Expansin
Encyclopedia
Expansin refers to a family of closely related nonenzymatic proteins found in the plant cell wall, with important roles in plant cell growth, fruit softening, abscission, emergence of root hairs, pollen tube invasion of the stigma and style, meristem function, and other developmental processes where cell wall loosening occurs. Expansins were originally discovered as mediators of acid growth
Acid growth
Acid growth refers to the ability of plant cells and plant cell walls to elongate or expand quickly at low pH. This form of growth does not involve an increase in cell number. During acid growth, plant cells enlarge rapidly because the cell walls are made more extensible by expansin, a...

, which refers to the widespread characteristic of growing plant cell walls to expand faster at low (acidic) pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 than at neutral pH. Expansins are thus linked to auxin
Auxin
Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...

 action. They are also linked to cell enlargement and cell wall changes induced by other plant hormones such as gibberellin
Gibberellin
Gibberellins are plant hormones that regulate growth and influence various developmental processes, including stem elongation, germination, dormancy, flowering, sex expression, enzyme induction, and leaf and fruit senescence....

, cytokinin
Cytokinin
Cytokinins are a class of plant growth substances that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence...

, ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...

 and brassinosteroids.

A subset of the β-expansins are also the major group-1 allergen
Allergen
An allergen is any substance that can cause an allergy. In technical terms, an allergen is a non-parasitic antigen capable of stimulating a type-I hypersensitivity reaction in atopic individuals....

s of grass pollens.

Families

So far, two large families of expansin genes have been discovered in plants, named alpha-expansins (given the gene symbol EXPA) and beta-expansins (EXPB). Both families of expansins have been identified in a wide range of land plants, from angiosperms and gymnosperms to ferns and mosses. The model plant Arabidopsis thaliana
Arabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...

contains around 26 different α-expansin genes and 6 β-expansin genes. A subset of β-expansins has evolved a special role in grass pollen, where they are known as group 1 grass pollen allergens. Plants also have a small set of expansin-like genes (named EXLA and EXLB) whose function has not been established. Some proteins in bacteria and fungi are known to have distant sequence similarity to plant expansins. Strong evidence that at least some of these sequences are indeed expansins came in 2008 when the crystal structure of the YOAJ protein from a bacterium (Bacillus subtilis
Bacillus subtilis
Bacillus subtilis, known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium commonly found in soil. A member of the genus Bacillus, B. subtilis is rod-shaped, and has the ability to form a tough, protective endospore, allowing the organism to tolerate...

) was shown to be very similar to the structure of plant expansins, despite the low sequence similarity. This study also noted that proteins related to YOAJ were found in diverse species of plant pathogenic bacteria, but not in related bacteria that did not attack or colonize plants, thus suggesting that these bacterial expansins have a role in plant-microbe interactions.
Some animals can too produce a functional expansin, such as Globodera rostochiensis
Globodera rostochiensis
Globodera rostochiensis, commonly known as the golden nematode, golden eelworm or yellow potato cyst nematode, is a plant pathogenic nematode...

, a plant-parasitic nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

, which uses it to loosen cell walls when invading its host plant.

To be designated as expansin or expansin-like, genes and their protein products must contain both domain I (N-terminal, catalytic, GH45-like - GH meaning glycoside-hydrolase) and domain II (C-terminal, distantly related to group-2 grass pollen allergens).
Non-plant expansins can be designated with the symbol EXLX (expansin-like X), but they do not constitute to a monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 group; distantly similar to plant expansins, they could have diverged prior to the origin of land plants, or else could have been acquired by horizontal transfer.

Nomenclature of genes and proteins of expansins and expansin-like: e.g., Arabidopsis thaliana EXPANSIN A1 is named "AtEXPA1" as for the gene, and "AtEXPA1" as for the protein; one adds "-1" for mutant allele 1.

Actions

Expansins characteristically cause wall stress relaxation
Wall stress relaxation
Wall stress relaxation refers to the reduction in tensile stress in cell walls of plants, fungi and bacteria, as a result of movement or rearrangement of the polymeric network that gives the wall its tensile strength. Wall stress usually comes from cell hydrostatic pressure or turgor pressure,...

 and irreversible wall extension (wall creep). This process is essential for cell enlargement. Expansins are also expressed in ripening fruit where they function in fruit softening, and in grass pollen, where they loosen stigmatic cell walls and aid pollen tube penetration of the stigmain germinating seeds for cell wall disassembly, in floral organs for their patterning, in developing nitrogen-fixing nodules in legumes, in abscissing leaves, in parasitic plants, and in ‘resurrection’ plants during their rehydration. No enzymatic activity has been found for expansin and in particular, no glucanase activity: they don't hydrolyze the matrix polysaccharides; the only definitive assay for expansin activity is thus to measure wall stress relaxation or wall extension.

Structure and regulation

Expansins are proteins; the two expansins initially uncovered had molecular weights of 29 kDa
KDA
KDA may refer to:* Karachi Development Authority* Kongsberg Defence & Aerospace* Kotelawala Defence Academy* Kramer Design Associates* Lithium diisopropylamide, KDA is the potassium analogue of lithium diisopropylamideOr kDa may refer to:...

