Unstructured Domains in Assembly Factors Promote the Hierarchy of Ribosome Maturation via Their Structural Plasticity.

Ribosomes are assembled with the help of a large machinery of assembly factors (AFs), whose roles remain incompletely characterized. Recent structural studies of assembly intermediates have provided tremendous context for such studies and helped reveal novel roles, including as RNA chaperones. These structures have also revealed that a subset of the AFs have entirely non-globular structures, or large non-globular extensions, which extend across the assembling ribosomal subdomains, contacting many rRNA regions, ribosomal proteins (RPs) and other AFs. How these unusual structures could help promote assembly has remained unclear although it has been suggested that their potential for making multiple interactions helps constrain early assembly steps. By studying the roles of the AF Ltv1, an entirely non-globular protein, during late maturation steps of the 40S subunits head, we show here how the structural plasticity that derives from the non-globular structure is used to communicate maturation steps across the nascent subunit, thereby establishing the previously described hierarchy in head assembly. Our data indicate that while this structural plasticity enables integration of distinct folding and assembly steps, it also creates the potential for mutations that allow for bypass of these QC steps. These mutations are pathogenic in humans, further demonstrating the importance of proper 40S subunit assembly for protein homeostasis.