Poster abstracts

Poster number 116 submitted by Ben Pockrass

Translation at the ER: Developing a smART-ER method

Ben Pockrass (Department of Biological Chemistry, University of Michigan Medical School), Rachel O. Niederer (Department of Biological Chemistry, University of Michigan Medical School)

Abstract:
It is generally understood that mRNAs encoding membrane/secreted proteins are targeted to the endoplasmic reticulum (ER) during early elongation via the signal recognition particle pathway. In this model, translation initiation occurs in the cytoplasm, and mRNA is only targeted to the ER upon emergence of an N-terminal signal sequence during early elongation. However, subsequent rounds of translation initiation on an already ER-localized mRNA would take place near the ER. Additionally, there is evidence that ribosomes remain in stable association with the ER following translation termination1 and that these ER-bound ribosomes are capable of direct translation initiation, including mRNAs encoding cytoplasmic proteins which do not encode a signal sequence2. Despite these findings, the nature of translation initiation at the ER and its regulation are largely unexplored. Because mRNA 5’ untranslated regions (5’ UTRs) are the site of translation initiation, and translation initiation is highly regulated, we hypothesize that 5’ UTR features regulate translation initiation at the ER. One prediction of this hypothesis is that 5’ UTRs from mRNAs encoding membrane proteins would have distinct features, as they uniquely require translation at the ER. A bioinformatic analysis of 5' UTR features from mRNAs encoding membrane proteins compared to those encoding non-membrane proteins revealed subtle but significant differences in their sequence composition, 5’-terminal nucleotide identity, and presence of RNA binding protein motifs. To systematically identify mRNA 5’ UTR features associated with enhanced/repressed translation initiation at the ER, we are developing a high-throughput method to measure translation initiation rates on ER-bound ribosomes for thousands of unique mRNAs. Discovery of these 5’ UTR features will guide further studies into the factors regulating translation initiation at the ER.

References:
1. Potter, M. D. & Nicchitta, C. V. Regulation of Ribosome Detachment from the Mammalian Endoplasmic Reticulum Membrane. Journal of Biological Chemistry 275, 33828–33835 (2000).

2. Jagannathan, S., Reid, D. W., Cox, A. H. & Nicchitta, C. V. De novo translation initiation on membrane-bound ribosomes as a mechanism for localization of cytosolic protein mRNAs to the endoplasmic reticulum. RNA 20, 1489–1498 (2014).

Keywords: translation initiation, ER, UTR