Aquaporins constitute a large and highly divergent protein family in maize

Aquaporins (AQPs) are an ancient family of channel proteins that transport water and neutral solutes through a Fore and are found in all eukaryotes an d most prokaryotes. A comparison of the amino acid sequences and phylogenet ic analysis of 31 full-length cDNAs of maize (Zen mays) AQPs shows that the y comprise four different groups of highly divergent proteins. We have clas sified them as plasma membrane intinsic proteins (PIPs), tonoplast intrinsi c proteins, Nod26-like intrinsic proteins, and small and basic intrinsic pr oteins. Amino acid sequence identities vary from 16% to 100%, but all seque nces share structural motifs and conserved amino acids necessary to stabili ze the two loops that form the aqueous pore. Most divergent are the mall an d basic integral proteins in which the first of the two highly conserved As n-Pro-Ala motifs of the pore is not conserved, but is represented by alanin e-proline-threonine or alanine-proline-serine. We present a model of ZmPIP1 -2 based on the three-dimensional structure of mammalian AQP1. Tabulation o f the number of times that the AQP sequences are found in a collection of d atabases that comprises about 470,000 maize cDNAs indicates that a few of t he maize AQPs are very highly expressed and many are not abundantly express ed. The phylogenetic analysis supports the interpretation that the divergen ce of PIPs through gene duplication occurred more recently than tile diverg ence of the members of the ether three subfamilies. This study opens the wa y to analyze the function of the proteins in Xenopus laevis oocytes, determ ine the tissue specific expression of the genes, recover insertion mutants, and determine the in planta function.