THE HUMAN AQP4 GENE - DEFINITION OF THE LOCUS ENCODING 2 WATER CHANNEL POLYPEPTIDES IN BRAIN

The aquaporin family of membrane water transport proteins are expresse d in diverse tissues, and in brain the predominant water channel prote in is AQP4. Here we report the isolation and characterization of the h uman AQP4 cDNAs and genomic DNA, Two cDNAs were isolated corresponding to the two initiating methionines (M1 in a 323-aa polypeptide and M23 in a 301-aa polypeptide) previously identified in rat [Jung, J. S., B hat, R. V., Preston, G. M., Guggino, W. B. & Agre, P. (1994) Proc, Nat l, Acad. Sci, USA 91, 13052-13056]. Similar to other aquaporins, the A QP4 gene is composed of four exons encoding 127, 55, 27, and 92 amino acids separated by introns of 0.8, 0.3, and 5.2 kb. Unlike other aquap orins, an alternative coding initiation sequence (designated exon 0) w as located 2.7 kb upstream of exon 1. When spliced together, M1 and th e subsequent 10 amino acids are encoded by exon 0; the next 11 amino a cids and M23 are encoded by exon 1. Transcription initiation sites hav e been mapped in the proximal promoters of exons 0 and 1. RNase protec tion revealed distinct transcripts corresponding to M1 and M23 mRNAs, and AQP4 immunoblots of cerebellum demonstrated reactive polypeptides of 31 and 34 kDa. Using a P1 and a lambda EMBL subclone, the chromosom al site of the human AQP4 gene was mapped to chromosome 18 at the junc tion of q11.2 and q12.1 by fluorescence in situ hybridization. These s tudies may now permit molecular characterization of AQP4 during human development and in clinical disorders.