The structure of AQP1 water channel was determined to 2.2 Å resolution. The channel consists of three topological elements, an extracellular and a cytoplasmic vestibule connected by an extended narrow pore or selectivity filter. At the extracellular end of the selectivity filter is the constriction region, which establishes the steric upper limit of the channel and has an effective solvent accessible diameter of ~2.8 Å. Within the selectivity filter, four bound waters are localized along three hydrophilic nodes which punctuate an otherwise extremely hydrophobic pore segment. This novel combination of a long hydrophobic pore and a minimal number of solute binding sites facilitates rapid water transport. Residues of the constriction region, in particular histidine 182 which is conserved among all known water specific channels, are critical in establishing water specificity. Analysis of the AQP1 pore also indicates that the transport of protons through this channel is highly energetically unfavorable.

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␥-Secretase is a membrane protein complex that cleaves the ␤-amyloid precursor protein (APP) within the transmembrane region, after prior processing by ␤-secretase, producing amyloid ␤-peptides A␤40 and A␤42. Errant production of A␤-peptides that substantially increases A␤ 42 production has been associated with the formation of amyloid plaques in Alzheimer's disease patients. Biophysical and genetic studies indicate that presenilin-1, which contains the proteolytic active site, and three other membrane proteins [nicastrin, anterior pharynx defective-1 (APH-1), and presenilin enhancer-2 (PEN-2)] are required to form the core of the active ␥-secretase complex. Here, we report the purification of the native ␥-secretase complexes from HeLa cell membranes and the identification of an additional ␥-secretase complex subunit, CD147, a transmembrane glycoprotein with two Ig-like domains. The presence of this subunit as an integral part of the complex itself was confirmed through coimmunoprecipitation studies of the purified protein from HeLa cells and of solubilized complexes from other cell lines such as neural cell HCN-1A and HEK293. Depletion of CD147 by RNA interference was found to increase the production of A␤ peptides without changing the expression level of the other ␥-secretase components or APP substrates whereas CD147 overexpression had no statistically significant effect on A␤-peptide production, other ␥-secretase components or APP substrates, indicating that the presence of the CD147 subunit within the ␥-secretase complex down-modulates the production of A␤-peptides.aspartyl protease ͉ membrane protein complex ͉ membrane protein purification