Integrin-linked kinase explained

Integrin-linked kinase is an enzyme that in humans is encoded by the ILK gene involved with integrin-mediated signal transduction. Mutations in ILK are associated with cardiomyopathies.^{[1] [2]} It is a 59kDa protein originally identified in a yeast-two hybrid screen with integrin β1 as the bait protein. Since its discovery, ILK has been associated with multiple cellular functions including cell migration, proliferation, and adhesion.

Integrin-linked kinases (ILKs) are a subfamily of Raf-like kinases (RAF). The structure of ILK consists of three features: 5 ankyrin repeats in the N-terminus, Phosphoinositide binding motif and extreme N-terminus of kinase catalytic domain.^[3] Integrins lack enzymatic activity and depend on adapters to signal proteins. ILK is linked to beta-1 and beta-3 integrin cytoplasmic domains and is one of the best described integrins.^[4] Although first described as a serine/threonine kinase by Hannigan,^[5] important motifs of ILK kinases are still uncharacterized. ILK is thought to have a role in development regulation and tissue homeostasis, however it was found that in flies, worms and mice ILK activity isn't required to regulate these processes.

Animal ILKs have been linked to the pinch- parvin complex which control muscle development. Mice lacking ILK were embryonic lethal due to lack of organized muscle cell development. In mammals ILK lacks catalytic activity but supports scaffolding protein functions for focal adhesions. In plants, ILKs signal complexes to focal adhesion sites. ILKs of plants contain multiple ILK genes. Unlike animals that contain few ILK genes ILKs have been found to possess oncogenic properties. ILKs control the activity of serine/threonine phosphatases.

Principle Features

Transduction of extracellular matrix signals through integrins influences intracellular and extracellular functions, and appears to require interaction of integrin cytoplasmic domains with cellular proteins. Integrin-linked kinase (ILK), interacts with the cytoplasmic domain of beta-1 integrin. Multiple alternatively spliced transcript variants encoding the same protein have been found for this gene.^[6] Recent results showed that the C-terminal kinase domain is actually a pseudo-kinase with adaptor function.^{[7] [8] [9]}

In 2008, ILK was found to localize to the centrosome and regulate mitotic spindle organization.^[10]

Integrin-linked kinase has been shown to interact with:

• ACP6,^[11]
• AKT1,^{[12] [13] [14]}
• ILKAP,^[15] and
• LIMS1,^{[16] [17]}

Function of Plant ILK1

ILKs function by interacting with the many transmembrane receptors to regulate different signaling cascades. ILK1 has been found in the root system of most plants where they are co-localized on the plasma membrane and endoplasmic reticulum where they transport ions across the plasma membrane^[18] ILK1 is responsible for the control of osmotic and salt stress, control of the uptake of nutrients based on availability and pathogen detection.^[19]

Osmotic and salt stress

ILK1 is linked to hyperosmotic stress sensitivity. ILK1 reduced salt stress in seedlings placed in solution with increased concentrations of salt. ILK1 concentrations remain fairly constant throughout development regardless of a high salt exposure. Previously, it was believed that K⁺ accumulation was reduced in increased salt concentration.^[20] K⁺ homeostasis is not affected in high salt concentrations. During periods of high salt stress, K⁺ concentrations in the presence of ILK1 was maintained at the existing level. Potassium transport is required for flg22 root growth inhibition and potassium transport was affected by flg22.

Potassium levels modulate the activation of flg22, a flagellin peptide composed of 22 amino acids that triggers pathogen-associated molecular patterns (PAMPs). PAMPs functions by activating regulators of bacterial pathogen alert system.^[21] Ion concentration levels of Mn²⁺, Mg²⁺, S and Ca²⁺ were also affected after PAMP regulators were mobilized.

