Protein phosphorylation regulation in Arabidopsis
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Protein phosphorylation and dephosphorylation are fundamental mechanisms involved in many cell-signaling events. Coordinated actions of protein kinases and protein phosphatases help maintain the signaling through reversible protein phosphorylation and dephosphorylation. In Arabidopsis, both protein kinases and protein phosphatases exist as large protein families. Here the recent progress in understanding the roles of plant protein phosphatases and mitogen-activated protein kinases (MAPKs) is reviewed. To further understand the regulatory functions of protein phosphatases and kinases, I carried out detailed functional analyses of kinase associated protein phosphatase (KAPP) and a MAPK cascade involving MAPK3 and MAPK6. KAPP is the only protein phosphatase that has been shown to physically interact with multiple receptor like protein kinases (RLKs). To understand the role of KAPP in coordinating RLK signal transduction, we carried out detailed functional analyses of KAPP in Arabidopsis. We demonstrated the KAPP protein phosphatase 2C (PP2C) domain is functional in vivo; overexpressing KAPP with a mutation in the PP2C domain (D346G) creates a dominant negative effect and the transgenic plants have a pleiotropic phenotype. Fork-head associated (FHA) domain-mediated protein-protein interaction is responsible for this dominant negative effect. Multiple null alleles of KAPP were isolated and no obvious growth or developmental defects were observed. However, double mutants of kapp3/bri1-5 show increased sensitivity to brassinolide treatment. In vivo and in vitro experiments showed that KAPP interacts with BRI1 in a phosphorylation-dependent manner. These results suggest that KAPP is a negative regulator of the BRI1 signaling pathway MAPK cascades are evolutionarily conserved three-tier protein kinase modules. They function in diverse cellular signal transduction processes including, stress responses, hormonal responses, cell proliferation and cell differentiation. Multi-gene families have been identified encoding each of the three tiers of the MAPK cascades in plants, animals and yeast. The results presented here establish that the MAPK cascade YDA-MKK4/MKK5- MPK3/MPK6 is a key pathway in regulating asymmetric division and coordinating cell fate specification during stomata development and patterning. A lateral inhibition model of stomatal cell fate coordination is proposed. As asymmetric division is correlated with multiple cell fate specifications in plants, understanding the molecular mechanism of asymmetric division in stomatal cell fate coordination will have a significant impact on our understanding of plant cell fate specification in general.
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