Assembly of mRNP Complexes During Stress and Nonsense-Mediated mRNA Decay Quality Control in Saccharomyces cerevisiae

Abstract

In eukaryotes, mRNA is in constant flux between an actively translating state and translationally repressed states. Specifically, mRNA degradation and repression factors compete with translation factors to direct mRNAs out of translation for storage or decay. This process often leads to formation of cytoplasmic aggregates. P-bodies are granules that contain mRNA and degradation factors, suggesting they are sites of mRNA decay or storage. Stress granules form in response to stress conditions and contain mRNAs and translation factors.P-bodies and stress granules consist of mRNPs of different compositions, believed to mature and transition between the states. It is proposed that mRNAs transition between the two granules. In the work described below, we use Saccharomyces cerevisiae to demonstrate that a decay factor, Dhh1 is capable of existing in both P-body and stress granule mRNPs. This suggests that a decay factor can be part of two different mRNP complexes. Additionally, we identify two novel components of the stress granule mRNPs, Pbp4 and Lsm12, and determine that they are not essential for stress granule formation. Lastly, we show that the stress granule mRNP factor, Pab1, is not absolutely required for stress granule formation.An important aspect of cytoplasmic mRNA regulation is mRNA quality control. One example of this is nonsense-mediated mRNA decay (NMD), whereby aberrant mRNAs containing premature termination codons are targeted for decay, and can be localized to P-bodies. Upf1-3 and the mRNA decapping complex, Dcp2/Dcp1 are essential for NMD, which requires Upf1 interaction with stalled ribosomal/mRNA complexes to target aberrant mRNA for decapping and degradation. How Dcp2/Dcp1 is recruited to aberrant mRNA is poorly understood.Here, we show by yeast two-hybrid assays that an interaction between Dcp2 and Upf1 is mediated by the decapping stimulator Edc3. Interestingly, Edc3 and Upf2 share overlapping binding sites on the Upf1 N-terminal domain. The decapping stimulator, Pat1, also interacts on the Upf1 N-terminus, but Edc3 and Pat1 are not essential for NMD. Surprisingly, the Upf1-Edc3 interaction does not promote or negatively regulate NMD. Thus, the Upf1-Edc3 and Upf1-Pat1 interactions likely regulate a subset of mRNA transcripts, or are essential for proper NMD under different environmental conditions.