Nutrient uptake

Potassium (K⁺) is responsible for osmoregulation, membrane potential maintenance and turgor pressure of plant cells which in turn mediates stomata movement and growth of tubules within the plant. Photosynthesis and other metabolic pathways are controlled by potassium.^[22] When sufficient K⁺ uptake is not met, PAMPs are activated. Calmodulins, specifically CML9, have appeared as important genes to interact with ILK1 and regulate potassium levels within the cell. While CLM9 primarily regulates Ca²⁺ it is linked to a yet identified K⁺/Ca²⁺ influx channel. While interactions are known to occur between CML9 and ILK1, ILK1 Is not a direct phosphorylation target of CML9. With the addition of CML9, autophosphorylation of ILK1 is diminished, the present irrespective of calcium available for uptake.ILK1 is also affected by presence or absence of manganese (Mn²⁺). Autophosphorylation and substrate phosphorylation occurred when exposed to both Mn²⁺ and Mg^2+. Mn²⁺ and was dose dependent where Mg²⁺ was not. Specific ILK autophosphorylation sites were found in the presence of Mn²⁺ but not in the presence of Mg²⁺ which supports the ILK1 dependent phosphorylation suggested above. Mass spectrometry revealed no other kinases were present to trigger this response.

Pathogen detection

ILK1 has been found to promote resistance in bacterial pathogens. ILK1 is required for flg22 sensitivity in seedlings. A catalytically inactive version of ILK1 was compared with catalytically active versions of ILK1 to see the level of resistance when challenged with bacterial pathogens. Plants inoculated with inactive ILK1 were more susceptible to bacterial infection than active ILK1 suggesting that ILK1 is needed for bacterial pathogen detection. While ILK1 is involved in bacterial pathogen detection it is not used for effect induced defenses.

ILK1 increases PAMP response and basal immunity through phosphorylation of MPK3 and MPK6 and operates independently in reactive oxygen species (ROS) production. High Affinity Potassium uptake mediators such as HAK5 have also been found to be integral in the signaling of flg22. HAK5 function when potassium levels are low. Flg22 has been shown to depolarize the cell's plasma membrane with HAK5 and ILK1 working together to mediate ion homeostasis to assist with both short and long term actions such as growth and suppression thereof.

