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.

Endosperm of cereal grains is one of the most important renewable resources for food, feed, and industrial raw material. It consists of four triploid cell types, i.e., aleurone, starchy endosperm, transfer cells, and cells of the embryo surrounding region. In maize, the aleurone layer is one cell layer thick and covers most of the perimeter of the endosperm. Specification of maize aleurone cell fate is proposed to occur through activation of the tumor necrosis factor receptor-like receptor kinase CRINKLY4. A second maize gene essential for aleurone cell development is defective kernel 1 (dek1). Here we show that DEK1 shares high homology with animal calpains. The predicted 2,159-aa DEK1 protein has 21 transmembrane regions, an extracellular loop, and a cysteine proteinase domain that shares high homology with domain II of m-calpain from animals. We propose that DEK1 functions to maintain and restrict the aleurone cell fate imposed by CR4 through activation of its cysteine proteinase by contact with the outer endosperm surface. DEK1 seems to be the only member of the calpain superfamily in plants, Arabidopsis DEK1 sharing 70% overall identity with maize DEK1. The expression of dek1 in most plant tissues in maize and Arabidopsis, as well as its presence in a variety of higher plants, including angiosperms and gymnosperms, suggests that DEK1 plays a conserved role in plant signal transduction.

Maize starchy endosperm mutants have kernel phenotypes that include a brittle texture, susceptibility to insect pests, and inferior functional characteristics of products made from their flour. At least 18 such mutants have been identified, but only in the cases of opaque2 ( o2 ) and floury2 ( fl2 ), which affect different aspects of storage protein synthesis, is the molecular basis of the mutation known. To better understand the relationship between the phenotypes of these mutants and their biochemical bases, we characterized the protein and amino acid composition, as well as the mRNA transcript profiles, of nearly isogenic inbred lines of W64A o1 , o2 , o5 , o9 , o11 , Mucuronate (

The transport of water through membranes is regulated in part by aquaporins or water channel proteins. These proteins are members of the larger family of major intrinsic proteins (MIPs). Plant aquaporins are categorized as either tonoplast intrinsic proteins (TIPs) or plasma membrane intrinsic proteins (PIPs). Sequence analysis shows that PIPs form several subclasses. We report on the characterization of three maize (Zea mays) PIPs belonging to the PIP1 and PIP2 subfamilies (ZmPIP1a, ZmPIP1b, and ZmPIP2a). The ZmPIP2a clone has normal aquaporin activity in Xenopus laevis oocytes. ZmPIP1a and ZmPIP1b have no activity, and a review of the literature shows that most PIP1 proteins identified in other plants have no or very low activity in oocytes. Arabidopsis PIP1 proteins are the only exception. Control experiments show that this lack of activity of maize PIP1 proteins is not caused by their failure to arrive at the plasma membrane of the oocytes. ZmPIP1b also does not appear to facilitate the transport of any of the small solutes tried (glycerol, choline, ethanol, urea, and amino acids). These results are discussed in relationship to the function and regulation of the PIP family of aquaporins.

We have investigated the transport of storage proteins, their processing proteases, and the Vacuolar Sorting Receptor-1/ Epidermal Growth Factor Receptor-Like Protein1 (VSR-1/ATELP1) receptor during the formation of protein storage vacuoles in Arabidopsis thaliana embryos by means of high-pressure freezing/freeze substitution, electron tomography, immunolabeling techniques, and subcellular fractionation. The storage proteins and their processing proteases are segregated from each other within the Golgi cisternae and packaged into separate vesicles. The storage protein-containing vesicles but not the processing enzyme-containing vesicles carry the VSR-1/ATELP1 receptor. Both types of secretory vesicles appear to fuse into a type of prevacuolar multivesicular body (MVB). We have also determined that the proteolytic processing of the 2S albumins starts in the MVBs. We hypothesize that the compartmentalized processing of storage proteins in the MVBs may allow for the sequential activation of processing proteases as the MVB lumen gradually acidifies.