Abstract
Transmembrane signaling is crucial for organisms to perceive and respond to their environment. It allows the organisms to adapt to changing environments, coordinate growth and development, and respond to pathogens. In plants, transmembrane signaling is majorly mediated by receptor kinases (RKs) and receptor proteins that cooperate with RKs. Consequently, phosphorylation is crucial for the intracellular signal activation and propagation process. Interestingly, despite lacking bona fide tyrosine kinases, plant transmembrane signaling involves phosphorylated tyrosine residues. A conserved tyrosine residue in kinase subdomain VIa (VIa-Tyr) is phosphorylated and critical for function in multiple Arabidopsis RKs. Yet, a mechanistic explanation for the importance of tyrosine phosphorylation in signaling activation is lacking. In choanozoans, tyrosine phosphorylation serves as an anchor for the recruitment of downstream signaling components and is negatively regulated by protein tyrosine phosphatases (PTPs). Here, it was tested if VIa-Tyr phosphorylation in plants fulfills a similar mechanistic role as choanozoan pTyr and if it is regulated by PTPs. Despite testing an extensive set of candidate phosphatases and applying two different approaches for affinity purifying VIa-pTyr interacting proteins, neither a regulatory phosphatase nor a VIa-pTyr binding protein could be identified. In contrast, the importance of the unphosphorylated VIa-Tyr for conformational switching was demonstrated, suggesting no functional importance of VIa-pTyr. Homology-based design of intragenic EFRY836F suppressor mutations identified an activating mutation that could also partially restore the function of the signaling-deficient BAK1Y403F. These results suggest that EFR allosterically activates BAK1 by supporting BAK1’s transition into an active conformation.
Mühlenbeck, Henning