It is vital for cells to rapidly adjust protein levels in response to environmental changes. A major way cells modulate the concentration of proteins is by regulating their synthesis. There are three main stages of translation: initiation, elongation, and termination. Translational control occurs mainly during the rate-limiting step of initiation, in which a large number of eukaryotic initiation factors (eIFs) work together in a multi-stage process. The complex nature of translation initiation in eukaryotes offers a number of targets for regulation, many of which rely on the modulation of phosphorylation states in eIFs. One of the most commonly used mechanisms for regulating translation is through phosphorylation of eIF2 upon activation of stress-sensing kinases (PERK, PKR, HRI, GCN2). Each of these kinases is regulated by a different stress signal — unfolded proteins in the ER lumen, viral infection, heme deficiency, and amino acid starvation, respectively). These pathways converge in the phosphorylation of the same residue in eIF2 and are collectively called the integrated stress response (ISR). Paradoxically, under conditions of reduced protein synthesis, translation of a small group of mRNAs containing upstream open reading frames (uORFs) is selectively upregulated. Translation of these mRNAs is critical for mitigating cell stress but their prolonged expression can lead to activation of apoptotic pathways.
Our lab is interested in studying how translational reprogramming upon stress can determine cell fate, a process that is often deregulated in disease. Standing questions include: the extent of translational reprogramming and whether it changes based on the severity or duration of the stress encountered; the mechanism by which uORFs induce translation of the downstream coding sequence upon eIF2 phosphorylation in higher eukaryotes; and the role of non-canonical translation initiation upon ISR induction and reduction of conventional initiation. We seek to understand if novel proteins are produced by these alternative initiation events under stress and their importance for cell physiology.
We are also investigating the prevalence of uORF translation under normal growth conditions by developing sensitive tools to directly detect the resulting peptides produced from these small open reading frames. These tools will allow us to systematically and carefully measure changes in their expression upon different cellular conditions.