Integrin-like receptors (ILRs) are found in plants and carry unique functional properties similar to true

integrin Integrins are transmembrane receptors that facilitate cell-cell and cell-extracellular matrix (ECM) adhesion. Upon ligand binding, integrins activate signal transduction pathways that mediate cellular signals such as regulation of the cell cycle, ...

proteins. True

homologs A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points alon ...

of integrins exist in mammals, invertebrates, and some fungi but not in plant cells. Mammalian integrins are

heterodimer In biochemistry, a protein dimer is a macromolecular complex formed by two protein monomers, or single proteins, which are usually non-covalently bound. Many macromolecules, such as proteins or nucleic acids, form dimers. The word ''dimer'' has ...

transmembrane protein A transmembrane protein (TP) is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequentl ...

s that play a large role bidirectional signal transduction. As transmembrane proteins, integrins connect the

extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide stru ...

(ECM) to the

plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...

of the animal cell. The extracellular matrix of plant cells, fungi, and some protist is referred to as the

cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...

. The plant cell wall is composed of a tough cellulose polysaccharide rather than the collagen fibers of the animal ECM. Even with these differences, research indicates that similar proteins involved in the interaction between the ECM and animals cells are also involved in the interaction of the cell wall and plant cells. Integrin-like receptors and integrin-linked kinases together have been implicated in surface adhesion, immune response, and ion accumulation in plant cells in a manner akin to the family of integrin proteins.

Structure

ILRs contain a transmembrane region with a large extracellular portion and a smaller intracellular section. Most commonly, ILRs resembles the β₁ subunit found in integrin proteins. This structural similarity between ILRs and integrins was determined through various imaging techniques, SDS-PAGE, western blotting, and kinetic studies. These proteins are around 55 to 110 kDa and some studies have found them to react with animal anti-β₁ antibodies suggesting the structural similarity between animal integrins and these plant integrin-like receptors. Some ILRs mimic the α-subunit of integrin proteins containing the ligand binding region known as the I-domain. The I-domain functions primarily in the recognition and binding of a ligand. Conformational changes in the I-domain leads to ILR activation and is dependent on metal ion interaction at metal-ion-dependent adhesion sites (MIDAS). Activation of these sites occur in the presence of Mg²⁺, Mn²⁺, and Ca²⁺. The extracellular domain of most ILRs contain the highly conserved tripepetid sequence Arg-Gly-Asp (RGD). This sequence is commonly found in integrins and other molecules that attach to the

for cell adhesion. The discovery of the RGD sequence in many proteins suggest the same adhesive ability. While the RGD sequence is the most common, some ILRs have been found with sequences that are similar but differ in one amino acid. A plant protein with structural similarity to integrins contains the amino acid sequence Asn-Gly-Asp (NGD).

Function

Plants

ILRs play a role in protein-protein interaction and are found in the plasma membrane of plant cells in the leaf, root and vasculature of plants. Plants produce a physiological response that is dependent on information obtained from the environment. The majority of this information is received through mechanical signals which include touch, sound, and gravity. Therefore, the interaction between the ECM and the internal cell response is incredibly important for receiving and interpreting information. The specific functionality of ILRs in plants is not well characterized but in addition to mechanical signaling transduction, they are believed to have some role in plant immune response, osmotic stress sensitivity, and ion regulation within the cell.

Surface-Adhesion

Some β₁ integrin-like receptors on the root caps of Tabaco plants are found to play a role in the plant’s ability to detect gravitational pull and aid in root elongation in a process known as

gravitropism Gravitropism (also known as geotropism) is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general featu ...

