The L1 stalk is required for efficient export of nascent large ribosomal subunits in yeast

Nuclear export of pre-60S particles through the nuclear pore complex

The nuclear pore complex (NPC) is the bidirectional gate that mediates the exchange of macromolecules or their assemblies between nucleus and cytoplasm^1-3. The assembly intermediates of the ribosomal subunits, pre-60S and pre-40S particles, are among the largest cargoes of the NPC and the export of these gigantic ribonucleoproteins requires numerous export factors^4,5. Here we report the cryo-electron microscopy structure of native pre-60S particles trapped in the channel of yeast NPCs. In addition to known assembly factors, multiple factors with export functions are also included in the structure. These factors in general bind to either the flexible regions or subunit interface of the pre-60S particle, and virtually form many anchor sites for NPC binding. Through interactions with phenylalanine-glycine (FG) repeats from various nucleoporins of NPC, these factors collectively facilitate the passage of the pre-60S particle through the central FG repeat network of the NPC. Moreover, in silico analysis of the axial and radial distribution of pre-60S particles within the NPC shows that a single NPC can take up to four pre-60S particles simultaneously, and pre-60S particles are enriched in the inner ring regions close to the wall of the NPC with the solvent-exposed surface facing the centre of the nuclear pore. Our data suggest a translocation model for the export of pre-60S particles through the NPC.

A stem cell roadmap of ribosome heterogeneity reveals a function for RPL10A in mesoderm production

Recent findings suggest that the ribosome itself modulates gene expression. However, whether ribosomes change composition across cell types or control cell fate remains unknown. Here, employing quantitative mass spectrometry during human embryonic stem cell differentiation, we identify dozens of ribosome composition changes underlying cell fate specification. We observe upregulation of RPL10A/uL1-containing ribosomes in the primitive streak followed by progressive decreases during mesoderm differentiation. An Rpl10a loss-of-function allele in mice causes striking early mesodermal phenotypes, including posterior trunk truncations, and inhibits paraxial mesoderm production in culture. Ribosome profiling in Rpl10a loss-of-function mice reveals decreased translation of mesoderm regulators, including Wnt pathway mRNAs, which are also enriched on RPL10A/uL1-containing ribosomes. We further show that RPL10A/uL1 regulates canonical and non-canonical Wnt signaling during stem cell differentiation and in the developing embryo. These findings reveal unexpected ribosome composition modularity that controls differentiation and development through the specialized translation of key signaling networks.

Misregulation of the Nucleoporins 98 and 96 lead to defects in protein synthesis that promote hallmarks of tumorigenesis

Nucleoporin 98KD (Nup98) is a promiscuous translocation partner in hematological malignancies. Most disease models of Nup98 translocations involve ectopic expression of the fusion protein under study, leaving the endogenous Nup98 loci unperturbed. Overlooked in these approaches is the loss of one copy of normal Nup98 in addition to the loss of Nup96 – a second Nucleoporin encoded within the same mRNA and reading frame as Nup98 – in translocations. Nup98 and Nup96 are also mutated in a number of other cancers, suggesting that their disruption is not limited to blood cancers. We found that reducing Nup98-96 function in Drosophila melanogaster (in which the Nup98-96 shared mRNA and reading frame is conserved) de-regulates the cell cycle. We found evidence of overproliferation in tissues with reduced Nup98-96, counteracted by elevated apoptosis and aberrant signaling associated with chronic wounding. Reducing Nup98-96 function led to defects in protein synthesis that triggered JNK signaling and contributed to hallmarks of tumorigenesis when apoptosis was inhibited. We suggest that partial loss of Nup98-96 function in translocations could de-regulate protein synthesis, leading to signaling that cooperates with other mutations to promote tumorigenesis.

Editor's choice: Compromising Nucleoporins 98 and 96 leads to defects in protein synthesis and stress signaling via JNK that triggers compensatory and apoptosis-induced proliferation.