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Abstract
The yeast nucleolar protein Nop8p has previously been shown to interact with Nip7p and to be required for 60S ribosomal subunit formation. Although depletion of Nop8p in yeast cells leads to premature degradation of rRNAs, the biochemical mechanism responsible for this phenotype is still not known. In this work, we show that the Nop8p amino-terminal region mediates interaction with the 5.8S rRNA, while its carboxyl-terminal portion interacts with Nip7p and can partially complement the growth defect of the conditional mutant strain Δnop8/GAL::NOP8. Interestingly, Nop8p mediates association of Nip7p to pre-ribosomal particles. Nop8p also interacts with the exosome subunit Rrp6p and inhibits the complex activity in vitro, suggesting that the decrease in 60S ribosomal subunit levels detected upon depletion of Nop8p may result from degradation of pre-rRNAs by the exosome. These results strongly indicate that Nop8p may control the exosome function during pre-rRNA processing.
Highlights
Eukaryotic ribosome maturation is a complex pathway that requires at least 200 transacting factors in addition to the ribosomal protein subunits (r-proteins) and the ribosomal RNAs
To understand how Nop8p acts during ribosome biogenesis, we explored the contribution of its amino- and carboxyl-portions to its function
Similar to the conditional strain Dnop8/GAL1::NOP8 depleted of Nop8p, expression of the N-Nop8p deletion mutant leads to decreased levels of mature ribosomal RNAs (rRNAs)
Summary
Eukaryotic ribosome maturation is a complex pathway that requires at least 200 transacting factors in addition to the ribosomal protein subunits (r-proteins) and the ribosomal RNAs (rRNAs). 35S pre-rRNA processing involves several endo- and exonucleolytic cleavage reactions, as well as nucleotide modifications at specific positions. Pre-rRNA processing starts co-transcriptionally in the nucleolus, continues through the nucleoplasm, and the final reactions take place in the cytoplasm [1]. Throughout pre-rRNA processing and assembly of r-proteins, precursor ribosome particles are formed, that are going to originate the 40S and 60S mature ribosome subunits. While in the nucleolus, associated factors participate in the pre-60S subunit processing reactions for prerRNA 27S maturation. During transport of pre-60S ribosomes to the nucleoplasm, the protein composition of the particles undergoes great changes, by removal of some factors and joining of others. Similar changes occur during transport of pre-60S from the nucleoplasm to the cytoplasm, where final rearrangements will give rise to mature 60S ribosomal subunits [2]
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