Staufen1 (STAU1)-mediated mRNA decay (SMD) is an mRNA degradation process in mammalian cells that is mediated by the binding of STAU1 to a STAU1-binding site (SBS) within the 3'-untranslated region (3'UTR) of target mRNAs. During SMD, STAU1, a double-stranded (ds) RNA-binding protein, recognizes dsRNA structures formed either by intramolecular base-pairing of 3'UTR sequences or by intermolecular base-pairing of 3'UTR sequences with a long noncoding RNA (lncRNA) via partially complementary Alu elements. Recently, STAU2, a paralog of STAU1, has also been reported to mediate SMD. Both STAU1 and STAU2 interact directly with the ATP-dependent RNA helicase UPF1, a key SMD factor, enhancing its helicase activity to promote effective SMD. Moreover, STAU1 and STAU2 form homodimeric and heterodimeric interactions via domain-swapping. Since both SMD and the mechanistically related nonsense-mediated mRNA decay (NMD) employ UPF1, SMD and NMD are competitive pathways. Competition contributes to cellular differentiation processes, such as myogenesis and adipogenesis, placing SMD at the heart of various physiologically important mechanisms.

Staufen (STAU)1-mediated mRNA decay (SMD) is a posttranscriptional regulatory mechanism in mammals that degrades mRNAs harboring a STAU1-binding site (SBS) in their 3′-untranslated regions (3′ UTRs). We show that SMD involves not only STAU1 but also its paralog STAU2. STAU2, like STAU1, is a double-stranded RNA-binding protein that interacts directly with the ATP-dependent RNA helicase up-frameshift 1 (UPF1) to reduce the half-life of SMD targets that form an SBS by either intramolecular or intermolecular base-pairing. Compared with STAU1, STAU2 binds ∼10-fold more UPF1 and ∼two-to fivefold more of those SBS-containing mRNAs that were tested, and it comparably promotes UPF1 helicase activity, which is critical for SMD. STAU1-or STAU2-mediated augmentation of UPF1 helicase activity is not accompanied by enhanced ATP hydrolysis but does depend on ATP binding and a basal level of UPF1 ATPase activity. Studies of STAU2 demonstrate it changes the conformation of RNA-bound UPF1. These findings, and evidence for STAU1−STAU1, STAU2−STAU2, and STAU1−STAU2 formation in vitro and in cells, are consistent with results from tethering assays: the decrease in mRNA abundance brought about by tethering siRNA-resistant STAU2 or STAU1 to an mRNA 3′ UTR is inhibited by downregulating the abundance of cellular STAU2, STAU1, or UPF1. It follows that the efficiency of SMD in different cell types reflects the cumulative abundance of STAU1 and STAU2. We propose that STAU paralogs contribute to SMD by "greasing the wheels" of RNA-bound UPF1 so as to enhance its unwinding capacity per molecule of ATP hydrolyzed.protein-protein interactions | protein-RNA interactions M ammalian cells contain two Staufen (STAU) paralogs, STAU1 and STAU2. Each derives from a separate gene that produces multiple protein isoforms via alternative pre-mRNA splicing and/or polyadenylation. Every STAU1 and STAU2 isoform contains multiple conserved dsRNA-binding domains (RBDs), of which dsRBD3 and dsRBD4 constitute the major if not sole active dsRNA-binding sites (1, 2).Although STAU1 and probably STAU2 are expressed ubiquitously, STAU2 is most abundant in heart and brain (1, 3). Both paralogs are present in ribonucleoprotein particles (RNPs) within neuronal cell bodies and dendrites (1, 4-7). Both are also involved in the microtubule-dependent transport of RNAs to dendrites of polarized neurons (5, 8), presumably via the tubulinbinding domain (2). Moreover, both paralogs play a crucial role in the formation and maintenance of the dendritic spines of hippocampal neurons and are required for synaptic plasticity and memory (7, 9-11), although the detection of paralog-specific RNA granules in the distal dendrites of rat hippocampal neurons has been used to argue for paralog-distinct functions (1). Additional support for paralog-specific roles derives from the demonstration using rat hippocampal slice cultures that STAU1 but not STAU2 functions in the late phase of forskolin-induced longterm potentiation (10-12), whereas STAU2 but not STAU1 is involved in metabotropic ...