Talk on Friday 04:45-05:00pm submitted by Justin Mabin
Alternate Exon Junction Complexes potentiate branched nonsense-mediated mRNA decay pathways
Justin Mabin (Department of Molecular Genetics, The Ohio State University), Lauren Woodward (Department of Molecular Genetics, The Ohio State University), Mengxuan Jia and Vicki Wysocki (Department of Chemistry and Biochemistry, The Ohio State University), Robert Patton (Department of Physics, The Ohio State University), Ralf Bundschuh (Department of Physics, Division of Hematology, College of Medicine, Center for RNA Biology, The Ohio State University), Guramrit Singh (Department of Molecular Genetics, Center for RNA Biology, The Ohio State University)
Pre-mRNA splicing deposits the exon junction complex (EJC) ~24 nucleotides upstream of most exon-exon junctions in a sequence-independent manner. The EJC core consists of eIF4AIII, Y14 and Magoh. This core stably associates with RNA and serves as an interaction platform for more dynamic peripheral EJC proteins that direct mRNA export, localization, translation and nonsense-mediated mRNA decay (NMD). To date, the specificity of the peripheral EJC proteins in target RNA selection and the resulting impact on mRNA fate remains largely unknown. We recently reported that MLN51, a protein presumed to be an EJC core factor, and several peripheral EJC proteins are sub-stoichiometric in EJCs purified from human cells. We have now discovered that MLN51 and RNPS1, two EJC proteins previously implicated in NMD, exist in two mutually exclusive stable EJCs in human and mouse cells. Biochemical and proteomic analysis of the purified MLN51 and RNPS1 complexes from HEK293 cells shows that the two alternate EJCs share the three EJC core proteins but have many unique components. Importantly, alternate EJCs associate differentially with NMD proteins Upf2 and Upf3b suggesting that these alternate complexes could lie at the root of previously documented Upf2- and/or Upf3-independent NMD branches. Surprisingly, we find that the alternate EJCs have very similar binding patterns on endogenous mRNAs including those targeted for NMD, and yet many NMD targeted mRNAs are more sensitive to RNPS1 depletion than MLN51 depletion. Taken together, our data suggests that EJCs undergo sequential remodeling from RNPS1- to MLN51-containing complexes. While both complexes are likely to be active in NMD, RNPS1-EJC may represent the early-acting NMD branch and MLN51-EJC may define the late-acting branch. These branches could also differ in Upf factor requirements. We are currently testing these and other models to understand how the unexpected EJC composition dynamics that we discovered impacts mRNA fate.
Keywords: Exon-Junction Complex, Nonsense-mediated mRNA decay, Post-transcriptional gene regulation