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HandWiki. Cell Wall Associated Kinase. Encyclopedia. Available online: (accessed on 14 June 2024).
HandWiki. Cell Wall Associated Kinase. Encyclopedia. Available at: Accessed June 14, 2024.
HandWiki. "Cell Wall Associated Kinase" Encyclopedia, (accessed June 14, 2024).
HandWiki. (2022, November 03). Cell Wall Associated Kinase. In Encyclopedia.
HandWiki. "Cell Wall Associated Kinase." Encyclopedia. Web. 03 November, 2022.
Cell Wall Associated Kinase

Cell wall associated kinases (WAKs) are receptor-like protein kinases, found in plant cell walls, that have the capability to transmit signals directly by their cytoplasmic kinase domains. They usually link the plasma membrane to the protein and carbohydrate that composed the cell wall. 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. WAKs are found in various plants and crops like rice, and maize. In plants genome like Arabidopsis, WAKs, are encoded by five highly similar genes clustered in a 30-kb locus, among them WAK1 & WAK2 are highly distributed. They are primarily involved in regulating plant cell wall functions including cell expansion, bind as well as response to pectins, pathogen response and also protects plants from detrimental effects.

plant cell wall plant cell serine threonine kinase

1. Structure

All the five WAK proteins have highly conserved serine/threonine protein kinase domain (86% similarity) on the cytoplasmic side and an extracellular domain (only 40% to 64% similarity in amino acid sequences).[1][2][3] Moreover, All of the isomers of WAK proteins have epidermal growth factor (EGF) like repeats located at the amino-terminal side.[3] Six cysteins (located in the EGF repeats) positions are well-kept in all the five WAKs, however, protein-protein interactions of WAks are still unknown.[4]

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.[5] Hydroxyl group may be involved in hydrogen bonding in protein-protein interactions mediated by the EGF-like domain.[6]

WAKs' association with cell wall is very strong (having covalent link to pectin), such that its release from the cell wall requires enzymatic digestion.[7] Under conditions that collapse the turgor of a plant cell so as to separate the membrane from the wall (plasmolysis), the WAKs-wall association is so strong that they remain in the cell wall. There are five WAK's isoform in Arabidopsis with variable extracellular domain within these isoform, all of which contain at least two epidermal growth factor (EGF). 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.[7]

2. Interaction

WAK1 is crosslinked in endomembranes, and its transport to the cell surface requires correct cell-wall synthesis.[8] The interaction between WAK1 and pectins (Pectins are complex oligopolysaccharides formed a hydrophilic gel-like matrix between the cellulose microfibrils, and can be concentrated in different regions of the cell wall)[9] was confirmed by using anti-WAK1 and anti-pectin JIM5 and JIM7 antibodies recognized the same 68 kDa protein band in western blots of the cell wall proteins extracted from pectinase-treated cell walls.[10] 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.[11][12] The interaction of pectin polyanion with the cell wall or plasmalemma could induce conformational changes in the pectin polymers that affect their gelling and swelling behavior in the presence of the calcium[13] and the binding of pectins to WAK1 in the presence of calcium could result in muro disturbances of the pectin network that could generate signals within the cell wall.[13]

3. Function

Wall associated Kinases (WAKs) contribute several functions (cell division or growth) as other plant receptors like cell wall sensors, however, the unique characteristics is to bind directly to pectin that postulates a WAK-dependent signaling pathway regulating cell expansion.[3] They are also contributed to the pathogen and stress responses,[3] heavy metal tolerance,[14] and plant development.[3]

