Aquaporins Constitute a Large and Highly Divergent Protein Family in Maize

TLDR

Aquaporins are ancient channel proteins that transport water and neutral solutes, found in all eukaryotes and most prokaryotes, with maize AQPs showing 16–100 % sequence identity yet sharing conserved motifs that stabilize the pore loops. This study aims to enable functional analysis of maize AQPs in Xenopus oocytes, determine tissue‑specific expression, recover insertion mutants, and elucidate their in planta roles. The authors modeled ZmPIP1‑2 on mammalian AQP1 and surveyed ~470,000 maize cDNA sequences to identify highly expressed AQPs versus low‑expressed ones. Phylogenetic analysis of 31 maize AQP cDNAs reveals four divergent subfamilies—PIPs, TIPs, NIPs, and SIPs—with the SIPs being the most divergent, and indicates that PIP diversification via gene duplication is more recent than that of the other subfamilies.

Abstract

Abstract Aquaporins (AQPs) are an ancient family of channel proteins that transport water and neutral solutes through a pore and are found in all eukaryotes and most prokaryotes. A comparison of the amino acid sequences and phylogenetic analysis of 31 full-length cDNAs of maize (Zea mays) AQPs shows that they comprise four different groups of highly divergent proteins. We have classified them as plasma membrane intinsic proteins (PIPs), tonoplast intrinsic proteins, Nod26-like intrinsic proteins, and small and basic intrinsic proteins. Amino acid sequence identities vary from 16% to 100%, but all sequences share structural motifs and conserved amino acids necessary to stabilize the two loops that form the aqueous pore. Most divergent are the small and basic integral proteins in which the first of the two highly conserved Asn-Pro-Ala motifs of the pore is not conserved, but is represented by alanine-proline-threonine or alanine-proline-serine. We present a model of ZmPIP1-2 based on the three-dimensional structure of mammalian AQP1. Tabulation of the number of times that the AQP sequences are found in a collection of databases that comprises about 470,000 maize cDNAs indicates that a few of the maize AQPs are very highly expressed and many are not abundantly expressed. The phylogenetic analysis supports the interpretation that the divergence of PIPs through gene duplication occurred more recently than the divergence of the members of the other three subfamilies. This study opens the way to analyze the function of the proteins in Xenopus laevis oocytes, determine the tissue specific expression of the genes, recover insertion mutants, and determine the in planta function.