Eukaryotic gene expression is a fundamental cellular activity that is critical for cellular identity, function, and physiology. During gene expression, a messenger RNA (mRNA) is generated by transcription and undergoes a number of different steps, including splicing and nuclear processing, nucleocytoplasmic export and localization, translation, and decay (Figure 1). These steps result in dynamic changes to the RNA sequence, structure, and the cohort of proteins bound to the mRNA. Furthermore, these changes need to occur with the proper timing and in the correct sequence to avoid aberrant expression. Therefore elaborate regulation of mRNP dynamics is required for proper gene expression.

At virtually every step in gene expression, members of a highly conserved protein family called the DEAD-box proteins are required for facilitating mRNP transitions by acting as RNA helicases and ribonucleoprotein (RNP) remodeling enzymes. Therefore, DEAD-box helicases function as master regulators of RNA and RNA-protein interactions during gene expression (and other RNA-dependent cellular processes), exerting overarching control of RNP dynamics. However, how they control different subsets of mRNAs, particularly in response to different cellular conditions, is still largely unclear.

We and others have described a novel role for Ded1 in regulating translation during cellular stresses such as lack of nutrients, extreme temperatures, or oxidative stress. Interestingly, Ded1 represses translation in stress, unlike its stimulatory role in pro-growth conditions. Our working model is that Ded1 causes dissociation of its binding partner eIF4G1 from translation complexes, leading to degradation of both eIF4G1 and Ded1 (Figure 3). Current work in this area includes understanding how this stress response mechanism occurs, what the translational targets of Ded1 and its associated factors are, and whether there are differences in this mechanism between different kinds of stresses. 

Many cellular pathways and processes are involved in responding to stress conditions. In order to examine how translation regulation is connected to these other pathways, we have recently conducted multiple screens. Intriguingly, we identified stress-dependent physical interactions between Ded1 and cell cycle components via mass spectrometry, suggesting a functional link between translation and the cell cycle during stress. We also identified a large number of genes that interact with ded1 mutants via a synthetic genetic array (e.g. Figure 4). We are currently working to follow up both of these results in order to better understand the cellular stress response.

Genome sequencing studies have found frequent mutations in DDX3X in medulloblastoma, the most common pediatric brain cancer. However, it remains largely unclear how these mutations contribute to cancer biogenesis and/or progression. We have taken a broad-based approach and have analyzed the effects of a large set of the mutants in yeast. This work has indicated that the mutations cause specific changes in translation, affecting stress responses. We are now moving to a tissue culture approach in order to examine the phenotypes in medulloblastoma cell lines