Utp22p acts in concert with Utp8p to channel aminoacyl-tRNA from the nucleolus to the nuclear tRNA export receptor Los1p but not Msn5p.

Utp8p is an essential nucleolar protein that channels aminoacyl-tRNAs from aminoacyl-tRNA synthetases in the nucleolus to the nuclear tRNA export receptors located in the nucleoplasm and nuclear pore complex in Saccharomyces cerevisiae. Utp8p is also part of the U3 snoRNA-associated protein complex involved in 18S rRNA biogenesis in the nucleolus. We report that Utp22p, which is another member of the U3 snoRNA-associated protein complex, is also an intranuclear component of the nuclear tRNA export machinery. Depletion of Utp22p results in nuclear retention of mature tRNAs derived from intron-containing and intronless precursors. Moreover, Utp22p copurifies with the nuclear tRNA export receptor Los1p, the aminoacyl-tRNA synthetase Tys1p and Utp8p, but not with the RanGTPase Gsp1p and the nuclear tRNA export receptor Msn5p. Utp22p interacts directly with Utp8p and Los1p in a tRNA-independent manner in vitro. Utp22p also interacts directly with Tys1p, but this binding is stimulated when Tys1p is bound to tRNA. However, Utp22p, unlike Utp8p, does not bind tRNA saturably. These data suggest that Utp22p recruits Utp8p to aminoacyl-tRNA synthetases in the nucleolus to collect aminoacyl-tRNA and then accompanies the Utp8p-tRNA complex to deliver the aminoacyl-tRNAs to Los1p but not Msn5p. It is possible that Nrap/Nol6, the mammalian orthologue of Utp22p, plays a role in channelling aminoacyl-tRNA to the nuclear tRNA export receptor exportin-t.

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