Dual function of eIF3j/Hcr1p in processing 20 S pre-rRNA and translation initiation

eIF3j/Hcr1p, a protein associated with eIF3, was shown to bind to, and stab ilize, the multifactor complex containing eIFs 1, 2, 3, and 5 and Met-tRNA( i)(Met), whose formation is required for an optimal rate of translation ini tiation. Here we present evidence that eIF3j/Hcr1p is an RNA binding protei n that enhances a late step in 40 S ribosome maturation involving cleavage of the 20 S precursor of 18 S rRNA in the cytoplasm. Immunofluorescence sta ining shows that eIF3j/Hcr1p is localized predominantly in the cytoplasm. T he herl A mutant exhibits a decreased amount of 40 S subunits, hypersensiti vity to paromomycin, and increased levels of 20 S pre-rRNA. Combining the h cr1 Delta mutation with drs2 Delta or rps0a Delta, deletions of two other g enes involved in the same step of 40 S subunit biogenesis, produced a synth etic growth defect. p35, the human ortholog of eIF3j/Hcr1p, partially compl emented the slow growth phenotype conferred by hcr1 Delta when overexpresse d in yeast. heIF3j/p35 was found physically associated with yeast eIF3 and 43 S initiation complexes in vitro and in vivo. Because it did not compleme nt the 40 S biogenesis defect of hcr1 Delta, it appears that heIF3j can sub stitute for eIF3j/Hcr1p only in translation initiation. We conclude that eI F3j/Hcr1p is required for rapid processing of 20 S to IS S rRNA besides its role in translation initiation, providing an intriguing link between ribos ome biogenesis and translation.