Characterization Of Novel Rna Protein Regulatory Interactions In Saccharomyces Cerevisiae

The dynamic processes of a living cell depend on the coordinated temporal and spatial regulation of the many steps of gene expression. Transcription regulation is one control point of gene expression, and a gene can also be regulated post-transcriptionally, by RNA-binding proteins (RBPs). The biological significance of post-transcriptional regulation is especially evident in cases, where RBP binding controls the temporal precision of suppression and activation of important cellular stress responses. We developed a proteome-wide experimental approach for in vitro identification of novel RBPs and RNA-protein interactions in Saccharomyces cerevisiae. We found 12 novel RNA-binding proteins, the majority of which, surprisingly, are currently annotated as enzymes with roles in metabolic processes. We next used this proteomic approach to screen for proteins specifically interacting with the HAC1 RNA, which mediates activation of the yeast unfolded protein response (UPR). We found that HAC1 associated reproducibly with four small yeast GTPases, three of which are of the Ypt family of ras-GTPases. We further characterized one of them, the yeast Rab1 homolog Ypt1, and showed that Ypt1 interacted with unspliced HAC1 RNA only in the absence of ER stress. Selective Ypt1 depletion increased HAC1 RNA stability and expression, and also affected timely recovery from UPR. By developing and applying a novel proteomic approach for studying RNA-protein interactions, we established Ypt1 as an important regulator of HAC1 expression and UPR signaling. This unexpected protein-RNA interaction provides a biochemical mechanism for coordinating the key cellular processes of vesicle trafficking and ER homeostasis.