Evidence against a role of mouse, rat, and two cloned human T1α isoforms as a water channel or a regulator of aquaporin-type water channels

T1 alpha 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 T1 alpha functions as a water channel or a regulator of aqua-porin-type water channels that are coexpressed with T1 alpha. Two complementary DNAs (cDNAs) (hT1 alpha-1 and hT1 alpha-2) encoding human isoforms of Tla: were cloned by homology to the rat Tla coding sequence. The cDNAs encoded 164 (hT1 alpha-1) and 162 (hT1 alpha-2) amino acid proteins with high homology to rat T1 alpha in a putative membrane-spanning domain. hT1 alpha-1 transcripts of 2.6 and 1.3 kb were detected in human lung, heart, and skeletal muscle, and a single hT1 alpha-2 transcript of 1.2 kb was detected in human lung. Rat and mouse T1 alpha were isolated by reverse transcription-polymerase chain reaction and confirmed by DNA sequence analysis. Expression of mouse, rat, and human T1 alpha isoforms in Xenopus oocytes did not increase osmotic water permeability (P-f) above that in water-injected oocytes, nor was there an effect of protein kinase A or C activation; P-f was increased > 10-fold in positive control oocytes expressing aquaporin (AQP)1 or AQP5. Coexpression of AQP1 or AQP5 with excess T1 alpha gave Pf not different From that in oocytes expressing AQP1 or AQP5 alone. Oocyte plasma membrane localization of epitope-tagged Tla protein was confirmed and quantified by immunoprecipitation of microdissected plasma membranes. Quantitative densitometry indicated that the single-channel water permeability of Tloc is under 2 X 10(-16) cm(3)/s, suggesting that T1 alpha is not involved in the high transalveolar water permeability in intact lung. The cloning of hT1 alpha isoforms may permit the development of an assay of type I cell antigen in airspace fluid as a marker of human lung injury.