dimensions a = b = 9.6 nm and a p422(1) symmetry. The 3-D map revealed that CHIP tetramers contain central stain-filled depressions about the fourfold axis. These cavities extend from both sides into the transbilayer domain of the molecule leaving only a thin barrier to be penetrated by the water pores. Although CHIP monomers behave as independent pores, we propose that their particular structure requires tetramerization for stable integration into the bilayer.