WAKs may contribute to cell elongation since they have an active cytoplasmic protein kinase domain that span the plasma membrane, and contain an N terminus which binds the cell wall whether WAK2 can regulate invertase at the transcriptional level.[15] WAKs can also regulate cell expansion through a control of sugar concentration and thus turgor control where wak2-1 phenotype could be rescued by the expression of sucrose phosphate synthase that alters sugar sinks.[16] However, Antisense WAK RNA can be induced using the Dex system which contributes to a 50% reduction in WAK protein levels as well as a smaller cell size, rather than fewer cells.[17][18][19] A wak2-1 (WAK2 null allele) causes a loss of cell expansion in roots, but only under limiting sugar and salt conditions,[16] however, Individual loss of function alleles in any of the four other WAKs do not result in an obvious phenotype.[1] 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.[8] WAKs are released from pectinase of the cell wall material where they are bound to pectins.[17][19] Therefore, WAK1 or 2 binds to pectin have a higher affinity for de-esterified pectin than to esterified molecules. Moreover, short pectin fragments of a degree of polymerization (dp) 9–15 effectively competed with longer pectins for WAK binding.[16][20] Both WAK1 and WAK 2 bind to a variety of pectins including polymers of homogalacturonan (HA), OGs, and to rhamnogalacturonans (RG) I and II.[20] The binding requirements are not to a simple polymer of HA, but perhaps the presence of galacturonic acid.[20] The biological activity of pectin fragments, or OGs, contributes to defense and stress responses, and in developmental processes where WAKs function as the receptor.[21][22][23][24]

Wall-associated kinases are involved in pathogen and stress responses.[16]

4. Signal Transduction Pathway

Kohorn (2016) suggested that "pectin polymers can be cross-linked in the cell wall with Ca+, and WAKs bind these pectins and signal via the activation of vacuolar invertase and numerous other induced proteins to aid in cell expansion. The methyl esterification state of the pectin is modulated by pectin methylesterases (PMEs) and WAKs bind de-methylated pectin with higher affinity. 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".[25]

5. History

The association of WAKs with The Plant Cell wall was first compromised by immunolocalization technique using antiserum where epitome of WAK are found to be tightly bound with cell wall fragments so that they can not be separated using detergent, however, WAKs could be released by boiling the walls with SDS, dithiothreitol (a strong thiol reductant), protoplasting enzymes or pectinase.[10][26]

6. Gene

WAKs protein composed of five types of highly similar genes located tightly in a 30 kb clusters of Arabidopsis genome.[1][26] Most of the WAKs are expressed throughout the plant whether WAK1, 2, 3 and 5 are expressed in green organs, WAK1 and 2 weakly expressed in flowers and siliques and WAK2 is also expressed in roots, however, WAK4 is only expressed in siliques.[5] There are also 21 WAK like gene in Arabitopsis genome known as WAKLs which have a little sequence similarity with WAKs.[27]

The Ara WAK and WAKL genes are distributed among all five chromosomes of Arabidopsis

Chromosome number Genes localized
I WAKL1-13, WAKL22, WAK1-5


WAK/WAKL gene family members in Arabidopsis were divided into four groups based on the pair-wise comparisons of their predicted protein sequences. WAK1 to WAK5 containing EGF-Ca2+ domain with an overlapping Asp/Asn hydroxylation site and an EGF-2 domain were placed in Group I. Both the EGF domains were predicted to be completely encoded by the second exon. Seven WAKL members that include WAKL1 to WAKL6 and WAKL22 were placed in Group II. In all these group II genes, the EGF-Ca2+ and EGF-2 domains are separated by a short gap of 15-18 aa and were reversed in order relative to group I. The EGF-Ca2+ domain is encoded by first exon and EGF-Ca2+ domain is encoded by second exon. No Asn/asp hydroxylation site was predicted.[14]

Group III contains six members: WAKL9, WAKL10, WAKL11, WAKL13, WAKL17, and WAKL18. Their corresponding proteins all contain EGF-Ca2+ and EGF2 domains, and they are structurally similar to the Group II WAKLs. In WAKL13, the EGF-Ca2+ domain is degenerate. With the exception of WAKL17, all have degenerate EGF2 domains.[14]

