Direct Phosphorylation and Activation of a Mitogen-Activated Protein
Kinase by a Calcium-Dependent Protein Kinase in Rice |
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Authors: | Kabin Xie Jianping Chen Qin Wang Yinong Yang |
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Institution: | Department of Plant Pathology and Environmental Microbiology, The Huck Institutes of the Life Sciences, Pennsylvania State University, University Park, Pennsylvania 16802 |
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Abstract: | The mitogen-activated protein kinase (MAPK) is a pivotal point of convergence for
many signaling pathways in eukaryotes. In the classical MAPK cascade, a signal is
transmitted via sequential phosphorylation and activation of MAPK kinase kinase, MAPK
kinase (MKK), and MAPK. The activation of MAPK is dependent on dual phosphorylation
of a TXY motif by an MKK, which is considered the sole kinase to phosphorylate and
activate MAPK. Here, we report a novel regulatory mechanism of MAPK phosphorylation
and activation besides the canonical MAPK cascade. A rice (Oryza
sativa) calcium-dependent protein kinase (CDPK), CPK18, was identified as
an upstream kinase of MAPK (MPK5) in vitro and in vivo. Curiously, CPK18 was shown to
phosphorylate and activate MPK5 without affecting the phosphorylation of its TXY
motif. Instead, CPK18 was found to predominantly phosphorylate two Thr residues
(Thr-14 and Thr-32) that are widely conserved in MAPKs from land plants. Further
analyses reveal that the newly identified CPK18-MPK5 pathway represses defense gene
expression and negatively regulates rice blast resistance. Our results suggest that
land plants have evolved an MKK-independent phosphorylation pathway that directly
connects calcium signaling to the MAPK machinery. |
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