Coordinated Ribosomal L4 Protein Assembly into the Pre-Ribosome Is Regulated by Its Eukaryote-Specific Extension

Eukaryotic ribosome biogenesis requires nuclear import and hierarchical incorporation of ∼80 ribosomal proteins (RPs) into the ribosomal RNA core. In contrast to prokaryotes, many eukaryotic RPs possess long extensions that interdigitate in the mature ribosome. RpL4 is a prime example, with an ∼80-residue-long surface extension of unknown function. Here, we identify assembly chaperone Acl4 that initially binds the universally conserved internal loop of newly synthesized RpL4 via its superhelical TPR domain, thereby restricting RpL4 loop insertion at its cognate nascent rRNA site. RpL4 release from Acl4 is orchestrated with pre-ribosome assembly, during which the eukaryote-specific RpL4 extension makes several distinct interactions with the 60S surface, including a co-evolved site on neighboring RpL18. Consequently, mutational inactivation of this contact site, on either RpL4 or RpL18, impairs RpL4-Acl4 disassembly and RpL4 pre-ribosome incorporation. We propose that hierarchical ribosome assembly can be achieved by eukaryotic RP extensions and dedicated assembly chaperones.