Nog2p, a putative GTPase associated with pre-60S subunits and required forlate 60S maturation steps

Eukaryotic ribosome maturation depends on a set of well ordered processing steps. Here we describe the functional characterization of yeast Nog2p (Ynr 053cp), a highly conserved nuclear protein. Nog2p contains a putative GTP-b inding site, which is essential in vivo. Kinetic and steady-state measureme nts of the levels of pre-rRNAs in Nog2p-depleted cells showed a defect in 5 .8S and 25S maturation and a concomitant increase in the levels of both 27S B(S) and 7S(S) precursors. We found Nog2p physically associated with large pre-60S complexes highly enriched in the 27SB and 7S rRNA precursors. These complexes contained, besides a subset of ribosomal proteins, at least two additional factors, Nog1p, another putative GTP-binding protein, and Rlp24p (Ylr009wp), which belongs to the Rp124e family of archaeal and eukaryotic ribosomal proteins. In the absence of Nog2p, the pre-60S ribosomal complexe s left the nucleolus, but were retained in the nucleoplasm. These results s uggest that transient, possibly GTP-dependent association of Nog2p with the pre-ribosomes might trigger late rRNA maturation steps in ribosomal large subunit biogenesis.