Cloned Human Aquaporin-1 Is a Cyclic GMP-Gated Ion Channel

ABSTRACT Aquaporin-1 (AQP1) is a member of the membrane intrinsic protein (MIP) gene family and is known to provide pathways for water flux across cell membranes. We show here that cloned human AQP1 not only mediates water flux but also serves as a cGMP-gated ion channel. Two-electrode voltage-clamp analyses showed consistent activation of an ionic conductance in wild-type AQP1-expressing oocytes after the direct injection of cGMP (50 nl of 100 mM). Current activation was not observed in control (water-injected) oocytes or in AQP5-expressing oocytes with osmotic water permeabilities equivalent to those seen with AQP1. Patch-clamp recordings revealed large conductance channels (150 pS in K ϩ saline) in excised patches from AQP1-expressing oocytes after the application of cGMP to the internal side. Amino acid sequence alignments between AQP1 and sensory cyclic-nucleotide-gated channels showed similarities between the cyclic-nucleotide-gated binding domain and the AQP1 carboxyl terminus that were not present in AQP5. Competitive radioligand-binding assays with Aquaporin-1 (AQP1) channels provide high transmembrane water movement for a specialized subset of mammalian tissues, including red blood cells, kidney (proximal tubule and descending Henle's loop), eye (ciliary epithelia, trabecular meshwork, and canals of Schlemm), brain (choroid plexus), lung (peribronchial vasculature), and other tissues, including lymph vessels and muscle Several members of the membrane intrinsic protein (MIP) family of channels have been shown to carry ionic currents. Reconstitution of MIP into lipid bilayers yielded large-conductance ion channels with single-channel events showing unitary conductances of 180 and 380 pS, long open times, and symmetrical activation over a range of voltages in 100 mM KCl salin