Studies of signal transduction networks such as the inositol signaling pathway can provide important insights for our understanding of the regulation of various biological events, including growth and development, disease and stress responses. Recently, we have identified a myo-inositol polyphosphate 5-phosphatase (5PTase13, At1g05630) that hydrolyzes the second messenger inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and also interacts with the sucrose nonfermenting-1-related kinase (SnRK1.1) in the yeast two hybrid system and in vitro. Plant SnRK1 proteins coordinate nutrient and developmental signals to regulate plant survival under stress, darkness and sugar deprivation conditions. Using mutants defective in 5PTase13, we showed that 5PTase13 can act as a regulator of SnRK1 activity, and that regulation differs with nutrient availability. Specifically, we showed that 5PTase13 acts as a positive regulator of SnRK1 activity by preventing SnRK1.1 from proteasomal degradation in the presence of low nutrients or 6% glucose. In contrast, under severe starvation conditions, 5PTase13 acts as a negative regulator of SnRK1 activity. We present here a model of 5PTase13 regulatory interaction with SnRK1.1 and further discuss its importance for balancing inositol signaling and metabolism
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