Circadian Clock Control of Translation Initiation Factor eIF2 Activity Requires eIF2-Dependent Recruitment of Rhythmic PPP-1 Phosphatase in Neurospora crassa.
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abstract
The circadian clock controls the phosphorylation and activity of eukaryotic translation initiation factor 2 (eIF2). In Neurospora crassa, the clock drives a daytime peak in the activity of the eIF2 kinase CPC-3, the homolog of yeast and mammalian GCN2 kinase. This leads to increased levels of phosphorylated eIF2 (P-eIF2) and reduced mRNA translation initiation during the day. We hypothesized that rhythmic eIF2 activity also requires dephosphorylation of P-eIF2 at night by phosphatases. In support of this hypothesis, we show that mutation of N. crassa PPP-1, a homolog of the yeast eIF2 phosphatase GLC7, leads to high and arrhythmic P-eIF2 levels, while maintaining core circadian oscillator function. PPP-1 levels are clock-controlled, peaking in the early evening, and rhythmic PPP-1 levels are necessary for rhythmic P-eIF2 accumulation. Deletion of the N terminus of N. crassa eIF2, the region necessary for eIF2 interaction with GLC7 in yeast, led to high and arrhythmic P-eIF2 levels. These data supported that N. crassa eIF2 functions to recruit PPP-1 to dephosphorylate eIF2 at night. Thus, in addition to the activity of CPC-3 kinase, circadian clock regulation of eIF2 activity requires dephosphorylation by PPP-1 phosphatase at night. These data show how the circadian clock controls the activity a central regulator of translation, critical for cellular metabolism and growth control, through the temporal coordination of phosphorylation and dephosphorylation events.IMPORTANCE Circadian clock control of mRNA translation contributes to the daily cycling of a significant proportion of the cellular protein synthesis, but how this is accomplished is not understood. We discovered that the clock in the model fungus Neurospora crassa regulates rhythms in protein synthesis by controlling the phosphorylation and dephosphorylation of a conserved translation initiation factor eIF2. During the day, N. crassa eIF2 is phosphorylated and inactivated by CPC-3 kinase. At night, a clock-controlled phosphatase, PPP-1, dephosphorylates and activates eIF2, leading to increased nighttime protein synthesis. Translation requires significant cellular energy; thus, partitioning translation to the night by the clock provides a mechanism to coordinate energy metabolism with protein synthesis and cellular growth.
