CLONING AND FUNCTIONAL EXPRESSION OF A NEW WATER CHANNEL ABUNDANTLY EXPRESSED IN THE TESTIS PERMEABLE TO WATER, GLYCEROL, AND UREA

A new member of the aquaporin (AQP) family has been identified from ra t testis. This gene, referred as aquaporin 7 (AQP7), encodes a 269-ami no acid protein that contained the conserved NPA motifs of MLP family proteins, AQP7 has the amino acid sequence homology with other aquapor ins (similar to 30%), and it is highest with AQP3 (48%), suggesting th at both AQP3 and AQP7 belong to a subfamily in the MIP family, Injecti on of AQP7-cRNA into Xenopus oocytes expressed a 26-kDa protein detect ed by immunoblotting, The expression of AQP7 in oocytes stimulated the osmotic water permeability by 10-fold which was not inhibited by 0.3 mm mercury chloride, The Arrhenius activation energy for the stimulate d water permeability was low (2.1 kcal/mol), AQP7 also facilitated gly cerol and urea transport by 5- and 9-fold, respectively. The activatio n energy for glycerol was also low (5.3 kcal/mol after the correction of the endogenous glycerol permeability of oocytes), Northern blot ana lysis revealed a 1.5-kilobase pair transcript expressed abundantly in testis. In situ hybridization of testis revealed the expression of AQP 7 at late spermatids in seminiferous tubules. The immunohistochemistry of testis localized the AQP7 expression at late spermatids and at mat uring sperms, AQP7 may play an important role in sperm function.