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T1α is a protein of unknown function that is expressed at the plasma membrane in epithelia involved in fluid transport, including type I alveolar epithelial cells, choroid plexus, and ciliary epithelium. The purpose of this study was to test the hypothesis that T1α functions as a water channel or a regulator of aquaporin-type water channels that are coexpressed with T1α. Two complementary DNAs (cDNAs) (hT1α-1 and hT1α-2) encoding human isoforms of T1α were cloned by homology to the rat T1α coding sequence. The cDNAs encoded 164 (hT1α-1) and 162 (hT1α-2) amino acid proteins with high homology to rat T1α in a putative membrane-spanning domain. hT1α-1 transcripts of 2.6 and 1.4 kb were detected in human lung, heart, and skeletal muscle, and a single hT1α-2 transcript of 1.2 kb was detected in human lung. Rat and mouse T1α were isolated by reverse transcription-polymerase chain reaction and confirmed by DNA sequence analysis. Expression of mouse, rat, and human T1α isoforms in Xenopus oocytes did not increase osmotic water permeability (Pf) above that in water-injected oocytes, nor was there an effect of protein kinase A or C activation; Pf was increased > 10-fold in positive control oocytes expressing aquaporin (AQP)1 or AQP5. Coexpression of AQP1 or AQP5 with excess T1α gave Pf not different from that in oocytes expressing AQP1 or AQP5 alone. Oocyte plasma membrane localization of epitope-tagged T1α protein was confirmed and quantified by immunoprecipitation of microdissected plasma membranes. Quantitative densitometry indicated that the single-channel water permeability of T1α is under 2 × 10-16 cm3/s, suggesting that T1α is not involved in the high transalveolar water permeability in intact lung. The cloning of hT1α isoforms may permit the development of an assay of type I cell antigen in airspace fluid as a marker of human lung injury.