Harnessing the power of translation: Selective advantages in mRNA translation by endoplasmic reticulum-bound ribosomes

In eukaryotic cells, the endoplasmic reticulum (ER) is the exclusive site of secretory and integral membrane protein synthesis. The process of compartmentalization protein synthesis is guided by a positive selection mechanism, the signal recognition particle (SRP) pathway, which recognizes a signal sequence on the ribosome-bound nascent polypepide chain of secretory/membrane proteins. This sequence directs the ribosome/nascent chain complex to the ER and stimulates translocation of the nascent chain across the ER membrane. In addition to this well-established role, ER-bound ribosomes are postulated to function in the synthesis of cytosolic and nucleoplasmic proteins; numerous studies have demonstrated that mRNAs encoding cytosolic and nucleoplasmic proteins are well represented, and in some cases, highly enriched on the ER membrane. Although the cellular basis for this non-canonical mRNA partitioning to the ER has yet to be determined, these observations suggest that in addition to being the entry point for the secretory pathway, the ER may serve additional, perhaps distinct, biological roles in protein synthesis.;To gain insight into the regulatory function(s) of the ER regarding protein synthesis, I examined ribosome and mRNA partitioning during the unfolded protein response (UPR), key elements of which include suppression of the initiation stage of protein synthesis and a global remodeling of polyribosomes. Under these conditions, cytosolic and ER polyribosome assembly was differentially affected; cytosolic polyribosomes disassembled into component mRNA and ribosomes, whereas on the ER, small polyribosomes containing stress response mRNAs (XBP-1 and ATF4) were preserved. This study suggested that the ER membrane is used as the primary site of stress response (UPR) protein synthesis. In a companion study, I examined the kinetics of ribosome-bound nascent chain synthesis and turnover as a direct measure of protein synthesis activity. Under both homeostatic growth conditions and cell stress (UPR), cytosolic and ER ribosomes exhibited distinct protein synthesis capacities. Enhanced mRNA translation rates were observed for ER-bound ribosomes (2-3-fold), which were enabled in part, by tRNA channeling; aminoacyl-tRNA turnover and protein synthesis were tightly coupled on the ER and only weakly coupled in the cytosol. Emerging from these studies is a new paradigm of post-transcriptional gene control: As a privileged environment for the efficient assembly of translation complexes, the ER membrane is the major site of synthesis of both secretory/membrane and cytosolic/nucleoplasmic proteins.