Expansins are small extracellular proteins that promote turgordriven extension of plant cell walls. EXPB1 (also called Zea m 1) is a member of the ␤-expansin subfamily known in the allergen literature as group-1 grass pollen allergens. EXPB1 induces extension and stress relaxation of grass cell walls. To help elucidate expansin's mechanism of wall loosening, we determined the structure of EXPB1 by x-ray crystallography to 2.75-Å resolution. EXPB1 consists of two domains closely packed and aligned so as to form a long, shallow groove with potential to bind a glycan backbone of Ϸ10 sugar residues. The structure of EXPB1 domain 1 resembles that of family-45 glycoside hydrolase (GH45), with conservation of most of the residues in the catalytic site. However, EXPB1 lacks a second aspartate that serves as the catalytic base required for hydrolytic activity in GH45 enzymes. Domain 2 of EXPB1 is an Ig-like ␤-sandwich, with aromatic and polar residues that form a potential surface for polysaccharide binding in line with the glycan binding cleft of domain 1. EXPB1 binds to maize cell walls, most strongly to xylans, causing swelling of the cell wall. Tests for hydrolytic activity by EXPB1 with various wall polysaccharides proved negative. Moreover, GH45 enzymes and a GH45-related protein called ''swollenin'' lacked wall extension activity comparable to that of expansins. We propose a model of expansin action in which EXPB1 facilitates the local movement and stress relaxation of arabinoxylancellulose networks within the wall by noncovalent rearrangement of its target.plant cell wall

Group I grass pollen allergens make up a subgroup of the b-expansin family of cell wall loosening proteins in plants. A recent study reported that recombinant Phl p 1, the group I allergen from timothy grass pollen, was associated with papain-like proteinase activity and suggested that expansins loosen the plant cell wall via proteolysis. We tested this idea with three experimental approaches. First, we evaluated three purified native group I allergens from timothy grass, ryegrass and maize (Phl p 1, Lol p 1, Zea m 1) using five proteinase assays with a variety of substrates. The proteins had substantial wall loosening activity, but no detectable proteolytic activity. Thus we cannot confirm proteolytic activity in the pollen allergen class of b-expansins. Second, we tested the ability of proteinases to induce cell wall extension in vitro. Tests included cysteine proteinases, serine proteinases, aspartic proteinases, metallo proteinases, and aggressive proteinase mixtures, none of which induced wall extension in vitro. Thus, wall proteins are unlikely to be important load-bearing components of the plant cell wall. Third, we tested the sensitivity of b-expansin activity and native wall extension activity to proteinase inhibitors. The results show that a wide range of proteinase inhibitors (phenylmethanesulfonyl fluoride, N-ethylmaleimide, iodoacetic acid, Pefabloc SC, and others) inhibited neither activity. From these three sets of results we conclude proteolysis is not a likely mechanism of plant cell wall loosening and that the pollen allergen class of b-expansins do not loosen cell walls via a proteolytic mechanism.

The proteome of the Arabidopsis flower has not been extensively studied previously. Here, we report a proteomic analysis of the wild type Arabidopsis flower. Using both two-dimensional electrophoresis/mass spectrometry (2-DGE/MS) and multi-dimensional protein identification technology (MudPIT) approaches, we identified 2,446 proteins. Although a single experiment or analysis uncovered only a subset of the proteins we identified, a combination of multiple experiments and analyses facilitated the detection of a greater number of proteins. When proteins are grouped according to RNA expression levels revealed by microarray experiments, we found that proteins encoded by genes with relatively high levels of expression were detected with greater frequencies. On the other hand, at the level of the individual gene/protein, there was not a good correlation between protein spot intensity and microarray values. We also obtained strong evidence for post-translational modification from 2-DGE and MudPIT data. We detected proteins that are annotated to function in protein synthesis, folding, modification, and degradation, as well as the presence of regulatory proteins such as transcription factors and protein kinases. Finally, sequence and evolutionary analysis of genes for active methyl group metabolisms suggests that these genes are highly conserved. Our results allow the formulation of hypotheses regarding post-translational regulation of proteins in the flower, providing new understanding about Arabidopsis flower development and physiology.

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Ascoviruses are a family of insect viruses with circular, double-stranded DNA genomes. With the sequencing of the Trichoplusia ni ascovirus 2c (TnAV-2c) genome, the virion structural proteins were identified by using tandem mass spectrometry. From at least eight protein bands visible on a Coomassie blue-stained gel of TnAV-2c virion proteins, seven bands generated protein sequences that matched predicted open reading frames (ORFs) in the genome, i.e. ORFs 2, 43, 115, 141, 142, 147 and 153. Among these ORFs, only ORF153, encoding the major capsid protein, has been characterized previously.