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Tyrosine phosphorylation of protein kinase complex BAK1/BIK1 mediates Arabidopsis innate immunity

The innate immune response is initiated by the recognition of conserved microbial signatures via membrane-resident receptor complex. The dimerization and phosphorylation of Arabidopsis protein kinase complex FLS2/BAK1/BIK1 is essential to initiate and transduce immune signaling to bacterial flagellin

Wenwei Lina,1, Bo Lia,b,c,1, Dongping Lub, Sixue Chend, Ning Zhud, Ping Heb, and Libo Shana,2

 

Author Affiliations

 

1.Departments of aPlant Pathology and Microbiology and
    2.bBiochemistry and Biophysics, Institute for Plant Genomics and Biotechnology, Texas A&M University, College Station, TX 77843;
         3.cProvincial Key Laboratory of Plant Pathology of Hubei Province, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, Hubei 430070, People’s Republic of China; and
              4.dDepartment of Biology, Genetics Institute, Plant Molecular and Cellular Biology Program, University of Florida, Gainesville, FL 32610

1. Edited by Frederick M. Ausubel, Harvard Medical School; Massachusetts General Hospital, Boston, MA, and approved January 13, 2014 (received for review October 7, 2013)

 

Significance

 

The innate immune response is initiated by the recognition of conserved microbial signatures via membrane-resident receptor complex. The dimerization and phosphorylation of Arabidopsis protein kinase complex FLS2/BAK1/BIK1 is essential to initiate and transduce immune signaling to bacterial flagellin. BIK1, a classic serine/threonine kinase, was found here to be autophosphorylated and transphosphorylated by BAK1 at multiple tyrosine residues to relay plant immune signaling. The essential function of tyrosine kinase activity of BIK1 in plants echoes the function of nonreceptor tyrosine kinases that transduce receptor tyrosine kinase signaling via dimerization and phosphorylation in metazoans. Thus, despite lack of classical tyrosine kinases, tyrosine phosphorylation is also an important regulatory mechanism to control membrane-resident receptor signaling in plants.

 

Abstract

 

The sessile plants have evolved a large number of receptor-like kinases (RLKs) and receptor-like cytoplasmic kinases (RLCKs) to modulate diverse biological processes, including plant innate immunity. Phosphorylation of the RLK/RLCK complex constitutes an essential step to initiate immune signaling. Two Arabidopsis plasma membrane-resident RLKs, flagellin-sensing 2 and brassinosteroid insensitive 1-associated kinase 1 (BAK1), interact with RLCK Botrytis-induced kinase 1 (BIK1) to initiate plant immune responses to bacterial flagellin. BAK1 directly phosphorylates BIK1 and positively regulates plant immunity. Classically defined as a serine/threonine kinase, BIK1 is shown here to possess tyrosine kinase activity with mass spectrometry, immunoblot, and genetic analyses. BIK1 is autophosphorylated at multiple tyrosine (Y) residues in addition to serine/threonine residues. Importantly, BAK1 is able to phosphorylate BIK1 at both tyrosine and serine/threonine residues. BIK1Y150 is likely catalytically important as the mutation blocks both tyrosine and serine/threonine kinase activity, whereas Y243 and Y250 are more specifically involved in tyrosine phosphorylation. The BIK1 tyrosine phosphorylation plays a crucial role in BIK1-mediated plant innate immunity as the transgenic plants carrying BIK1Y150F, Y243F, or Y250F (the mutation of tyrosine to phenylalanine) failed to complement the bik1 mutant deficiency in immunity. Our data indicate that plant RLCK BIK1 is a nonreceptor dual-specificity kinase and both tyrosine and serine/threonine kinase activities are required for its functions in plant immune signaling. Together with the previous finding of BAK1 to be autophosphorylated at tyrosine residues, our results unveiled the tyrosine phosphorylation cascade as a common regulatory mechanism that controls membrane-resident receptor signaling in plants and metazoans.

 

http://www.pnas.org/content/111/9/3632.abstract.html?etoc

PNAS March 4, 2014 Vol. 11 no. 9:3632–3637

 

Figure 1: BIK1 interacts with BAK1 in vivo and in vitro.

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