Group IV contains four members: WAKL14, WAKL15, WAKL20, and WAKL21. Each has an EGF2 domain encoded by the first exon. This domain is degenerate in both WAKL20 and WAKL21. All four members lack the EGF-Ca2+ domain. In addition, each has a cytoplasmic protein kinase ATP-binding domain (PS00107). The remaining sequences (WAKL7, WAKL8, WAKL12, WAKL16, and WAKL19) are predicted to encode abbreviated WAKL proteins. WAKL7, WAKL8 and WAKL19 are similar to various other WAKLs in their extracellular regions, and lack a transmembrane domain. WAKL8 and WAKL9 both contain an EGF-Ca2+ domain and WAKL19 contains a degenerate EGF2 domain. Neither of these domains is present in WAKL7. WAKL12 also contains an EGF-Ca2+ domain, but unlike WAKL8, it contains a trans-membrane domain. WAKL16 contains a transmembrane domain, an STK domain that is most similar to WAK3, and a short extracellular domain of eight amino acids that lacks both of the EGF-like domains.[14]

7. Families

Wall-Associated Kinases (WAKs) are a subfamily of receptor-like kinases (RLKs) associated with the cell wall.[26] They were described in Arabidopsis thaliana as a cluster of five (WAK1-5),[1] and 22 (WAKL1-WAKL22) genes.[27]

WAK/WAKL (OsWAK) gene Family in Rice[28]

  • OsWAK-RLKs (Receptor-like kinases)- contain both extracellular EGF-like domains and an intracellular kinase domain[28]
  • OsWAK-RLCKs (Receptor-like cytoplasmic kinases)- contain only the kinase domain[28]
  • OsWAK-RLPs (receptor-like proteins)- Contain only the extracellular EGF-like domains[28]
  • OsWAK short genes- lacking both the domains but has > 40% identity at the aminoacid level with other OsWAK members[28]
  • pseudogenes (with either stop codons or frame shifts in coding region)[28]


