Abstract
SNF1-related kinase (SnRK1) in plants belongs to a conserved family that includes sucrose non-fermenting 1 kinase (SNF1) in yeast and AMP-activated protein kinase (AMPK) in animals. These kinases play important roles in the regulation of cellular energy homeostasis and in response to stresses that deplete ATP, they inhibit energy consuming anabolic pathways and promote catabolism. Energy stress is sensed by increased AMP:ATP ratios and in plants, 5′-AMP inhibits inactivation of phosphorylated SnRK1 by phosphatase. In previous studies, we showed that geminivirus pathogenicity proteins interact with both SnRK1 and adenosine kinase (ADK), which phosphorylates adenosine to generate 5′-AMP. This suggested a relationship between SnRK1 and ADK, which we investigate in the studies described here. We demonstrate that SnRK1 and ADK physically associate in the cytoplasm, and that SnRK1 stimulates ADK in vitro by an unknown, non-enzymatic mechanism. Further, altering SnRK1 or ADK activity in transgenic plants altered the activity of the other kinase, providing evidence for in vivo linkage but also revealing that in vivo regulation of these activities is complex. This study establishes the existence of SnRK1-ADK complexes that may play important roles in energy homeostasis and cellular responses to biotic and abiotic stress.
Highlights
The evolutionarily conserved sucrose non-fermenting 1 kinase (SNF1)/AMPK/SnRK1 family of protein kinases includes SNF1 kinase in yeast, AMPK (AMP-activated protein kinase) in animals, and SnRK1 (SNF1-related kinase 1) in plants
In addition to altering enzyme activities, SnRK1 integrates energy and stress signals by inducing extensive changes in the transcriptome that promote catabolism and inhibit anabolic pathways [9,10]. Given their dependence on the host both for biosynthetic machinery and energy, it is not surprising that viruses have been linked to these kinases. This was first recognized in plants, where SnRK1-mediated responses were identified as a component of innate antiviral defense that is antagonized by the geminivirus pathogenicity proteins AL2 and L2, which interact with SnRK1 [11,12]
Instead, reduced SnRK1 activity mediated by the antisense SnRK1 transgenes caused dramatic increases in adenosine kinase (ADK) activity (Table S6D). The results of these experiments show that an increase in cellular ADK activity leads to a parallel increase in SnRK1 activity, while reducing SnRK1 activity causes a substantial increase in ADK activity. These studies demonstrate that SnRK1 and ADK activities are linked in vivo, and indicate that the relationship between them is more complex than suggested by in vitro experiments
Summary
The evolutionarily conserved SNF1/AMPK/SnRK1 family of protein kinases includes SNF1 kinase (sucrose non-fermenting 1) in yeast, AMPK (AMP-activated protein kinase) in animals, and SnRK1 (SNF1-related kinase 1) in plants These serine/threonine kinases play a central role in the regulation of metabolism by responding to cellular energy charge, as sensed by relative AMP and ATP concentrations [1,2,3,4,5]. In addition to altering enzyme activities, SnRK1 integrates energy and stress signals by inducing extensive changes in the transcriptome that promote catabolism and inhibit anabolic pathways [9,10] Given their dependence on the host both for biosynthetic machinery and energy, it is not surprising that viruses have been linked to these kinases. Interactions between AMPK and several mammalian viruses have been described, including HIV, SV40, hepatitis C virus, and human cytomegalovirus [14,15,16,17]
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