. ILRs are found on the cellular membrane of plant

protoplast Protoplast (), is a biological term coined by Hanstein in 1880 to refer to the entire cell, excluding the cell wall. Protoplasts can be generated by stripping the cell wall from plant, bacterial, or fungal cells by mechanical, chemical or enzy ...

s. The dispersion of the ILRs on these protoplasts can vary from species to species. The variation in the ILR surface placement has been correlated to species growth behavior. For example, ''

Rubus fruticosus ''Rubus fruticosus'' L. is the ambiguous name of a European blackberry species in the genus ''Rubus'' in the rose family. The name has been interpreted in several ways: *The species represented by the type specimen of ''Rubus fruticosus'' L., ...

'' cells have a uniformed distribution of ILRs on their cellular membrane while ''

Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...

'' contains ILRs that cluster resulting in cell growth clusters.

Immunology

Integrin-like receptors have the capability to relay messages from inside the cell to the outside of the cell and vice versa. This is an important factor in the initiation and sustaining of an immunological response. A good body of research has found ILR proteins that model the glycoproteins

vitronectin Vitronectin (VTN or VN) is a glycoprotein of the hemopexin family which is abundantly found in serum, the extracellular matrix and bone. In humans it is encoded by the ''VTN'' gene. Vitronectin binds to integrin alpha-V beta-3 and thus promotes c ...

and

fibronectin Fibronectin is a high- molecular weight (~500-~600 kDa) glycoprotein of the extracellular matrix that binds to membrane-spanning receptor proteins called integrins. Fibronectin also binds to other extracellular matrix proteins such as collage ...

, two important molecules in membrane stability and homeostasis. These virtonectin-like and fibronectin-like protein provide further support that compounds in the cell membrane of plant cells have important regulatory functions in the immune response such as the activation of immune cells. The non-race specific disease resistance-1 (NDR1) primarily discovered to have a large function in plant immune response. This protein shares functional homology with mammalian integrins in that it connects the ECM to the intracellular matrix to both stabilize the cell structure and allow for signal exchange. NDR1 is also believed to be involved in cell wall adhesion to the plasma membrane and fluid retention of the cell.

Fungi

In addition to adhesive properties, integrin-like receptors with RGD-binding sites have special functions in fungi. Using peptides that inhibit the activity of proteins with RGD activation, ILR were discovered in '' Magnaporthe oryzae'' to initiate fungal conidial adhesion and appressorium formation needed for host infection. ''

Candida albicans ''Candida albicans'' is an opportunistic pathogenic yeast that is a common member of the human gut flora. It can also survive outside the human body. It is detected in the gastrointestinal tract and mouth in 40–60% of healthy adults. It is us ...

'' is an opportunistic fungi with an integrin-like receptor protein known as αInt1p. This protein maintains structural similarity and sequence homology to the α-subunits of human leukocyte integrins. The αInt1p protein contains an RGD extracellular binding site and allows the organism to attach to epithelial cells in the host organism to begin the infection process. Once bound, the protein then assists in the morphogenesis of the fungi into a tube-like structure.

Invertebrates

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s, protein structures with the RGD-binding sequence assist in an array of different functions such as the repairing of wounds and cell adhesion. Integrin-like receptors are found in

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

and have a part in the spreading of

hemocytes A blood cell, also called a hematopoietic cell, hemocyte, or hematocyte, is a cell produced through hematopoiesis and found mainly in the blood. Major types of blood cells include red blood cells (erythrocytes), white blood cells (leukocytes), a ...

to damaged locations in the cellular system. Studies that block the RGD-binding site of these integrin-like receptors indicate a reduction in hemocyte aggregation and spreading suggesting the RGD-binding site on integrin-like receptors is a necessary component in organismal immune response. Further support for this calm shows RGD-binding inhibition reduces nodule formation and encapsulation in invertebrate immune response.{{cite journal , vauthors = Johansson MW , title = Cell adhesion molecules in invertebrate immunity , journal = Dev. Comp. Immunol. , volume = 23 , issue = 4–5 , pages = 303–15 , date = 1999 , pmid = 10426424, doi = 10.1016/S0145-305X(99)00013-0

References

Transmembrane receptors