  1. He, Zheng-Hui; Cheeseman, Iain; He, Deze; Kohorn, Bruce D (1999). "A cluster of five cell wall-associated receptor kinase genes, Wak1-5, are expressed in specific organs of Arabidopsis". Plant Molecular Biology 39 (6): 1189–96. doi:10.1023/A:1006197318246. PMID 10380805.
  2. 3.0.CO;2-N. PMID 10813841." id="ref_2">Sampoli Benitez, Benedetta A; Komives, Elizabeth A (2000). "Disulfide bond plasticity in epidermal growth factor". Proteins: Structure, Function, and Genetics 40 (1): 168–74. doi:10.1002/(SICI)1097-0134(20000701)40:1<168::AID-PROT180>3.0.CO;2-N. PMID 10813841.
  3. Kohorn, Bruce D; Kohorn, Susan L (2012). "The cell wall-associated kinases, WAKs, as pectin receptors". Frontiers in Plant Science 3: 88. doi:10.3389/fpls.2012.00088. PMID 22639672.
  4. Sivaguru, M; Ezaki, B; He, Z. H; Tong, H; Osawa, H; Baluska, F; Volkmann, D; Matsumoto, H (2003). "Aluminum-Induced Gene Expression and Protein Localization of a Cell Wall-Associated Receptor Kinase in Arabidopsis". Plant Physiology 132 (4): 2256–66. doi:10.1104/pp.103.022129. PMID 12913180.
  5. Decreux, Annabelle; Messiaen, Johan (2005). "Wall-associated Kinase WAK1 Interacts with Cell Wall Pectins in a Calcium-induced Conformation". Plant and Cell Physiology 46 (2): 268–78. doi:10.1093/pcp/pci026. PMID 15769808.
  6. Stenflo, Johan; Stenberg, Yvonne; Muranyi, Andreas (2000). "Calcium-binding EGF-like modules in coagulation proteinases: Function of the calcium ion in module interactions". Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology 1477 (1–2): 51–63. doi:10.1016/S0167-4838(99)00262-9. PMID 10708848.
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  8. Kohorn, Bruce D.; Kobayashi, Masaru; Johansen, Sue; Friedman, Henry Perry; Fischer, Andy; Byers, Nicole (2006). "Wall-associated kinase 1 (WAK1) is crosslinked in endomembranes, and transport to the cell surface requires correct cell-wall synthesis". Journal of Cell Science 119 (11): 2282–90. doi:10.1242/jcs.02968. PMID 16723734.
  9. Carpita, Nicholas C; Gibeaut, David M (1993). "Structural models of primary cell walls in flowering plants: Consistency of molecular structure with the physical properties of the walls during growth". The Plant Journal 3 (1): 1–30. doi:10.1111/j.1365-313X.1993.tb00007.x. PMID 8401598.
  10. Wagner, Tanya A.; Kohorn, Bruce D. (2001). "Wall-associated kinases are expressed throughout plant development and are required for cell expansion". The Plant Cell 13 (2): 303–18. doi:10.1105/tpc.13.2.303. PMID 11226187.
  11. Decreux, A; Thomas, A; Spies, B; Brasseur, R; Cutsem, P; Messiaen, J (2006). "In vitro characterization of the homogalacturonan-binding domain of the wall-associated kinase WAK1 using site-directed mutagenesis". Phytochemistry 67 (11): 1068–79. doi:10.1016/j.phytochem.2006.03.009. PMID 16631829.
  12. Deeks, Michael J; Hussey, Patrick J; Davies, Brendan (2002). "Formins: Intermediates in signal-transduction cascades that affect cytoskeletal reorganization". Trends in Plant Science 7 (11): 492–8. doi:10.1016/S1360-1385(02)02341-5. PMID 12417149.
  13. MacDougall, Alistair J; Brett, Gary M; Morris, Victor J; Rigby, Neil M; Ridout, Michael J; Ring, Stephen G (2001). "The effect of peptide–pectin interactions on the gelation behaviour of a plant cell wall pectin". Carbohydrate Research 335 (2): 115–26. doi:10.1016/S0008-6215(01)00221-X. PMID 11567642.
  14. Kanneganti, Vydehi; Gupta, Aditya K (2008). "Wall associated kinases from plants — an overview". Physiology and Molecular Biology of Plants 14 (1–2): 109–18. doi:10.1007/s12298-008-0010-6. PMID 23572878.
  15. Wagner, T.A.; Kohorn, B.D. (2001). "Wall-associated kinases are expressed throughout plant development and are required for cell expansion". Plant Cell 13 (2): 303–318. doi:10.1105/tpc.13.2.303. PMID 11226187.
  16. Kohorn B. D., Kobayashi M., Johansen S., Riese J., Huang L. F., Koch K., Fu S., Dotson A., Byers N. (2006b). An Arabidopsis cell wall-associated kinase required for invertase activity and cell growth. Plant J. 46 307–316
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  18. Lally, D.; Ingmire, P.; Tong, H. Y.; He, Z. H. (2001). "Antisense expression of a cell wall-associated protein kinase, WAK4, inhibits cell elongation and alters morphology". Plant Cell 13 (6): 1317–1331. doi:10.2307/3871298. PMID 11402163.
  19. Kohorn, B. D. (2001). "WAKs; cell wall associated kinases". Curr. Opin. Cell Biol. 13 (5): 529–533. doi:10.1016/s0955-0674(00)00247-7. PMID 11544019.
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  22. "Pectin: cell biology and prospects for functional analysis". Plant Mol. Biol. 47 (1–2): 9–27. September 2001. doi:10.1023/A:1010662911148. PMID 11554482.
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  24. "Biosynthesis of pectin". Plant Physiol. 153 (2): 384–95. June 2010. doi:10.1104/pp.110.156588. PMID 20427466.
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