 (kiloDalton
KDA
KDA may refer to:* Karachi Development Authority* Kongsberg Defence & Aerospace* Kotelawala Defence Academy* Kramer Design Associates* Lithium diisopropylamide, KDA is the potassium analogue of lithium diisopropylamideOr kDa may refer to:...

) and 30 kDa, which would correspond to around 270 aminoacids on average. Generally speaking, α- and β-expansins and expansin-like are composed of approximately 300 aminoacids, with a MW of ~25–28 kDa for the mature protein. The peptidic sequence of an expansin consists, in particular, of: a signal peptide of around 20–30 aminoacides at the N-terminal end, the putative catalytic domain, a His-Phe-Asp (HFD) motif in central region (except EXL), and the C-terminal putative cellulose-binding domain with conserved Trp (tryptophane) residues. Sequence analysis of expansin genes shows seven introns named A, B, C, D, E, F, and G; sequences from different expansin genes show good correspondence, the exon/intron organization being conserved among α- and β-expansins, and expansin-like genes, although the number of introns and the length of each intron differ among genes. In the N-terminal signal sequences of α-expansin genes, the general absence of endoplasmic reticulum retention signal (KDEL or HDEL) confirms that the proteins are targeted to the cell wall.
A promoter analysis of expansin genes indicates that expression of these genes may be regulated by auxin, gibberellin, cytokinin or ethylene, this being more frequent in α-expansins than in β-expansins; semi-aquatic plants such as Rumex palustris
Rumex palustris
Rumex palustris or Marsh Dock is a plant species of the genus Rumex.The species was mentioned by Leonard Plukenet, based on a collection made by Isaac Rand. The authority of the accepted description, Rumex palustris, was James Edward Smith...

, which are induced to grow rapidly by submergence, show a transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...

 induction by submergence, the same as in rice where hypoxia and submergence increase α-expansin mRNA levels.

Mechanism

The plant cell wall has high tensile strength
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...

 and must be loosened to enable the cell to grow (enlarge irreversibly). Within the cell wall, this expansion of surface area involves slippage or movement of cellulose microfibril
Microfibril
The microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, examples are hair and sperm tail. Its most frequently observed structural pattern is 9+2...

s, which normally is coupled to simultaneous uptake of water. In physical terms, this mode of wall expansion requires cell turgor pressure
Turgor pressure
Turgor Pressure or turgidity is the main pressure of the cell contents against the cell wall in plant cells and bacteria cells, determined by the water content of the vacuole, resulting from osmotic pressure, i.e...

 to stretch the cell wall and put the network of interlinked cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 microfibrils under tension. By loosening the linkages between cellulose microfibrils, expansins allow the wall to yield to the tensile stresses created in the wall through turgor pressure. The molecular mechanism by which expansin loosens the cellulosic network within the cell wall is not yet established in detail. However, expansin is hypothesized to disrupt the non-covalent adhesion or entrapment of hemicellulose
Hemicellulose
A hemicellulose is any of several heteropolymers , such as arabinoxylans, present along with cellulose in almost all plant cell walls. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength...

 on the surface of cellulose microfibrils. Hemicelluloses can tether cellulose microfibrils together, forming a strong load-bearing network. Expansin is thought to disrupt the cellulose-hemicellulose association transiently, allowing slippage or movement of cell wall polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s before the association reforms and the integrity of the cell wall network is reestablished.

Turning to the function of bacterial expansins, the bacterial protein named YOAJ or BsEXLX1 binds to plant and bacterial cell walls and has weak but significant expansin activity, that is, it induces plant cell wall extension in vitro. Moreover, B. subtilis mutants lacking BsEXLX1 were defective in colonizing plant roots, suggesting that this protein facilitates plant-bacterium interactions.

Allergenicity

In grass pollens, the major allergens (groupe-1 allergen
Allergen
An allergen is any substance that can cause an allergy. In technical terms, an allergen is a non-parasitic antigen capable of stimulating a type-I hypersensitivity reaction in atopic individuals....

s, main causative agents of hay fever
Hay Fever
Hay Fever is a comic play written by Noël Coward in 1924 and first produced in 1925 with Marie Tempest as the first Judith Bliss. Laura Hope Crews played the role in New York...

 and of seasonal asthma
Asthma
Asthma is the common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include wheezing, coughing, chest tightness, and shortness of breath...

) are structurally linked to a sub-group of the β-expansins. These expansins appear specialized in pollination, likely in loosening the cell walls of the maternal tissues during penetration of the pollen tube into the stigma and style, as is suggested by their potent rheological effect on grass style and stigma walls, where they are abundantly released by the pollen. Expansin-like are implicated in group-2 and -3 grass allergenes, less important than those of groupe-1. These three allergens groups share a carbohydrate-binding module
Carbohydrate-binding module
In molecular biology, a carbohydrate-binding module is a protein domain found in carbohydrate-active enzymes . The majority of these domains have carbohydrate-binding activity. Some of these domains are found on cellulosomal scaffoldin proteins. CBMs were previously known as cellulose-binding...

(CBM), which could be responsible for the binding to the IgE antibody. The expansin domain II, causative of the allergenic effects, could be related to the competition between pollens for access to ovules.

Sources

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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