THE 3-DIMENSIONAL STRUCTURE OF HUMAN ERYTHROCYTE AQUAPORIN CHIP

Water-permeable membranes of several plant and mammalian tissues conta in specific water channel proteins, the 'aquaporins'. The best charact erized aquaporin is CHIP, a 28 kDa red blood cell channel-forming inte gral protein. Isolated CHIP and Escherichia coli lipids may be assembl ed into 2-D crystals for structural analyses. Here we present (i) a st ructural characterization of the solubilized CHIP oligomers, (ii) proj ections 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 electr on microscopy. Reconstituted into lipid bilayers, CHIP formed 2-D squa re lattices with unit cell dimensions a = b = 9.6 nm and a p422(1) sym metry, 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 mono mers behave as independent pores, we propose that their particular str ucture requires tetramerization for stable integration into the bilaye r.