Inside the 40S ribosome assembly machinery

Ribosome assembly involves rRNA transcription, modification, folding and cleavage from precursor transcripts, and association of ribosomal proteins (Rps). In bacteria, this complex process requires only a handful of proteins in addition to those needed for rRNA transcription, modification and cleavage, while in eukaryotes a large machinery comprising ∼200 proteins in the yeast S. cerevisiae has been identified. Furthermore, while the bacterial assembly factors generally produce only cold-sensitive phenotypes upon deletion, most of the eukaryotic assembly factors are essential, comprising ∼20% of essential yeast proteins. This review explores recent rapid progress in the structural and functional dissection of the 40S assembly machinery.

► snoRNAs prevent formation of secondary structure, may modulate insertion of proteins.
► Co-transcriptional binding of proteins to the body and platform.
► Conserved pathway of protein assembly.
► Co-transcriptional and cytoplasmic maturation steps separated by conformational switch.
► Cytoplasmic assembly comprises many novel steps.