Poster abstracts
Poster number 108 submitted by Shyama Nandakumar
Micropeptide-mediated regulation of mRNA translation during ER stress
Shyama Nandakumar (Cell Biology, University of Pittsburgh School of Medicine), Deepika Vasudevan (Cell Biology, University of Pittsburgh School of Medicine)
Abstract:
The eukaryotic transcription factor ATF4 regulates the Integrated Stress Response, which responds to various stressors including ER stress. When ISR is activated, global protein synthesis is swiftly dampened due to the phospho-inactivation of the initiator methionine complex, eIF2, by stress-responsive kinases. However, ATF4 translation is paradoxically induced due to its specialised 5' leader. Our work uncovers a novel trans-acting micropeptide, µP, that regulates translation of the ATF4 main ORF.
The ATF4 5’ leader contains multiple short upstream ORFs (uORFs), with the ultimate uORF (encoding µP) sequence overlapping the ATF4-encoding main ORF.µP. Under homeostasis, due to termination events at the uORF1 stop codon, the ribosome occasionally “reinitiates” translation at the uORF2 start codon to synthesize µP. However, under conditions of stress, reinitiation at µP decreases due to reduced initiator methionine, thus the ribosome can reinitiate at the ATF4 start codon implying a trade-off between synthesis of µP and ATF4, such that µP favored under homeostasis and ATF4 under stress.
To test if µP regulates Atf4 activity, we made fly and mammalian models with inducible, ectopic expression of µP. Our data show that expression of µP reduces ATF4 activity in both systems by reducing ATF4 protein. Using an ATF4-5’leader reporter, we find that µP regulates the translation ATF4 without affecting transcript levels. Further, the mechanism of µP action is not dependent on known regulators of translation reinitiation such as DENR. We aim to understand µP-mediated translation regulation of the ATF4 5’leader. Since ATF4 mRNA is ubiquitously transcribed in nearly all cells, there is near-constitutive expression of µP in homeostasis. Intriguingly, our data indicate µP may have independent physiological functions, since µP expression in fly adipocytes results in developmental acceleration and increased lipid reserves. Thus we present a novel modality of ATF4 regulation.
Keywords: ATF4, Stress, translation