AQP-MEDIATED PATHWAYS carboxy-terminal ends are both in the cell interior. The molecule consists of two repeats, which are oriented oppositely Water can cross cellular plasma membranes through the and contain the conserved sequence motif asparagine-pro-lipid portion of the bilayer by a diffusion mechanism or line-alanine. AQPs are assembled into homotetramers, and through AQP water channels. 5 Diffusional water movement, each monomer forms a functional water pore that would which occurs in all cells, is constrained by membrane lipid transport water in either direction. 6, 7 Most of the AQPs dis-composition and fluidity, and is characterized by a high actiplay a specific and reversible inhibition by mercurials (eg, vation energy (ie, Ea ú 10 kcal/mol). This indicates that HgCl2). The mercurial sensitivity is due to a cysteine essen-less water transport will occur at lower temperatures because tial for the water channel activity. 4, 5 No other functional the lipid packing is tighter. AQP-expressing cells have meminhibitors of AQPs have been identified. brane water permeabilities well above that expected from Table 1 summarizes several characteristics of the known lipid-phase water permeation alone. The AQP-mediated wamammalian AQPs as well as their expression in liver and ter transport is relatively unaffected by changes of temperaother tissues. AQP1, initially called CHIP 28 (channel-form-ture; as mentioned above, it can be reversibly blocked by ing integral membrane protein of 28 kd), was the first AQP HgCl2. Thus, the effect of temperature and mercurial agents identified by Agre et al. 8 AQP1 is expressed in erythrocytes on osmotic water transport are commonly used to distinguish and in fluid transporting epithelia throughout the body at diffusional and AQP-mediated membrane water pathways. sites of constitutive (not regulated) water transport. How-