Novel Stress-responsive Genes EMG1 and NOP14 Encode Conserved, Interacting Proteins Required for 40S Ribosome Biogenesis

Abstract

Under stressful conditions organisms adjust the synthesis, processing, and trafficking of molecules to allow survival from and recovery after stress. In baker's yeast Saccharomyces cerevisiae, the cellular production of ribosomes is tightly matched with environmental conditions and nutrient availability through coordinate transcriptional regulation of genes involved in ribosome biogenesis. On the basis of stress-responsive gene expression and functional studies, we have identified a novel, evolutionarily conserved gene, EMG1, that has similar stress-responsive gene expression patterns as ribosomal protein genes and is required for the biogenesis of the 40S ribosomal subunit. The Emg1 protein is distributed throughout the cell; however, its nuclear localization depends on physical interaction with a newly characterized nucleolar protein, Nop14. Yeast depleted of Nop14 or harboring a temperature-sensitive allele of emg1 have selectively reduced levels of the 20S pre-rRNA and mature18S rRNA and diminished cellular levels of the 40S ribosomal subunit. Neither Emg1 nor Nop14 contain any characterized functional motifs; however, isolation and functional analyses of mammalian orthologues of Emg1 and Nop14 suggest that these proteins are functionally conserved among eukaryotes. We conclude that Emg1 and Nop14 are novel proteins whose interaction is required for the maturation of the 18S rRNA and for 40S ribosome production.

A Noc Complex Specifically Involved in the Formation and Nuclear Export of Ribosomal 40 S Subunits

“…This conclusion is also supported by the observation that early pre-rRNA processing at sites A 0 , A 1 , and A 2 is inhibited in both noc4 and nop14 ts mutants (see also Ref. 21). Moreover, rRNA processing and Table I.…”

“…2, lane 3). Mass spectrometry analysis identified this protein as Nop14p, which was shown previously to play a role in 40 S subunit biogenesis (21). Subsequently, we generated a chromosomally integrated Nop14p-ProtA and affinity-purified it under the same stringent conditions.…”

“…Here, we analyze the properties of Noc4p and show that it is required for maturation and transport of the 40 S, but not the 60 S, subunit. Noc4p localizes to the nucleolus and forms a stable heterodimer with Nop14, which was recently described to be involved in 40 S subunit biogenesis (21). Apparently, formation of different pairs of related Noc proteins represents a common theme in ribosome biogenesis; they participate in distinct steps of either pre-40 S or pre-60 S ribosome assembly, which is directly linked to ribosomal precursor transport.…”

“…11 and 15). Another candidate that could follow pre-40 S particles through different maturation steps is Emg1p, a 40 S biogenesis factor, which was described recently to interact with Nop14p (21). Interestingly, the nuclear distribution of Emg1p is dependent on the presence of functional Nop14p (21), which indicates that at least some components of the network involved in nuclear pre-40 S biogenesis also have the potential to shuttle between cytoplasm and nucleus.…”

Is Ribosome Synthesis Controlled by Pol I Transcription?

“…(2) Group B (see Table 1) includes eight genes whose products are involved in ribosome synthesis, assembly and/or function: Utp22 is involved in the 35S primary transcript processing, 18 Emg1 is required for the maturation of the 18S rRNA and for 40S ribosome production, 19 Stm1 directly binds to mature 80S ribosomes and polysomes, 20 Rpg1 is a translation initiation factor, 21 Asc1 acts as a negative regulator of translation, 22 and Cdc60 aminoacylates leucyl-tRNA. 23 Finally, SDC1 and SPP1, whose products are two subunits of the COMPASS (Set1C) complex which methylates lysine 4 of histone H3, and which is involved in rDNA silencing, 24,25 are also specifically deregulated in CARA cells during the rapamycin treatment.…”