Identification of a 60S preribosomal particle that is closely linked to nuclear export

A nuclear GTPase, Nug1p, was identified in a genetic screen for components linked to 60S ribosomal subunit export. Nug1p cosedimented with nuclear 60S preribosomes and was required for subunit export to the cytoplasm. Tagged Nug1p coprecipitated with proteins of the 60S subunit, late precursors to t he 25S and 5.8S rRNAs, and at least 21 nonribosomal proteins. These include d a homologous nuclear GTPase, Nug2p, the Noc2p/Noc3p heterodimer, Rix1p, a nd Rlp7p, each of which was implicated in 60S subunit export. Other known r ibosome synthesis factors and proteins of previously unknown function, incl uding the 559 kDa protein Ylr106p, also copurified. Eight of these proteins were copurified with nuclear pore complexes, suggesting that this complex represents the transport intermediate for 60S subunit export.