Cloning and Functional Characterization of a Novel Aquaporin fromXenopus laevis Oocytes

We have cloned a novel aquaporin (AQP) from Xenopus laevis oocytes, which we have provisionally named AQPxlo. The predicted protein showed highest homology (39-50%) to aquaglyceroporins. Northern blot analysis showed strong hybridization to an similar to1.4-kb transcript in X. laevis fat body and oocytes, whereas a weaker signal was obtained in kidney. We injected in vitro transcribed cRNA encoding AQPxlo into Xenopus oocytes for functional characterization. AQPxlo expression increased osmotic water permeability (P-f), as well as the uptake of glycerol and urea. However, AQPxlo excluded larger polyols and thiourea. An alkaline extracellular pH (pH(o)) increased P-f and to a lesser extent urea uptake but not glycerol uptake. Remarkably, low HgCl2, concentrations (0.3-10 mum) reduced P-f and urea uptake, whereas high concentrations (300-1000 mum) reversed the inhibition. We propose that AQPxlo is a new AQP paralogue unknown in mammals.