A Local Role for the Small Ribosomal Subunit Primary Binder rpS5 in Final 18S rRNA Processing in Yeast

Abstract

In vivo depletion of the yeast small ribosomal subunit (SSU) protein S5 (rpS5) leads to nuclear degradation of nascent SSUs and to a perturbed global assembly state of the SSU head domain. Here, we report that rpS5 plays an additional local role at the head/platform interface in efficient SSU maturation. We find that yeast small ribosomal subunits which incorporated an rpS5 variant lacking the seven C-terminal amino acids have a largely assembled head domain and are exported to the cytoplasm. On the other hand, 3′ processing of 18S rRNA precursors is inhibited in these ribosomal particles, although they associate with the putative endonuclease Nob1p and other late acting 40S biogenesis factors. We suggest that the SSU head component rpS5 and platform components as rpS14 are crucial constituents of a highly defined spatial arrangement in the head – platform interface of nascent SSUs, which is required for efficient processing of the therein predicted SSU rRNA 3′ end. Positioning of rpS5 in nascent SSUs, including its relative orientation towards platform components in the head-platform cleft, will depend on the general assembly and folding state of the head domain. Therefore, the suggested model can explain 18S precursor rRNA 3′ processing phenotypes observed in many eukaryotic SSU head assembly mutants.

RNA Mimicry by the Fap7 Adenylate Kinase in Ribosome Biogenesis

“…However, although mutations in the C-terminal extension of Rps14 interfere with site-D cleavage [20], depletion of Rps14 leads to earlier defects in pre-rRNA processing [46]–[48]. Rps14 therefore has a dual role in ribosome biogenesis, acting in both early and late preribosomal particles, as is the case for Rps5 [45]. This indicates that Rps14 is bound to the ribosome throughout the maturation process, so its function in 20S processing must be triggered by an external factor or conformational change.…”

Ribosome Biogenesis in the YeastSaccharomyces cerevisiae

“…While depletion of S5 or S14 blocks early steps of pre-rRNA processing (Moritz et al 1990;Jakovljevic et al 2004;Ferreira-Cerca et al 2007), alanine substitution mutations in three of the last four amino acids of S14 or deletion of the last seven amino acids of S5 (Neueder et al 2010) block a later step, cleavage at site D in 20S pre-rRNA. Thus S5 and S14 may direct proper folding of pre-rRNA necessary to form stable early assembly intermediates, but they also are important to create a proper local environment at the head/cleft interface of SSUs, near the 39 end of 20S pre-rRNA, to enable a late step in pre-rRNA processing.…”

Rps5-Rps16 communication is essential for efficient translation initiation in yeastS. cerevisiae

“…The protein forms part of the exit (E) site, is essential for cellular viability and recent data suggest that yeast Rps5 functions in translation initiation (9,10) as well as 40S head formation (11). Rps5/S7 proteins possess conserved central and C-terminal regions and exhibit variability at the N-terminus, with fungi and fruit flies exhibiting the longest N-terminal tail regions as compared to bacteria and metazoans.…”