Spatial control of mRNA stability in yeast

The degradation of mRNA is an important modulator of gene expression and the ultimate fate of messenger mRNA. Important steps in the degradation of mRNA include initial shortening of its poly(A) tail followed by the subsequent removal of the m7G cap. These two processes are linked temporally as well as spatially. In addition to physical interactions between proteins involved in these two processes, deadenylation and decapping enzymes and accessory factors are found in P bodies. P bodies are aggregates of protein and mRNA that are induced upon stress in all eukaryotes examined. In this thesis, I examine the spatial localization of decapping factors and explore the role of P bodies in mRNA turnover in the yeast Saccharomyces cerevisiae. This thesis is based on three underlying principles. First, mRNA decapping factors are membrane associated. More so, we show that decapping factors can be co-localized with the endoplasmic reticulum and Golgi apparatus. Second, although P bodies were proposed as sites of mRNA decay, we found that they stabilize mRNA. We examined the role of P bodies in mRNA turnover using a mutant defective in their assembly, edc3∆ lsm4∆C. This strain is mutated in two decapping activators. It combines a deletion of the gene encoding the Edc3 protein and lacks the prion-like domain of Lsm4. Using the edc3∆ lsm4∆C mutant, we demonstrate that mRNA stability is significantly reduced in the absence of P bodies for longer-lived mRNA. The effect of mRNA destabilization was due to increased deadenylation and decapping dependence. Finally, the decapping factor usually found in the cytoplasm, but accumulates in the nucleus in the P body deficient strain (edc3∆ lsm4∆C). This implies a possible role in modulating transcription. A model for the functioning of P bodies that is consistent with our work is that P bodies serve a role as a cytoplasmic sink for degradation factors. By regulating the access of the cytosol to proteins involved in mRNA turnover, P bodies can modulate mRNA stability. This suggests a role for P bodies under stress and their potential importance in stress adaptation