Further reading

• Dedhar S . Cell-substrate interactions and signaling through ILK . Current Opinion in Cell Biology . 12 . 2 . 250–6 . April 2000 . 10712922 . 10.1016/S0955-0674(99)00083-6 .
• Persad S, Dedhar S . The role of integrin-linked kinase (ILK) in cancer progression . Cancer and Metastasis Reviews . 22 . 4 . 375–84 . December 2003 . 12884912 . 10.1023/A:1023777013659 . 24434819 .
• Srivastava D, Yu S . Stretching to meet needs: integrin-linked kinase and the cardiac pump . Genes & Development . 20 . 17 . 2327–31 . September 2006 . 16951248 . 10.1101/gad.1472506 . free .
• Hannigan GE, Bayani J, Weksberg R, Beatty B, Pandita A, Dedhar S, Squire J . Mapping of the gene encoding the integrin-linked kinase, ILK, to human chromosome 11p15.5-p15.4 . Genomics . 42 . 1 . 177–9 . May 1997 . 9177792 . 10.1006/geno.1997.4719 .
• Li F, Liu J, Mayne R, Wu C . Identification and characterization of a mouse protein kinase that is highly homologous to human integrin-linked kinase . Biochimica et Biophysica Acta . 1358 . 3 . 215–20 . October 1997 . 9366252 . 10.1016/S0167-4889(97)00089-X . free .
• Chung DH, Lee JI, Kook MC, Kim JR, Kim SH, Choi EY, Park SH, Song HG . ILK (beta1-integrin-linked protein kinase): a novel immunohistochemical marker for Ewing's sarcoma and primitive neuroectodermal tumour . Virchows Archiv . 433 . 2 . 113–7 . August 1998 . 9737788 . 10.1007/s004280050225 . 24150231 .
• Tu Y, Li F, Goicoechea S, Wu C . The LIM-only protein PINCH directly interacts with integrin-linked kinase and is recruited to integrin-rich sites in spreading cells . Molecular and Cellular Biology . 19 . 3 . 2425–34 . March 1999 . 10022929 . 84035 . 10.1128/mcb.19.3.2425 .
• Feng J, Ito M, Ichikawa K, Isaka N, Nishikawa M, Hartshorne DJ, Nakano T . Inhibitory phosphorylation site for Rho-associated kinase on smooth muscle myosin phosphatase . The Journal of Biological Chemistry . 274 . 52 . 37385–90 . December 1999 . 10601309 . 10.1074/jbc.274.52.37385 . free .
• Janji B, Melchior C, Vallar L, Kieffer N . Cloning of an isoform of integrin-linked kinase (ILK) that is upregulated in HT-144 melanoma cells following TGF-beta1 stimulation . Oncogene . 19 . 27 . 3069–77 . June 2000 . 10871859 . 10.1038/sj.onc.1203640 . 8798909 .
• Velyvis A, Yang Y, Wu C, Qin J . Solution structure of the focal adhesion adaptor PINCH LIM1 domain and characterization of its interaction with the integrin-linked kinase ankyrin repeat domain . The Journal of Biological Chemistry . 276 . 7 . 4932–9 . February 2001 . 11078733 . 10.1074/jbc.M007632200 . free .
• Matsumoto M, Ogawa W, Hino Y, Furukawa K, Ono Y, Takahashi M, Ohba M, Kuroki T, Kasuga M . Inhibition of insulin-induced activation of Akt by a kinase-deficient mutant of the epsilon isozyme of protein kinase C . The Journal of Biological Chemistry . 276 . 17 . 14400–6 . April 2001 . 11278835 . 10.1074/jbc.M011093200 . free .
• Nikolopoulos SN, Turner CE . Integrin-linked kinase (ILK) binding to paxillin LD1 motif regulates ILK localization to focal adhesions . The Journal of Biological Chemistry . 276 . 26 . 23499–505 . June 2001 . 11304546 . 10.1074/jbc.M102163200 . free .
• Tu Y, Huang Y, Zhang Y, Hua Y, Wu C . A new focal adhesion protein that interacts with integrin-linked kinase and regulates cell adhesion and spreading . The Journal of Cell Biology . 153 . 3 . 585–98 . April 2001 . 11331308 . 2190577 . 10.1083/jcb.153.3.585 .
• Yamaji S, Suzuki A, Sugiyama Y, Koide Y, Yoshida M, Kanamori H, Mohri H, Ohno S, Ishigatsubo Y . A novel integrin-linked kinase-binding protein, affixin, is involved in the early stage of cell-substrate interaction . The Journal of Cell Biology . 153 . 6 . 1251–64 . June 2001 . 11402068 . 2192033 . 10.1083/jcb.153.6.1251 .
• Chen R, Kim O, Yang J, Sato K, Eisenmann KM, McCarthy J, Chen H, Qiu Y . Regulation of Akt/PKB activation by tyrosine phosphorylation . The Journal of Biological Chemistry . 276 . 34 . 31858–62 . August 2001 . 11445557 . 10.1074/jbc.C100271200 . free.

Notes and References

1. Brodehl. Andreas. Rezazadeh. Saman. Williams. Tatjana. Munsie. Nicole M.. Liedtke. Daniel. Oh. Tracey. Ferrier. Raechel. Shen. Yaoqing. Jones. Steven J. M.. 2019-02-15. Mutations in ILK, encoding integrin-linked kinase, are associated with arrhythmogenic cardiomyopathy. Translational Research . 208. 15–29. 10.1016/j.trsl.2019.02.004. 1878-1810. 30802431. 7412573.
2. Knöll. Ralph. Postel. Ruben. Wang. Jianming. Krätzner. Ralph. Hennecke. Gerrit. Vacaru. Andrei M.. Vakeel. Padmanabhan. Schubert. Cornelia. Murthy. Kenton. 2007-07-31. Laminin-alpha4 and integrin-linked kinase mutations cause human cardiomyopathy via simultaneous defects in cardiomyocytes and endothelial cells. Circulation. 116. 5. 515–525. 10.1161/CIRCULATIONAHA.107.689984. 1524-4539. 17646580. free.
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