Arabidopsis DXO1 is an evolutionarily diverged homolog that impacts ribosome loading and mRNA surveillance

The Rai1/Dxo1/DXO protein family are involved in eukaryotic 5'-end RNA quality control by hydrolyzing non-canonical mRNA cap structures, such as the NAD+ 5'-cap. The plant DXO1 ortholog shows distinct biochemical properties, compared to its fungal and mammalian counterparts, and only minor phenotypic traits can be attributed to its NAD+-decapping (deNADding) activity. On the other hand, the chloroplast and growth defects observed in dxo1 null mutants appear linked to the presence of a serine-rich domain at its N-terminus. Here, we report that plant DXO1 is a distinct ortholog that likely diverged earlier than its fungal and mammalian counterparts. We attribute this to the presence of a serine-rich N-terminal domain, which is found almost exclusively in plants and likely arose during the emergence of angiosperms. Degradome profiling revealed that Arabidopsis DXO1 contributes to mRNA surveillance, via examination of exon junction complex and ribosome footprints, and this activity is reliant on the presence of the serine-rich N-terminal domain in Arabidopsis. Furthermore, polysome profiling revealed that plants lacking DXO1 have severe ribosome loading defects, which may explain the loss of mRNA surveillance and alterations in pre-rRNA processing. Taken together, our data leads us to propose a model whereby Arabidopsis DXO1 is involved in 5'-end RNA quality control, which is critical for ribosome loading that impacts translational fidelity and translation-dependent mRNA surveillance.