Staufen1 (STAU1)-mediated mRNA decay (SMD) degrades translationally active mRNAs that bind the double-stranded (ds)RNA binding protein STAU1 within their 3'-untranslated regions (3'UTRs)1,2. Earlier studies defined the STAU1 binding site (SBS) within ADP ribosylation factor 1 (ARF1) mRNA as a 19-base-pair stem with a 100-nucleotide apex2. However, we were unable to identify comparable structures within the 3'UTRs of other SMD targets. Here we report that SBSs can be formed by imperfect base-pairing between an Alu element within the 3'UTR of an SMD target and another Alu element within a cytoplasmic and polyadenylated long noncoding RNA (lncRNA). Individual lncRNAs can downregulate a subset of SMD targets, and distinct lncRNAs can downregulate the same SMD target. These are previously unappreciated functions for ncRNAs and Alu elements3–5. Not all mRNAs that contain a 3'UTR Alu element are targeted for SMD despite the presence of a complementary lncRNA that targets other mRNAs for SMD. Most known trans-acting RNA effectors consist of fewer than 200 nucleotides and include snoRNAs and microRNAs. Our finding that STAU1 binding to mRNAs can be transactivated by lncRNAs uncovers an unexpected strategy used by cells to recruit proteins to mRNAs and mediate their decay. We name these lncRNAs “half(½)-sbsRNAs”.

UPF1 functions in both Staufen 1 (STAU1)-mediated mRNA decay (SMD) and nonsense-mediated mRNA decay (NMD), which we show here are competitive pathways. STAU1-and UPF2-binding sites within UPF1 overlap so that STAU1 and UPF2 binding to UPF1 appear to be mutually exclusive. Furthermore, down-regulating the cellular abundance of STAU1, which inhibits SMD, increases the efficiency of NMD, whereas down-regulating the cellular abundance of UPF2, which inhibits NMD, increases the efficiency of SMD. Competition under physiological conditions is exemplified during the differentiation of C2C12 myoblasts to myotubes: The efficiency of SMD increases and the efficiency of NMD decreases, consistent with our finding that more STAU1 but less UPF2 bind UPF1 in myotubes compared with myoblasts. Moreover, an increase in the cellular level of UPF3X during myogenesis results in an increase in the efficiency of an alternative NMD pathway that, unlike classical NMD, is largely insensitive to UPF2 down-regulation. We discuss the remarkable balance between SMD and the two types of NMD in view of data indicating that PAX3 mRNA is an SMD target whose decay promotes myogenesis whereas myogenin mRNA is a classical NMD target encoding a protein required for myogenesis.[Keywords: Staufen1-mediated mRNA decay; nonsense-mediated mRNA decay; Staufen1; UPF proteins; premature termination codon; myogenesis] Supplemental material is available at http://www.genesdev.org.

Increasing evidence suggests that long non-coding RNAs (LncRNAs) represent a new class of regulators of stem cells. However, the roles of LncRNAs in stem cell maintenance and myogenesis remain largely unexamined. For this study, hundreds of intergenic LncRNAs were identified that are expressed in myoblasts and regulated during differentiation. One of these LncRNAs, termed LncMyoD, is encoded next to the Myod gene and is directly activated by MyoD during myoblast differentiation. Knockdown of LncMyoD strongly inhibits terminal muscle differentiation, largely due to a failure to exit the cell cycle. LncMyoD directly binds to IGF2-mRNA-binding protein 2 (IMP2) and negatively regulates IMP2-mediated translation of proliferation genes such as N-Ras and c-Myc. While the RNA sequence of LncMyoD is not well conserved between human and mouse, its locus, gene structure, and function are preserved. The MyoD-LncMyoD-IMP2 pathway elucidates a mechanism as to how MyoD blocks proliferation to create a permissive state for differentiation.

Staufen1-mediated mRNA decay (SMD) degrades mRNAs that harbor a Staufen1-binding site (SBS) in their 39 untranslated regions (UTRs). Human SBSs can form by intermolecular base-pairing between a 39 UTR Alu element and an Alu element within a long noncoding RNA (lncRNA) called a 1 ⁄ 2 -sbsRNA. Since Alu elements are confined to primates, it was unclear how SMD occurs in rodents. Here we identify mouse mRNA 39 UTRs and lncRNAs that contain a B1, B2, B4, or identifier (ID) element. We show that SMD occurs in mouse cells via mRNA-lncRNA base-pairing of partially complementary elements and that mouse 1 ⁄ 2 -sbsRNA (m 1 ⁄ 2 -sbsRNA)-triggered SMD regulates C2C12 cell myogenesis. Our findings define new roles for lncRNAs as well as B and ID short interspersed elements (SINEs) in mice that undoubtedly influence many developmental and homeostatic pathways.

Staufen (STAU)1-mediated mRNA decay (SMD) degrades mammalian-cell mRNAs that bind the double-stranded (ds)RNA-binding protein STAU1 in their 3′-untranslated region. We report a new motif, which typifies STAU homologs from all vertebrate classes, that is responsible for human (h)STAU1 homodimerization. Our crystal structure and mutagenesis analyses reveal that this motif, now named the Staufen-swapping motif (SSM), and dsRNA-binding domain 5 (‘RBD’5) mediate protein dimerization: the two SSM α-helices of one molecule interact primarily through a hydrophobic patch with the two ‘RBD’5 α-helices of a second molecule. ‘RBD’5 adopts the canonical α-β-β-β-α fold of a functional RBD, but it lacks residues and features needed to bind duplex RNA. In cells, SSM-mediated hSTAU1 dimerization increases the efficiency of SMD by augmenting hSTAU1 binding to the ATP-dependent RNA helicase hUPF1. Dimerization regulates keratinocyte-mediated wound-healing and, undoubtedly, many other cellular processes.