Switches in nutrient and inositol signaling

Abstract

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.

Keywords: 5PTase13; PRL; SnRK1; WD40 proteins; inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]; myo-inositol.

Figure 1

The role of 5PTase13:SnRK1.1 complex in balancing inositol signaling and metabolism. The 5PTase and MIOX2 genes are downregulated by 3% glucose, while MIPS and VTC4 are upregulated by this stimulus. The net result may increase myo-inositol synthesis, decrease myo-inositol oxidation, and decrease InsP₃ hydrolysis. 5PTase13 and SnRK1.1 can form a complex and under these conditions 5PTase13 protects SnRK1.1 from proteasomal destruction. Thus we propose that 5PTase13 likely regulates SnRK in a nutrient dependent manner to contribute to the fine-tuning of inositol signaling and metabolism. Abbreviations are in the text.

Figure 2

Model of Antagonistic SnRK1.1 regulation by 5PTase13 and PRL1. Under low-nutrient or 6% glucose, 5PTase13 acts to protect SnRK1.1 from proteasomal degradation and PRL1 acts to facilitate SnRK1.1 turnover by the proteasome. When SnRK is stable/active, transcription factors (TFs) are stimulated to carry out metabolic reprogramming events.