Wall-associated kinases (WAKs) are one of many classes of plant proteins known to serve as a medium between the extracellular matrix (ECM) and cytoplasm of cell walls.

[2] WAKs are under a group of receptor-like kinases (RLK) that are actively involved in sensory and signal transduction pathways especially in response to foreign attacks by pathogens[3] and in cell development.

[5] The receptor-like proteins contain a cytoplasmic serine threonine kinase and a less conserved region; bound to the cell wall and contains a series of epidermal growth factor repeats.

[13][9] WAKs protein composed of five types of highly similar genes located tightly in a 30 kb clusters of Arabidopsis genome.

[14][19][6] Moreover, All of the isomers of WAK proteins have epidermal growth factor (EGF) like repeats located at the amino-terminal side.

[20] All WAKs (WAKs 1-5) have Asp/Asn hydroxylation site (Cx[DN]x(4)[FY]xCxC; Prosite PS00010) overlapping with calcium binding EGF domains where both hydroxylated and nonhydroxylated forms of coagulation proteases have equal affinities for calcium at physiological concentrations.

[15] Hydroxyl group may be involved in hydrogen bonding in protein-protein interactions mediated by the EGF-like domain.

Of all these isoforms, WAK1 and WAK2 are most ubiquitous and their messenger RNA (mRNAs) and proteins are present in vegetative meristem and areas of cell expansion.

[22] WAK1 is crosslinked in endomembranes, and its transport to the cell surface requires correct cell-wall synthesis.

[9] This pectin-kinase hybrid located for reporting to the cytoplasm on the cell wall where WAK1 is bound in a calcium-induced conformation to polygalacturonic acid, oligogalacturonides and pectins and this interaction was prevented by methyl esterification, calcium chelators and pectin depolymerization.

[6] They are also contributed to the pathogen and stress responses,[6] heavy metal tolerance,[17] and plant development.

[14] Kohorn et al.(2006a) suggested that WAKs can be cross-linked to cell wall material, however, the assembly and crosslinking of WAKs begin at an early stage within a cytoplasmic compartment rather than in the cell wall itself and also coordinated with the synthesis of surface cellulose.

[29][33] Both WAK1 and WAK 2 bind to a variety of pectins including polymers of homogalacturonan (HA), OGs, and to rhamnogalacturonans (RG) I and II.

[33] The biological activity of pectin fragments, or OGs, contributes to defense and stress responses, and in developmental processes where WAKs function as the receptor.

Pectin is fragmented by biotic and abiotic events and the oligo-galacturonides (OGs), have a higher affinity for the WAKs and induce a stress response".

[38] Wall-associated kinases are receptors with a calcium mediated cross-link to the cell wall of plants.

This suggestion that charge is responsible for the preferable binding of WAKs to de-esterified pectins (negatively charged) was shown in a mutation in cationic residues in a WAK1 gene to neutral residues which resulted to the loss of binding properties to the de-esterified pectins.

Heat and salt do not have an effect in WAK1 production within tissues, however wounding is significant as it causes the expression of a WAK1 message by 2,2-dichloroisonicotinic acid (INA), a natural salicylate (SA) in the signal transduction pathway in the plant response to infection.

[45] This ultimately means that inducing WAK1 expression causes decreased SA levels and a reduced toxicity, hence protection, demonstrating a role of WAK1 in regulating pathogenic attacks.

In comparison to wak2-1, 13 out of the 50 upregulated genes in the wild-type was suppressed in wak2-1 and 37 were expressed similarly to the wild type.

20 genes within those downregulated showed reduced expression in wak2-1 cells, 24 were activated and the remainder had levels similar to the wild type.