Nsa2 is an unstable, conserved factor required for the maturation of 27 SB pre-rRNAs

In Saccharomyces cerevisiae, a large variety of pre-ribosomal factors have been identified recently, a number of which are still of unknown function. The essential pre-ribosomal 30-kDa protein, Nsa2, was characterized as one of the most conserved proteins from yeast to human. We show here that the expression of the human orthologue TINP1 complements the repression of NSA2 in yeast. Nsa2 was co-purified in several pre-ribosomal complexes and found to be essential for the large ribosomal subunit biogenesis. Like several other factors of the pre-60 S particles, the absence of Nsa2 correlated with a decrease in the 25 S and 5.8 S ribosomal RNA levels, and with an accumulation of 27 SB pre-ribosomal RNA intermediates. We show that Nsa2 is a functional partner of the putative GTPase Nog1. In the absence of Nsa2, Nog1 was still able to associate with pre-ribosomal complexes blocked in maturation. In contrast, in the absence of Nog1, Nsa2 disappeared from pre-60 S complexes. Indeed, when ribosome biogenesis was blocked upstream of Nsa2, this short half-lived protein was largely depleted, suggesting that its cellular levels are tightly regulated.