Regulation of gene expression is critical for the development and homeostasis of all living organisms, as mis-regulation can lead to loss of viability, and cancer formation in humans.  Gene expression is regulated at three basic levels, the transcriptional, post-transcriptional, and the post-translational level.  I am interested in understanding the mechanisms controlling gene expression at the post-transcriptional level, using the budding yeast, Saccharomyces cerevisiae.  Specifically, I am studying the transition from translation to mRNA degradation. 

There is an important relationship between translation and mRNA turnover that is critical to proper regulation of gene expression.  For instance, mRNAs that translate efficiently are more stable, whereas mRNAs that do not translate efficiently are less stable.  Additionally, mRNA half-lives can vary in response to hormones, during immune response, infection, changes in iron levels and differentiation signals.  The major pathway of mRNA degradation in yeast occurs by deadenylation, which leads to mRNA decapping and subsequent 5’ to 3’ exonucleolytic degradation of the mRNA body (Figure 1).  Prior to an mRNA being targeted for degradation, it first exits translation.  The mRNA subsequently localizes to a P-body, which is a cytoplasmic structure that contains mRNA degradation enzymes.  Interestingly, P-bodies are conserved from yeast to mammals.  Also, the proteins found in yeast and mammalian P-bodies are also present in maternal mRNA storage granules, stress granules and neuronal granules, suggesting that the mechanisms of translational repression and localization to P-bodies in yeast may be conserved in higher eukaryotes.

I identified Sbp1p, a nucleolin-like protein, which is involved in the translational repression of mRNAs.  One such condition where translational repression can be studied is under glucose deprivation, where translation decreases, and P-bodies increase in size and number.  Loss of Sbp1p function under glucose deprivation results in loss of translational repression as seen by increased translation rate and a reciprocal decrease in P-bodies.  Interestingly, recent work with nucleolin has delineated its role as a translational repressor of IL-2, and p53 mRNAs.  Specifically, I would like to understand the mechanisms involved in driving the transition from translation to mRNA degradation, and I would like to identify new factors involved in the process.  In order identify new proteins involved in the transition from translation to mRNA turnover, I will use a combination of both genetic screens, molecular approaches and a candidate protein approach.  Once new factors are identified, I will further determine their role in translational repression, P-body formation and mRNA degradation.  Additionally, I would like to determine the mechanisms by which Sbp1p promotes translational repression of mRNA, and I would like to determine whether mammalian nucleolin functions in a similar manner.  I will do this by using in vitro translation assays, and by structure function analysis both in vivo and in vitro.