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RACK1, scaffolding a heterotrimeric G protein and a MAPK cascade

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Scaffold proteins of mitogen-activated protein kinase (MAPK) cascades play crucial roles in determining signal specificity, amplitude, and duration in yeast and mammals. Recently, RACK1 was identified as the first plant MAPK scaffold protein that connects heterotrimeric G protein with a MAPK cascade to form a unique signaling pathway in plant immunity.

Section snippets

Scaffolding MAPK cascades in yeast and mammals

MAPK cascades are highly conserved signaling modules in eukaryotes. A canonical MAPK cascade consists of three sequentially acting kinases, with the upstream MAPK kinase kinases (MAPKKKs) receiving signals from the sensors/receptors and activating downstream MAPK kinases (MAPKKs) that then activate the bottom tier MAPKs through phosphorylation. Over the past two decades, scaffold proteins in yeast and mammals have been extensively studied and their roles extended from tethering to substrate

Emerging scaffold protein of a MAPK cascade in Arabidopsis

In Arabidopsis (Arabidopsis thaliana), MPK3 and MPK6 play diverse functions downstream of different sensors/receptors in many biological processes 3, 4. Signaling specificity can be achieved through (i) spatiotemporal expression and interaction of receptors and their ligands, and (ii) the availability of different downstream substrates. In addition, scaffold proteins have been speculated to mediate the distinct activation and functions of the same MAPK(s) in different pathways [4]. However,

Is OsRACK1 a MAPK scaffold protein in rice?

Research by Ko Shimamoto and colleagues demonstrates that rice (Orya sativa) heterotrimeric Gα acts upstream of the small GTPase OsRac1 in disease resistance. Later, they found that OsMAPK6 and OsRac1 exist in the same protein complex. Interestingly, OsMAPK6 protein level is much lower in OsGα mutant (drawf1) and OsRac1 RNAi lines, although its mRNA level is normal, suggesting a regulation at protein stability level [8]. However, the levels of MPK6 protein and flg22-induced MPK6 activation are

Concluding remarks

Identification of RACK1 as the first plant MAPK scaffold protein that links MEKK1-MKK4/MKK5-MPK3/MKP6 cascade to upstream heterotrimeric G protein helps unraveling the signaling specificity in plants. Although MAPK activation is recognized as an important signaling event in plant immunity and frequently used as a marker of plant defense, how the sensing of invading pathogens (through PAMPs and effectors) triggers the activation of a MAPK cascade is largely unknown. RACK1, originally identified

Acknowledgements

We apologize for not being able to cite all related references because of space limitations. The research in Xu and Zhang laboratories is supported by grants from the Natural Science Foundation of China (91317303 and 31300244), Zhejiang University 985 Project (Number 118000-193411801), and the National Science Foundation (IOS-0743957 and MCB-0950519).

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    These results indicate that UvCBP1 does not alter the localization of OsRACK1A or affect its stability. RACK1, present in many protein complexes (Gibson, 2012), has no enzymatic activity by itself but functions as a scaffold protein to bridge protein interactions in diverse cellular processes, such as protein translation (Nilsson et al., 2004) and mitogen-activated protein kinase (MAPK) cascade-mediated signaling (Su et al., 2015). The rice homolog OsRACK1A can be detected in the OsRac1-centered defensome complex (Nakashima et al., 2008), which plays an important role in rice immunity against M. oryzae (Akamatsu et al., 2013).

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    Rice RACK1 played a key role in reactive oxygen species (ROS) production and innate immunity by interacting with the Rac1 immune complex [49]. Guanine nucleotide-binding proteins (G proteins) were shown to associate with RACK1 and regulate the expression of RACK1 in rice [50]. In this study, the abundance of G protein was increased by 4 times under the individual UV-B radiation but didn't change under the combined UV + D treatment (Table S5).

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    Recently, the Arabidopsis Receptor for Activated C Kinase 1 (RACK1) proteins, encoded by RACK1A, RACK1B and RACK1C genes, have been shown to act as the first plant MAPK scaffold proteins that connect three-tier MAPK cascade to upstream heterotrimeric G proteins (Cheng et al., 2015). Identification of RACK1 as the MAPK scaffold protein unravels the molecular mechanisms underlying the crosstalk between the MAPK pathway and sensors/receptors for plant G proteins (Meng et al., 2015; Su et al., 2015). Except for the classical GPCRs, the core heterotrimeric G protein subunits are all present in plants, although the repertoire of the heterotrimeric G proteins in plant is much simpler than that in animals (Trusov et al., 2012).

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