Regulatory protein of the MTREX-exosome complex involved in the synthesis of the 60S ribosomal subunit. Participates in an early cleavage of the pre-rRNA processing pathway in cooperation with NVL. Isoform 1 interacts (through WDR repeats) with NVL; the interaction is independent of RNA or pre-60S ribosome particles. Isoform 2 does not interact with NVL. Interacts with MTREX; the interaction dissociation in a late stage of rRNA synthesis is required for appropriate maturation of pre-60S particles and depends on the ATPase activity of NVL. Nucleolar location depends on active PolI transcription of pre-rRNA.