The dedicated chaperone acl4 escorts ribosomal protein rpl4 to its nuclear pre-60s assembly site

摘要

Ribosomes are the highly complex macromolecular assemblies dedicated to the synthesisof all cellular proteins from mRNA templates. The main principles underlying the making ofribosomes are conserved across eukaryotic organisms and this process has been studiedin most detail in the yeast Saccharomyces cerevisiae. Yeast ribosomes are composed offour ribosomal RNAs (rRNAs) and 79 ribosomal proteins (r-proteins). Most r-proteins needto be transported from the cytoplasm to the nucleus where they get incorporated into theevolving pre-ribosomal particles. Due to the high abundance and difficult physicochemicalproperties of r-proteins, their correct folding and fail-safe targeting to the assembly sitedepends largely on general, as well as highly specialized, chaperone and transport systems. Many r-proteins contain universally conserved or eukaryote-specific internal loopsand/or terminal extensions, which were shown to mediate their nuclear targeting and association with dedicated chaperones in a growing number of cases. The 60S r-protein Rpl4 isparticularly interesting since it harbours a conserved long internal loop and a prominent Cterminal eukaryote-specific extension. Here we show that both the long internal loop andthe C-terminal eukaryote-specific extension are strictly required for the functionality of Rpl4.While Rpl4 contains at least five distinct nuclear localization signals (NLS), the C-terminalpart of the long internal loop associates with a specific binding partner, termed Acl4.Absence of Acl4 confers a severe slow-growth phenotype and a deficiency in the productionof 60S subunits. Genetic and biochemical evidence indicates that Acl4 can be consideredas a dedicated chaperone of Rpl4. Notably, Acl4 localizes to both the cytoplasm andnucleus and it has the capacity to capture nascent Rpl4 in a co-translational manner. Takentogether, our findings indicate that the dedicated chaperone Acl4 accompanies Rpl4 fromthe cytoplasm to its pre-60S assembly site in the nucleus