Within a single generation time a growing yeast cell imports ∼14 million ribosomal proteins (r-proteins) into the nucleus for ribosome production. After import, it is unclear how these intrinsically unstable and aggregation-prone proteins are targeted to the ribosome assembly site in the nucleolus. Here, we report the discovery of a conserved nuclear carrier Tsr2 that coordinates transfer of the r-protein eS26 to the earliest assembling pre-ribosome, the 90S. In vitro studies revealed that Tsr2 efficiently dissociates importin:eS26 complexes via an atypical RanGTP-independent mechanism that terminates the import process. Subsequently, Tsr2 binds the released eS26, shields it from proteolysis, and ensures its safe delivery to the 90S pre-ribosome. We anticipate similar carriers—termed here escortins—to securely connect the nuclear import machinery with pathways that deposit r-proteins onto developing pre-ribosomal particles.DOI: http://dx.doi.org/10.7554/eLife.03473.001

No abstract

Polymeric nanoparticles displaying tumor necrosis factor on their surface (TNF nanocytes) are useful carrier systems capable of mimicking the bioactivity of membrane-bound TNF. Thus, TNF nanocytes are potent activators of TNF receptor 1 and 2 leading to a striking enhancement of apoptosis. However, in vivo applications are hampered by potential systemic toxicity. Here, using TNF nanocytes as a model system, we developed a procedure to generate targeted lipid-coated particles (TLP) in which TNF activity is shielded. The TLPs generated here are composed of an inner single-chain TNF (scTNF)-functionalized, polymeric nanoparticle core surrounded by a lipid coat endowed with polyethylene glycol (PEG) for sterical stabilization and a single-chain Fv (scFv) fragment for targeting. Using a scFv directed against the tumor stroma marker fibroblast activation protein (FAP) we show that TLP and scTNF-TLP specifically bind to FAP-expressing, but not to FAP-negative cells. Lipid coating strongly reduced nonspecific binding of particles and scTNF-mediated cytotoxicity towards FAP-negative cells. In contrast, an increased cytotoxicity of TLP was observed for FAP-positive cells. Thus, through liposome encapsulation, nanoparticles carrying bioactive molecules, which are subject to nonselective uptake and activity towards various cells and tissues, can be converted into target cell-specific composite particles exhibiting a selective activity towards antigen-positive target cells. Besides safe and targeted delivery of death ligands such as TNF, TLP should be suitable for various diagnostic and therapeutic applications, which benefit from a targeted delivery of reagents embedded into the particle core or displayed on the core particle surface.

A combination of genetic trapping, affinity-capture and selected reaction monitoring mass spectrometry is used to characterize the dynamic proteome of pre-60S ribosomal particles after nuclear export. These results identify Bud20 as a novel shuttling factor for pre-60S export.

Eukaryotic ribosome precursors acquire translation competence in the cytoplasm through stepwise release of bound assembly factors, and proofreading of their functional centers. In case of the pre-60S, these steps include removal of placeholders Rlp24, Arx1 and Mrt4 that prevent premature loading of the ribosomal protein eL24, the protein-folding machinery at the polypeptide exit tunnel (PET), and the ribosomal stalk, respectively. Here, we reveal that sequential ATPase and GTPase activities license release factors Rei1 and Yvh1 to trigger Arx1 and Mrt4 removal. Drg1-ATPase activity removes Rlp24 from the GTPase Nog1 on the pre-60S; consequently, the C-terminal tail of Nog1 is extracted from the PET. These events enable Rei1 to probe PET integrity and catalyze Arx1 release. Concomitantly, Nog1 eviction from the pre-60S permits peptidyl transferase center maturation, and allows Yvh1 to mediate Mrt4 release for stalk assembly. Thus, Nog1 co-ordinates the assembly, maturation and quality control of distant functional centers during ribosome formation.

The transport receptor Crm1 mediates the export of diverse cargos containing leucine-rich nuclear export signals (NESs) through complex formation with RanGTP. To ensure efficient cargo release in the cytoplasm, NESs have evolved to display low affinity for Crm1. However, mechanisms that overcome low affinity to assemble Crm1-export complexes in the nucleus remain poorly understood. In this study, we reveal a new type of RanGTP-binding protein, Slx9, which facilitates Crm1 recruitment to the 40S pre-ribosome-associated NES-containing adaptor Rio2. In vitro, Slx9 binds Rio2 and RanGTP, forming a complex. This complex directly loads Crm1, unveiling a non-canonical stepwise mechanism to assemble a Crm1-export complex. A mutation in Slx9 that impairs Crm1-export complex assembly inhibits 40S pre-ribosome export. Thus, Slx9 functions as a scaffold to optimally present RanGTP and the NES to Crm1, therefore, triggering 40S pre-ribosome export. This mechanism could represent one solution to the paradox of weak binding events underlying rapid Crm1-mediated export.DOI: http://dx.doi.org/10.7554/eLife.05745.001

No abstract

Multiple export receptors passage bound pre-ribosomes through nuclear pore complexes (NPCs) by transiently interacting with the Phe-Gly (FG) meshwork of their transport channels. Here, we reveal how the non-FG interacting yeast mRNA export factor Gly-Leu-FG lethal 2 (Gle2) functions in the export of the large pre-ribosomal subunit (pre-60S). Structure-guided studies uncovered conserved platforms used by Gle2 to export pre-60S: an uncharacterized basic patch required to bind pre-60S, and a second surface that makes non-FG contacts with the nucleoporin Nup116. A basic patch mutant of Gle2 is able to function in mRNA export, but not pre-60S export. Thus, Gle2 provides a distinct interaction platform to transport pre-60S to the cytoplasm. Notably, Gle2’s interaction platforms become crucial for pre-60S export when FG-interacting receptors are either not recruited to pre-60S or are impaired. We propose that large complex cargos rely on non-FG as well as FG-interactions for their efficient translocation through the nuclear pore complex channel.