Coupled GTPase and remodelling ATPase activities form a checkpoint for ribosome export

Two proteins are identified in yeast that regulate the timing of pre-ribosome export from the nucleus; Nug2 binds pre-60S particles until they are ready for export, at which time Nug2 is replaced by the export adaptor Nmd3, enabling the export machinery to recognise the pre-ribosome that is ready to be transferred to the cytoplasm. The step-wise assembly of the eukaryotic ribosome, which translates messenger RNA into protein, initiates in the nucleus but is completed in the cytoplasm. The nuclear envelope separates these two compartments, so the assembling ribosome has to pass through this barrier at a particular stage of maturation. In this study, Ed Hurt and colleagues identify two proteins that regulate the timing of pre-ribosome export from the nucleus. Nug2 binds pre-60S particles until they are ready for export, at which time it is replaced by Nmd3, an export adaptor; this allows the export machinery to recognize that the pre-ribosome is ready to be transferred to the cytoplasm. Eukaryotic ribosomes are assembled by a complex pathway that extends from the nucleolus to the cytoplasm and is powered by many energy-consuming enzymes1,2,3. Nuclear export is a key, irreversible step in pre-ribosome maturation4,5,6,7,8, but mechanisms underlying the timely acquisition of export competence remain poorly understood. Here we show that a conserved Saccharomyces cerevisiae GTPase Nug2 (also known as Nog2, and as NGP-1, GNL2 or nucleostemin 2 in human9) has a key role in the timing of export competence. Nug2 binds the inter-subunit face of maturing, nucleoplasmic pre-60S particles, and the location clashes with the position of Nmd3, a key pre-60S export adaptor10. Nug2 and Nmd3 are not present on the same pre-60S particles, with Nug2 binding before Nmd3. Depletion of Nug2 causes premature Nmd3 binding to the pre-60S particles, whereas mutations in the G-domain of Nug2 block Nmd3 recruitment, resulting in severe 60S export defects. Two pre-60S remodelling factors, the Rea1 ATPase and its co-substrate Rsa4, are present on Nug2-associated particles, and both show synthetic lethal interactions with nug2 mutants. Release of Nug2 from pre-60S particles requires both its K+-dependent GTPase activity and the remodelling ATPase activity of Rea1. We conclude that Nug2 is a regulatory GTPase that monitors pre-60S maturation, with release from its placeholder site linked to recruitment of the nuclear export machinery.