植物细胞壁松弛蛋白——膨胀素

膨胀素(expansin,也称作扩张素或扩张蛋白)是一种引起植物细胞壁松弛的蛋白质,在植物细胞伸展以及一系列涉及细胞壁修饰的生命活动中起着关键作用。膨胀素由多基因家族编码,目前的研究表明膨胀素超家族由4个基因亚家族构成。膨胀素存在于不同的种属植物中,并克隆了大量的扩张蛋白基因。综述了近年来国内外有关膨胀素基因和蛋白的结构特征及作用机制等方面的研究进展。     

Genome histories clarify evolution of the expansin superfamily: new insights from the poplar genome and pine ESTs

Expansins comprise a superfamily of plant cell wall-loosening proteins that has been divided into four distinct families, EXPA, EXPB, EXLA and EXLB. In a recent analysis of Arabidopsis thaliana and Oryza sativa expansins, we proposed a further subdivision of the families into 17 clades, representing independent lineages in the last common ancestor of monocots and eudicots. This division was based on both traditional sequence-based phylogenetic trees and on position-based trees, in which genomic locations and dated segmental duplications were used to reconstruct gene phylogeny. In this article we review recent work concerning the patterns of expansin evolution in angiosperms and include additional insights gained from the genome of a second eudicot species, Populus trichocarpa, which includes at least 36 expansin genes. All of the previously proposed monocot-eudicot orthologous groups, but no additional ones, are represented in this species. The results also confirm that all of these clades are truly independent lineages. Furthermore, we have used position-based phylogeny to clarify the history of clades EXPA-II and EXPA-IV. Most of the growth of the expansin superfamily in the poplar lineage is likely due to a recent polyploidy event. Finally, some monocot-eudicot clades are shown to have diverged before the separation of the angiosperm and gymnosperm lineages. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Isolation and characterization of 18 genes encoding α- and β-expansins in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Expansins are thought to be key regulators of cell wall extension during plant growth. In this study, we isolated 18 expansin genes from wheat, nine of which encode α-expansins while the other nine code for β-expansins. The cysteine-rich and tryptophan-rich regions of the deduced amino acid sequences of all 18 expansins were highly conserved. Genomic sequences were obtained for 17 of the genes, and their intron patterns were determined. Four (A, C, D, E) of the six intron positions known in expansin genes from other species were found to be occupied in these wheat expansin genes. Five wheat expansin genes were mapped to chromosomes 1L, 2L, 5L and 6L respectively, by in silico and comparative mapping. The 18 wheat expansin genes were expressed in leaf, root and the developing seed. Moreover, it was demonstrated that four β-expansin genes were up-regulated in the internode tissue in F₁ hybrids, suggesting that changes in the regulation of these genes in hybrid might contribute to the heterosis observed in internode length and plant height. We therefore conclude that expansins are encoded by a multigene family in wheat, and could play important roles in growth and development. Z. Lin and Z. Ni contributed to this work equally     

Origin, distribution and 3D-modeling of Gr-EXPB1, an expansin from the potato cyst nematode Globodera rostochiensis

Southern analysis showed that Gr-EXPB1, a functional expansin from the potato cyst nematode Globodera rostochiensis, is member of a multigene family, and EST data suggest expansins to be present in other plant parasitic nematodes as well. Homology modeling predicted that Gr-EXPB1 domain 1 (D1) has a flat beta-barrel structure with surface-exposed aromatic rings, whereas the 3D structure of Gr-EXPB1-D2 was remarkably similar to plant expansins. Gr-EXPB1 shows highest sequence similarity to two extracellular proteins from saprophytic soil-inhabiting Actinobacteria, and includes a bacterial type II carbohydrate-binding module. These results support the hypothesis that a number of pathogenicity factors of cyst nematodes is of procaryotic origin and were acquired by horizontal gene transfer.     

小立碗藓扩展蛋白基因家族的鉴定与生物信息学分析

扩展蛋白(Expansins,EXP)是一类基因家族,几乎参与了植物发育的全过程,从种子萌发到果实成熟都有扩展蛋白的参与。该研究利用生物信息学的方法对小立碗藓(Physcomitrella patens) Expansin基因家族成员进行鉴定,分析了其基因结构、染色体定位以及系统发生关系。结果表明:小立碗藓基因组中含有Expansin A(EXPA) 32个、Expansin-like A(EXLA) 6个,并未发现Expansin-like B(EXLB)及Expansin B(EXPB)。扩展蛋白氨基酸序列长度在228～290 aa之间,编码蛋白质具有两个保守的结构域Pollen_allerg_1和DPBB_1。蛋白质亚细胞定位预测结果表明:运用CELLO在线工具预测发现小立碗藓中约4/5的EXP家族基因定位于细胞外;而Euk-mPLoc预测结果则显示小立碗藓EXP基因家族成员全定位于细胞外。基因结构分析表明,小立碗藓中约68%Expansin基因有含有1～3个内含子。以上结果可为深入研究小立碗藓扩展蛋白基因的分子进化与生物学功能奠定基础。     

Structure-function analysis of the bacterial expansin EXLX1

We made use of EXLX1, an expansin from Bacillus subtilis, to investigate protein features essential for its plant cell wall binding and wall loosening activities. We found that the two expansin domains, D1 and D2, need to be linked for wall extension activity and that D2 mediates EXLX1 binding to whole cell walls and to cellulose via distinct residues on the D2 surface. Binding to cellulose is mediated by three aromatic residues arranged linearly on the putative binding surface that spans D1 and D2. Mutation of these three residues to alanine eliminated cellulose binding and concomitantly eliminated wall loosening activity measured either by cell wall extension or by weakening of filter paper but hardly affected binding to whole cell walls, which is mediated by basic residues located on other D2 surfaces. Mutation of these basic residues to glutamine reduced cell wall binding but not wall loosening activities. We propose domain D2 as the founding member of a new carbohydrate binding module family, CBM63, but its function in expansin activity apparently goes beyond simply anchoring D1 to the wall. Several polar residues on the putative binding surface of domain D1 are also important for activity, most notably Asp82, whose mutation to alanine or asparagine completely eliminated wall loosening activity. The functional insights based on this bacterial expansin may be extrapolated to the interactions of plant expansins with cell walls.     

Variable expansin expression in Arabidopsis leads to different growth responses

Expansins have long been implicated in the control of cell wall extensibility. However, despite ample evidence supporting a role for these proteins in the endogenous mechanism of plant growth, there are also examples in the literature where the outcome of altered expansin gene expression is difficult to reconcile with a simplistic causal linkage to growth promotion. To investigate this problem, we report on the analysis of transgenic Arabidopsis plants in which a heterologous cucumber expansin can be inducibly overexpressed. Our results indicate that the effects of expansin expression on growth depend on the degree of induction of expansin expression and the developmental pattern of organ growth. They support the role of expansin in directional cell expansion. They are also consistent with the idea that excess expansin might itself impede normal activities of cell wall modifications, culminating in both growth promotion and repression depending on the degree of expression.