More than 170 factors required for eukaryotic ribosome biogenesis have already been identified, including endo- and exonucleases, RNA helicases, rRNA modifying enzymes, snoRNPs and non-ribosomal proteins that associate transiently with the pre-rRNA. S. cerevisiae Nip7p is a 21 kDa non-ribosomal nucleolar protein required for proper 27S pre-rRNA processing and 60S subunit biogenesis. Nip7p is conserved among eukaryotes and putative homologues are found in Archaea. Previous studies described Nip7p interaction with the nucleolar protein Nop8p and with Rrp43p, a component of the exosome complex. In the present study, we show that it also interacts with the proteins Nop53p and Rrp15p, both involved in 60S subunit biogenesis. In addition, Nip7p interacts with Cbf5p and Nop10p, two components of the box H/ACA pseudouridylation complex of S. cerevisiae. Functional analysis using primer extension is being performed to further investigate Nip7p effect on rRNA pseudouridylation. Primer extension analysis revealed defects in pre-rRNA cleavage in Nip7p-deficient cells and the cleavage defective precursors were apparently degraded. For structural analysis the Pyrococcus abyssi Nip7p homologue (PaNip7) was crystallized. The structure was solved to 1.8Å resolution using the SIRAS method. PaNip7 structure is composed of two alpha/beta domains and the C-terminal region corresponds to the PUA domain. Based on this structural arrangement, we tested which Nip7p domain would be responsible for the protein-protein interactions. Interestingly, the PUA domain is responsible for almost every interaction tested. These results suggest that Nip7p plays a central role in rRNA processing and modification and that the PUA domain mediates its interaction with other proteins.