UPF1 localises to mitochondria and is linked to mitochondrial gene expression and function in Drosophila

UPF1 is a conserved RNA helicase in eukaryotes typically defined by its role in nonsense-mediated mRNA decay (NMD). This study presents ChIP-seq evidence indicating an RNA-dependent interaction of UPF1 with mtDNA, consistent with an RNA-mediated association. Mitochondrial localisation was observed as discrete foci throughout the mitochondrial network, frequently in proximity to nucleoids, as confirmed microscopically by immunostaining and GFP tagging of UPF1 in different fly tissues and S2 cells. Depletion of UPF1, but not depletion of the other core NMD factors UPF2 or UPF3, in salivary glands results in smaller mitochondria and, unexpectedly, increased respiratory activity. This increase is observed specifically in salivary glands and correlates with elevated expression of nuclear genes involved in respiratory chain assembly and mitochondrial translation. Changes in mtDNA-encoded gene expression are also observed upon the depletion of UPF1. These findings indicate that UPF1 operates in mitochondria and, independently of canonical NMD, is linked to mitochondrial gene expression and may coordinate nuclear and mitochondrial gene expression in a tissue-specific manner.