Structural insights into RNA phosphorylation by the RNase PNK module of the human rixosome complex

The mammalian rixosome complex is a large multi-subunit complex that plays essential roles in ribosome assembly and heterochromatin maintenance. Three structural proteins form the stable core of the rixosome to which three enzymatic modules are flexibly tethered including an RNA processing module, AAA-ATPase, and SUMO protease. The RNA processing module is formed by RNase PNK, a tetrameric assembly comprising two copies each of the LAS1L endoribonuclease (RNase) and the NOL9 polynucleotide kinase (PNK). Using single particle cryo-EM, we determined ATPγS and AMP-PNP/RNA-bound structures of human RNase PNK. The structures revealed the overall butterfly-like architecture of the complex and provide new insights into the mechanism of RNA accommodation and 5' hydroxyl phosphorylation within the NOL9 active site. Through reconstitution studies and molecular modeling, we establish how RNase PNK is incorporated into the larger rixosome complex by a distinct domain of LAS1L. Finally, we show that the human 5'-3' exoribonuclease XRN2 directly associates with RNase PNK and selectively degrades NOL9-phosphorylated RNA in vitro, thereby linking ITS2 processing by the rixosome to processive exonucleolytic decay. Collectively this work establishes an updated model for how the rixosome integrates its diverse enzymatic activities to regulate ITS2 processing.