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Receptor kinases are the members of the receptor serine/threonine kinases, under the enzyme-linked cell surface receptor family and constitute the largest receptor kinases in plants. They are located in the plasma membrane and initiate the signaling in a variety of plant processes. Receptor kinases are composed of an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular kinase domain.
In plants, the best-studied receptor kinases are those that are activated by the plant growth hormone brassinosteroids (BR). Brassinosteroids not only promotes plant growth but also causes pathogen-associated molecular patterns (PAMPs) that lead to the immune responses.
Originally, it was supposed that tyrosine residues of the receptor kinase are phosphorylated, but recent studies show that in addition to the tyrosine phosphorylation, serine/threonine are also phosphorylated playing an important role in the receptor kinase activation and initiation of the subsequent signaling.
Tyrosine phosphorylation of the ligand-bound receptor kinase
Receptor kinases such as BRI1 contain leucine-rich repeats that are able to autophosphorylate the tyrosine residues as well as serine and threonine residues. In the BRI1, the tyrosine residue is located in the juxtamembrane domain where it plays an important role in the brassinosteroids signaling without affecting the overall kinase activity of the BRI1.
The same was observed with another receptor kinase BAK1 that acts as a coreceptor with BRI1. But in this case, the key tyrosine residue is located in the C-terminal domain and is autophosphorylated by the leucine-rich repeats of the BAK1.
In both cases, the BRI1 and the BAK1, autophosphorylation of the key tyrosine residue by the leucine-rich repeats play an important role in the activation of the receptor kinase and subsequent signaling pathway.
Tyrosine phosphorylation of the inhibitor proteins and protein phosphatases
In the absence of the ligand brassinosteroid (BR), receptor kinase BRI1 remains in an inactive form. The inactivation of the receptor kinase in the absence of the ligand is mediated by the interaction of the receptor kinase with an inhibitor protein BRI1 kinase inhibitor-1(BKI1) located in the plasma membrane.
Therefore, ligand binding causes autophosphorylation of the key tyrosine residues that leads to the release of the inhibitor protein BKI1 into the cytosol. Once BKI1 is released from the BRI1, the inhibition of the kinase activity is released and the affinity between the BRI1 and BAK1 kinase domains increases. Thereby the BR receptor complex (ligand-bound BRI1 together with BAK1) is fully activated.
Tyrosine phosphorylation of the receptor kinases of the immune system
In recent years, several other receptor kinases have been studied that are involved in the plant immune system. Plant pattern-recognition receptors (PRRs) that recognize the pathogen-associated molecular patterns (PAMPs) and trigger the immune signaling are also a type of receptor kinase. PRRs such as FLS2 and EFR are involved in pathogen recognition. FLS2 and EFR perceive the bacterial flagellin and elongation factor-Tu (EF0TU) respectively. In this case, also, the autophosphorylation of the key tyrosine residue is essential for the activation of the ligand-bound receptor and subsequent signaling pathway.
However, it should be noted that BAK1 also acts as a coreceptor with the PRRs during the perception of the PAMPs as well as a positive regulator of the subsequent PAMPs signaling. In the subsequent downstream signaling, the cytoplasmic kinase, botrytis-induced kinase-1 (BIK1) plays an important role. It interacts with the FLS2, EFR, and BAK1 and leads to several downstream signaling cascades.
Tyrosine phosphorylation not only activates the receptor kinase and initiates the subsequent signaling cascade but also inactivates the kinase inhibitors or protein phosphatases. Phosphorylation of the key tyrosine residues of the kinase inhibitors and protein phosphatases leads to the disruption of the inhibition of the receptor kinases by these inhibitory molecules.
In conclusion, tyrosine phosphorylation is an important mechanism for the activation of the receptor kinase and its subsequent signaling cascade in plants.
Reference: Trends in Plant Science (Importance of tyrosine phosphorylation in receptor kinase complexes)
Article DOI: 10.1016/j.tplants.2015.02.005