A nucleolar assembly module integrates an ancestral isoaspartylase to safeguard ribosome biogenesis
Eukaryotes inherited the core ribosome biogenesis apparatus from archaea. However, nucleocytoplasmic compartmentalisation and expansion to >200 assembly factors created the challenge of integrating this ancestral machinery into a complex maturation programme. One solution is the formation of transient modules in which newly acquired assembly factors support deeply conserved components. Here, we…
Eukaryotes acquired the essential ribosome biogenesis machinery from archaea. However, the addition of 200 assembly factors and the requirement for nucleocytoplasmic compartmentalization posed a challenge in integrating this ancestral system into a sophisticated maturation process. A potential solution involves the formation of temporary modules, where newly acquired assembly factors assist deeply conserved components.
In this investigation, researchers identified such a module, centered around the ancient isoaspartylase Fap7. This module links the modification of the ribosomal protein uS11 to its integration into pre-ribosomes. Fap7 collaborates with Krr1 to capture uS11, creating an initial checkpoint where uS11 loading allows for Kri1 engagement and assembly advancement. If uS11 modification is absent, a later checkpoint intervenes, preventing the creation of defective pre-ribosomes that would otherwise gain translational capability.
Through integrative structure-function analyses, the study reveals how the ancestral isoaspartylase is embedded within a conserved network of eukaryotic assembly factors. This network ensures the precise timing, sequencing, and accuracy of ribosome production.
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