SMG-8 and SMG-9, two novel subunits of the SMG-1 complex, regulate remodeling of the mRNA surveillance complex during nonsense-mediated mRNA decay

Abstract

Nonsense-mediated mRNA decay (NMD) is a surveillance mechanism that detects and degrades mRNAs containing premature translation termination codons (PTCs). SMG-1 and Upf1 transiently form a surveillance complex termed ''SURF'' that includes eRF1 and eRF3 on post-spliced mRNAs during recognition of PTC. If an exon junction complex (EJC) exists downstream from the SURF complex, SMG-1 phosphorylates Upf1, the step that is a rate-limiting for NMD. We provide evidence of an association between the SURF complex and the ribosome in association with mRNPs, and we suggest that the SURF complex functions as a translation termination complex during NMD. We identified SMG-8 and SMG-9 as novel subunits of the SMG-1 complex. SMG-8 and SMG-9 suppress SMG-1 kinase activity in the isolated SMG-1 complex and are involved in NMD in both mammals and nematodes. SMG-8 recruits SMG-1 to the mRNA surveillance complex, and inactivation of SMG-8 induces accumulation of a ribosome:Upf1:eRF1:eRF3:EJC complex on mRNP, which physically bridges the ribosome and EJC through eRF1, eRF3, and Upf1. These results not only reveal the regulatory mechanism of SMG-1 kinase but also reveal the sequential remodeling of the ribosome:SURF complex to the predicted DECID (DECay InDucing) complex, a ribosome:SURF:EJC complex, as a mechanism of in vivo PTC discrimination.[Keywords: NMD; mRNA surveillance; UPF1; SMG-1; PIKK; translation termination; mRNP remodeling] Supplemental material is available at http://www.genesdev.org.

Structural insights into nonsense-mediated mRNA decay (NMD) by electron microscopy

“…UPF2 and UPF3 required to bridge UPF1 and the EJC [17]. SMG1 kinase is regulated by SMG8 and SMG9 [30], but interactions of SMG1 and UPF2 have also been described [36]. Structural insights into the mechanism for detecting PTCs in the NMD pathway have been obtained mostly by X-ray crystallography studies.…”

“…In 2009, Shigeo Ohno, Akio Yamashita and colleagues at Yokohama University described two proteins, SMG8 and SMG9, that co-purified with SMG1 and which were essential for NMD [30]. In collaboration with Ohno and Yamashita, we have described the overall architecture of SMG1 and the modulation of its kinase activity by the interaction with SMG8 and SMG9 [31,32].…”

Rules that govern UPF1 binding to mRNA 3′ UTRs

“…When translation terminates at a PTC and an EJC is situated sufficiently downstream of the PTC so as not to be displaced by the terminating ribosome (8,9), a complex called suppressor with morphogenic effect on genitalia (SMG) 1−upframeshift 1 (UPF1)−eukaryotic release factor (eRF) 1−eRF3 (SURF) is thought to form at the PTC and, together with the downstream EJC, constitutes a decay-inducing complex (DECID) (10,11). The transient and/or weak interaction of mRNA capbound CBP80 with UPF1, which occurs on mRNAs even if they are not NMD targets, promotes UPF1 binding to eRF1−eRF3 and, subsequently, to an EJC (12).…”

“…The transient and/or weak interaction of mRNA capbound CBP80 with UPF1, which occurs on mRNAs even if they are not NMD targets, promotes UPF1 binding to eRF1−eRF3 and, subsequently, to an EJC (12). Although SMG8 and SMG9 tightly associate with SMG1 to suppress its kinase activity (11,13), the EJC constituents UPF2 and UPF3 or UPF3X (also called UPF3a or UPF3b, respectively) augment the SMG1-mediated phosphorylation of UPF1 (10). Hyperphosphorylated UPF1 then binds to eIF3 of a 43S preinitiation complex, positioned at the translation initiation codon of the NMD target, to suppress further translation initiation events (14) and promote mRNA decay (15)(16)(17)(18).…”

Global analysis of alternative splicing uncovers developmental regulation of nonsense-mediated decay in C. elegans

“…In a screen using a C. elegans RNAi library, two essential NMD factors with an embryonic lethality phenotype were identified as smg lethal (smgl), named smgl-1 and smgl-2 (Longman et al 2007). In a more recent study, SMG8 and SMG9, two proteins that associate with the SMG1 complex, were identified and shown to be required for NMD in both mammals and nematodes (Yamashita et al 2009). All 11 genes are found in both worms and mammals.…”