Nep1p (Emg1p), a novel protein conserved in eukaryotes and archaea, is involved in ribosome biogenesis

The essential Saccharomyces cerevisiae gene YLR186w [NEP1 (nucleolar essential protein 1), also recently named EMG1 (essential for mitotic growth 1)] is highly conserved in eukaryotes and archaea. In S. cerevisiae, Nep1p is localized in the nucleus with a rodshaped morphology, which first suggested a spindle/microtubule association for SeNEP1 (S. cerevisiae NEP1). However, in Candida albicans and in HeLa cells, Nep1p is a protein of the nucleoli. CaNEP1 (C. albicans NEP1) and HsNEP1 (Homo sapiens NEP1) heterologously complement the essential phenotype in a S. cerevisiae nep1 deletion mutant. However, the ScNEP1 spindle/microtubule phenotype is not found with HsNEP1 and CaNEP1, which shows that the spindle/microtubule association is specific for ScNep1p and that it is not the primary essential function of Nep1p. A temperature-sensitive ScNEP1(ts1) allele was isolated and revealed a strongly increased sensitivity to paromomycin, a translational inhibitor which binds to RNA, indicating that ribosome biogenesis within the nucleolus is probably affected. This was confirmed by polysome profile and ribosomal subunit analysis, showing a ribosomal subunit imbalance with a decrease in 40S subunits, due to reduced amounts of native 18S rRNA. Furthermore, ScSAM2, encoding s-adenosylmethionine synthetase, was isolated as a multicopy-suppressor of the ScNEP1(ts1) mutant allele and medium supplementation with s-adenosylmethionine restored its growth. For the first time, these results identified a novel protein with an essential function in ribosomal biogenesis which directly or indirectly interferes with a methylation reaction during the early steps of pre-rRNA processing necessary for the generation of 40S ribosomal subunits.