Expression cloning and characterization of an osmotic solute channel from rat liver

Transport of water and solutes across cell membranes is important in all living cells. To elucidate how transport of large amounts of solutes are coordinated in metabolically active cells like hepatocytes, we screened a rat liver cDNA library by monitoring urea uptake in Xenopusoocytes, and isolated an unique aqueous pore that confers high osmotic permeability for both water and solutes, called "osmotic" solute (aquaporin-like) channel (OS-AQP). OS-AQP is a 295-amino acid protein that has low-to-moderate homology with aquaporins AQP3 (48% identity), AQP7 (46%), AQP1 (30%). and with the bacterial glycerol facilitator GlpF (37%). OS-AQP mRNA is expressed in liver, testis, and brain. OS-AQP expression in oocytes significantly increased the permeability for carbamides (urea, thiourea), polyols (glycerol, mannitol, sorbitol). purines (adenine), pyrimidines (uracil and 5-fluorouracil), with permeability coefficient (P_s)=15 25 x 10^-6cm/s. Despite this promiscuous permselectivity, OS-AQP was impermeable to cyclic sugars, uridine, amino acids, and ions. The sensitivity of water and solute permeabilities to phloretin and HgCl₂ were very similar, while reflection coefficients of the substrates were close to zero, suggesting that water and solutes share a common pathway. Collectively, OS-AQP defines a distinct evolutionary branch of MIP family and exemplifies a paradigm of water and solute movement passing through a single pore, enabling exit or uptake of metabolites without osmotic perturbation.