In contemporary models, eukaryotic cells segregate the synthesis of different classes of proteins;secretory/integral membrane proteins are made on the endoplasmic reticulum (ER) and soluble proteins in the cytosol. Recent studies of mRNA partitioning between the cytosol and ER compartments have, however, identified a prominent role for the ER in the synthesis of both soluble and secretory/membane proteins alike. In addition to describing new functions for the ER in global protein synthesis, these findings identify a significant gap in our understanding of how eukaryotic cells partition mRNAs between the two compartments and in turn regulate the synthesis of these two broad classes of proteins. To address this gap, we are focusing our research efforts to 1) understand how eukaryotic cells partition mRNAs between the cytosol and the endoplasmic reticulum (ER) and 2) determine how eukaryotic cells regulate the protein synthesis activity of the cytosol and ER compartments during homeostasis and cell stress. This research is expected to provide significant insights into the regulatory mechanisms governing protein synthesis in health and disease. In addition, this research will serve as a significant contribution to our understanding of the mRNA localization and protein synthesis pathways that are essential to eukaryotic life. Three specific aims are proposed: i) Define the in vivo role of the SRP pathway in mRNA partitioning to the ER;ii) Identify the mRNA localization signals that confer non-canonical mRNA partitioning to the ER and iii) Define the global role of the ER compartment in cytosolic protein syntehsis. To address these specific aims, we will use mammalian tissue culture cell models and will emphasize established biochemical techniques of cell fractionation, cell biological analysis of reporter mRNA localization in situ and molecular biological analysis of the structure/function elements of mRNAs that confer ER localization. Many prominent human diseases, including obesity, diabetes and stroke activate cell stress responses that profoundly alter the types and amounts of proteins cells synthesize and consequently, cell viability. By understanding how cells decide which and how much of each protein to make, in health and disease, we will understand the basic mechanisms used by cells to respond to pathological stress. By understanding these mechanisms, we hope to identify new targets for therapeutic intervention.