Tuesday, 6 November 2018 | 12:00 noon
MRC Human Genetics UnitUniversity of Edinburgh, UK
Mechanism and function of the Nonsense-mediateddecay (NMD) pathway in nematodes and vertebrates
(Host: E. Buratti)
The Nonsense-mediated decay (NMD) pathway selectively degrades mRNAsharboring premature termination codons (PTCs) but also regulates theabundance of a large number of endogenous RNAs and fine-tunes manyphysiological processes. Using genome-wide RNAi screens, we identifiedseveral novel NMD factors, including the RNA helicase DHX34 and NBAS(Neuroblastoma Amplified Sequence) and demonstrated that they act inconcert with core NMD factors to co-regulate a large number of endogenousRNA targets (1,2). DHX34 acts to remodel mRNPs and triggers the conversionfrom the SURF complex to the DECID (Decay-induced) complex resulting inNMD progression and mRNA degradation (3). Using single-particle electronmicroscopy (EM), we demonstrated the existence of a complex comprisingSMG1, UPF1 and DHX34, with DHX34 functioning as a scaffold for UPF1 andSMG1 and promoting UPF1 phosphorylation leading to functional NMD (4).
Most studies to date have focused on cytoplasmic NMD; however, it is largelyunknown how this mechanism operates on transcripts encoding secreted orER-resident proteins. We have uncovered evidence that suggests the existenceof an NMD pathway dedicated to Endoplasmic Reticulum (ER)-localizedmRNAs, which involves two NMD factors that localize to the ER, NBAS andSEC13. Using biochemical approaches, as well as Proximity ligation assay (PLA),FRET-FLIM and Fluorescence correlation spectroscopy (FCS), we demonstratethat NBAS interacts directly with the core NMD factor, UPF1.
NBAS is an RNA-binding protein and its RNA targets are enriched for ER-associated transcripts.Interestingly, NBAS was also identified as a component of the Syntaxin 18complex that is localized at the Endoplasmic reticulum (ER) and is requiredfor vesicular trafficking and ER homeostasis. I will discuss current experimentsaiming to uncover whether NBAS has two separate functions as an NMD factorand as a component of the ER-Golgi tethering complex or whether these twofunctions are somehow linked. I will also discuss the functional characterizationof an ER-NMD pathway.