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WAKs - Cell wall associated kinase

Abstract

Cell wall associated kinases, or WAKs, are a group of glycoproteins present in the cell wall of vegetables . Cellulose, hemicellulose and pectin are the main fibrous carbohydrates present in the cell wall. Cell wall also contain protein on its composition, and both protein and carbohydrate groups are important on plant cell physiology. Cell wall associated kinases are linked to pectinin on cell wall and it is thought to be responsible for some cell function such as cell expansion, cell signaling and cell defense against pathogens

WAKs are receptor-like kinases linked to the pectin portion present in the wall of cell plants. Furthermore, they have link to protein kinase present on the cytoplasm. Recents findings indicates that cell wall associated kinases are necessary for cell wall expansion, are related to immune response against antigens and their synthesis seems to be stimulated by environment

Its mechanisms are not yet fully understood. However, much has been discovered about this class of protein in the last few years.

Introduction - The plant extracellular matrix

Students of animal biology picture the extracellular matrix (ECM, the space in between cells) as a substrate in which cells can interact. On the other hand, the plant cell wall is traditionally viewed as a cage that may as well limits the cell and therefore most students do not see the cell wall as its ECM. However, WAKs recent findings might help plant scientists to have a more broad view of plants ECM . Nevertheless, science understanding on how communication between external and internal compartments of the plant cell (cell wall and cytoplasm, respectively) is still little when compared to animals.

Proteins covalently bounded to carbohydrates form the cell walls of plant cells. Carbohydrates must provide structural support to the plant, but it also need to be loosely enough to enable individual cell growth. The carbohydrate composition of cell wall changes according to the age of the plant. Main carbohydrates present on the cell wall are cellulose, hemicelulose (both fibrous carbohydrate), pectin (non-fibrous carbohydrate) and lignin (non-carbohydrate phenol compound that provides physical support to cells and are in greater percentage in old cells).

On the other hand, very little is known about the protein composition of cell walls. Research has identified proteins rich in hydroxyproline, proline, glycine and arabinogalactan. Examples of these proteins are expansins, lectins, peroxidases, methyltransferases, galactosidases, glycanases, and proteases. Unexpectedly, integrins, selectins, cadherins and other adhesion molecules that help mediate cell to ECM communication in animal tissue, are not present on vascular plants. So how can communication in between external and internal compartments are able to happen?

Previous studies on Arabidopsis have identified several proteins classes on plants' cell wall, such as arabinogalactan proteins,  cellulose synthases,  hydrolytic enzyme, and some others; however, their mechanisms are yet to be established.

A WAK superfamily?

All five known WAK isoforms are clustered in a 30 kb locus of Arabidopsis. They have similar features such as ptn-kinase domain, trans membrane region and cell wall localization. There are also other proteins so similar to WAKs that were named WLKs or WAK-like. WAK and WLK are 20-30% alike in their extracellular domain – epidermal growth factors. On top of that, WAKs are only expressed in roots, but not into flowers. Therefore, there must be other proteins replacing WAK1-5 in flowers, though WAKs and ALKs seems to be the only receptor-like ptn-kinase in Arabidopsis containing EGF

Wall associated kinase

WAKs are responsible for linking the cell wall to the kinase domain in cytoplasm; naturally passing thought the cell membrane. It is strongly bounded to the cell wall and dissociation requires enzymatic digestion or usage of specific detergent. WAKs is thought to be covalently bound to pectin .

WAKs family are therefore a known cell-wall-cytoplasmic signaling molecule. WAKs of angiosperms physically link the plasma membrane to the fibrous carbohydrate matrix and are unique in that they have the potential to directly signal cellular events through their cytoplasmic kinase domain. There are five isoforms of wall-associated kinases, whereas WAK1 and WAK2 are the most common, as they are present in vegetative meristems, junctions of organ types and areas of cell expansion.

The WAK isoforms share the same structure and are 87% identical in their cytoplasmic kinase. Their amino termini are linked to the cell wall and all of them contain EGF repeats adjacent to the trans membrane domain. The extracellular regions are 40–64% identical among isoforms .

WAK genes

First, it was thought that WAK was encoded by a single gene copy. More recently, PCR analysis has identified WAK genes

WAKs bind cell wall glycine-rich proteins (GRPs)

Besides binding pectin, WAKs is very likely to bind some proteins and other uncharacterized component of the cell wall - that are yet to be discovered.  The external domain of WAKs isoforms sGRP3 (secreted glycine rich protein), which would requires a domais WAK EGF, indicating relation between WAKs and leaf cell morphology. GRP are secreted cell wall proteins are present in plant vascular tissue, therefore they are also classified as "structural proteins" . Secreted GRP are synthesided in roughly all plants organs and have as characteristic the high level of glycine. GRP seem to be bind by WAK 1, but not WAK 2.

Besides pectin, WAKs can also bind proteins present in the cell wall. The extracellular domain of WAKs binds sGRP3 (secreted glycine rich protein). GRP are cell wall proteins evolved on development of vascular tissue and because of that, they are called “structural proteins”. GRP are typically rich in glycine and are present in 30 different genes in Arabidopsis. Six GRP have similar aminoacidic composition outside of the glycine-rich domain, some of them might not bind WAK2, but they all bind WAK1.

A WAK superfamily?

The five WAKs isoforms have two different epidermal growth facture (EGF), with similar protein kinase domain, all of them have been identified on Arabidopsis genes. There are 16 other predicted by Arabidopsis genomes

Plant cell development

During its initial development, plants cell must be able to grow in size. Cell wall able to grow and to elongate because the wall polymers molecule are not linked. The are between cells is called lamelas and it is composed mainly by pectin substances. Pectin are a carbohydrate group made of galacturonan polymers with neutral sugars substitutions.