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CELL BIOLOGY AND SIGNAL TRANSDUCTION

Arabidopsis Protein Kinases GRIK1 and GRIK2 Specifically Activate SnRK1 by Phosphorylating Its Activation Loop^1,[W],[OA]

Department of Molecular and Structural Biochemistry, North Carolina State University, Raleigh, North Carolina 27695–7622

SNF1-related kinases (SnRK1s) play central roles in coordinating energy balance and nutrient metabolism in plants. SNF1 and AMPK, the SnRK1 homologs in budding yeast (Saccharomyces cerevisiae) and mammals, are activated by phosphorylation of conserved threonine residues in their activation loops. Arabidopsis (Arabidopsis thaliana) GRIK1 and GRIK2, which were first characterized as geminivirus Rep interacting kinases, are phylogenetically related to SNF1 and AMPK activating kinases. In this study, we used recombinant proteins produced in bacteria to show that both GRIKs specifically bind to the SnRK1 catalytic subunit and phosphorylate the equivalent threonine residue in its activation loop in vitro. GRIK-mediated phosphorylation increased SnRK1 kinase activity in autophosphorylation and peptide substrate assays. These data, together with earlier observations that GRIKs could complement yeast mutants lacking SNF1 activation activities, established that the GRIKs are SnRK1 activating kinases. Given that the GRIK proteins only accumulate in young tissues and geminivirus-infected mature leaves, the GRIK-SnRK1 cascade may function in a developmentally regulated fashion and coordinate the unique metabolic requirements of rapidly growing cells and geminivirus-infected cells that have been induced to reenter the cell cycle.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Wei Shen (wshen{at}ncsu.edu).

^[W] The online version of this article contains Web-only data.

^[OA] Open access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.132787

^* Corresponding author; e-mail wshen{at}ncsu.edu.

Received November 19, 2008; accepted March 26, 2009; published April 1, 2009.