H/ACA snRNP-dependent ribosome biogenesis regulates translation of polyglutamine proteins.

Stem cells in many systems, including Drosophila germline stem cells (GSCs), increase ribosome biogenesis and translation during terminal differentiation. Here, we show that the H/ACA small nuclear ribonucleoprotein (snRNP) complex that promotes pseudouridylation of ribosomal RNA (rRNA) and ribosome biogenesis is required for oocyte specification. Reducing ribosome levels during differentiation decreased the translation of a subset of messenger RNAs that are enriched for CAG trinucleotide repeats and encode polyglutamine-containing proteins, including differentiation factors such as RNA-binding Fox protein 1. Moreover, ribosomes were enriched at CAG repeats within transcripts during oogenesis. Increasing target of rapamycin (TOR) activity to elevate ribosome levels in H/ACA snRNP complex-depleted germlines suppressed the GSC differentiation defects, whereas germlines treated with the TOR inhibitor rapamycin had reduced levels of polyglutamine-containing proteins. Thus, ribosome biogenesis and ribosome levels can control stem cell differentiation via selective translation of CAG repeat-containing transcripts.

The Drosophila hnRNP F/H homolog Glorund recruits dFMRP to inhibit nanos translation elongation.

Translational control of maternal mRNAs generates spatial and temporal patterns of protein expression necessary to begin animal development. Translational repression of unlocalized nanos (nos) mRNA in late-stage Drosophila oocytes by the hnRNP F/H homolog, Glorund (Glo), is important for embryonic body patterning. While previous work has suggested that repression occurs at both the translation initiation and elongation phases, the molecular mechanism by which Glo regulates nos translation remains elusive. Here, we have identified the Drosophila fragile X mental retardation protein, dFMRP, as a Glo interaction partner with links to the translational machinery. Using an oocyte-based in vitro translation system, we confirmed that Glo regulates both initiation and elongation of a nos translational reporter and showed that dFMRP specifically represses translation elongation and promotes ribosome stalling. Furthermore, we combined mutational analysis and in vivo and in vitro binding assays to show that Glo's qRRM2 domain specifically and directly interacts with dFMRP. Our findings suggest that Glo regulates nos translation elongation by recruiting dFMRP and that Glo's RNA-binding domains can also function as protein-protein interaction interfaces critical for its regulatory functions. Additionally, they reveal a mechanism for targeting dFMRP to specific transcripts.