UPF1 shuttles between nucleus and cytoplasm independently of its RNA-binding and ATPase activities

The ATP-dependent RNA helicase Up-frameshift 1 (UPF1) is an essential protein in mammalian cells and a key factor in nonsense-mediated mRNA decay (NMD), a translation-dependent mRNA surveillance process. UPF1 is mainly cytoplasmic at steady state but accumulates in the nucleus after inhibiting CRM1-mediated nuclear export by leptomycin B (LMB), indicating that UPF1 shuttles between the nucleus and the cytoplasm. Consistent with its dual localization, there is evidence for nuclear functions of UPF1, for instance, in DNA replication, DNA damage response, and telomere maintenance. However, whether any of UPF1's biochemical activities are required for its nuclear-cytoplasmic shuttling remains unclear. To investigate this, we examined two UPF1 mutants: the well-described ATPase-deficient UPF1-DE (D636A/E637A) and a newly generated RNA-binding mutant UPF1-NKR (N524A/K547A/R843A). Biochemical assays confirmed that the UPF1-NKR mutant cannot bind RNA or hydrolyze ATP in vitro but retains interaction with UPF2, UPF3B, and SMG6. Overexpression of UPF1-NKR exerted a dominant-negative effect on endogenous UPF1 and inhibited NMD. Subcellular localization studies revealed that UPF1-DE accumulates in cytoplasmic granules (P-bodies), even in the presence of LMB, whereas UPF1-NKR shuttles normally. This indicates that UPF1's shuttling does not require its RNA-binding or ATPase activities. Notably, the UPF1-DE.NKR double mutant restored nuclear-cytoplasmic shuttling and prevented accumulation in P-bodies, suggesting that the shuttling defect of UPF1-DE arises from its tight binding to RNA. Overall, our findings demonstrate that UPF1's shuttling is independent of its ATPase and RNA-binding activities, with RNA binding itself being a key determinant of its cytoplasmic retention.