Staufen2 functions in Staufen1-mediated mRNA decay by binding to itself and its paralog and promoting UPF1 helicase but not ATPase activity

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 similar to 10-fold more UPF1 and similar to 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.