Core spliceosomal Sm proteins as constituents of cytoplasmic mRNPs in plants

Evaluation of Spliceosome Protein SmD2 as a Potential Target for Cancer Therapy

The core spliceosome Sm proteins are gaining attention as potential targets for cancer treatment. Here, we evaluate this, with focus on SmD2. A pan-cancer analysis including 26 solid tumor types revealed that the SmD2-encoding SNRPD2 gene was overexpressed in almost all cancers. In several cancers, high SNRPD2 expression was associated with a poor prognosis. To investigate the vulnerability of human cells to the loss of SmD2 expression, we silenced SNRPD2 using a short hairpin-expressing lentiviral vector in established cancer cell lines; in short-term cultured melanoma cells; and in several normal cell cultures, including cancer-associated fibroblasts cultured from non-small cell lung cancer resections. Additionally, we analyzed publicly available cell viability datasets for the dependency of cancer cell lines to SmD2 expression. Together, these studies clearly established SmD2 as a cancer-selective lethal target. Delving into genes with similar essentiality profiles to SNRPD2, we uncovered the intersected lethal stress between the loss of SmD2 and the loss of gene products participating in not only different mRNA processing steps including mRNA splicing, but also processes for coordinated protein production, as well as mitosis. Furthermore, we could correlate SNRPD2 expression to the responses of cancer cells to several FDA-approved anti-tumor drugs, especially to drugs inhibiting the cell cycle. Overall, our study confirms the anticipated role for targeting SmD2 in cancer treatment and reveals non-canonical SmD2 functions beyond mRNA splicing that could contribute to the dependency of cancer cells to high SNRPD2 expression.

Impact of protein domains on the MEL2 granule, a cytoplasmic ribonucleoprotein complex maintaining faithful meiosis progression in rice

Cytoplasmic ribonucleoprotein (RNP) granules are membraneless structures composed of various RNAs and proteins that play important roles in post-transcriptional regulation. While RNP granules are known to regulate the meiotic entry in some organisms, little is known about their roles in plants. In this study, we observed the cytoplasmic granular structures of rice RNA-binding protein MEIOSIS ARRESTED AT LEPTOTENE2 (MEL2), which contributes to the control of meiotic entry timing, in leaf protoplasts and spore mother cells. We performed colocalization analysis with known cytoplasmic RNP factors, and domain deletion analysis to assess their impact on granule formation and meiosis progression. Conservation of MEL2 domains across plant species was also explored. Our results indicated that MEL2 granules colocalized with processing body and stress granule factors. The maintenance of granule properties modulated by LOTUS domain and the intrinsically disordered region (IDR) is essential for proper MEL2 function in meiosis progression. MEL2-like proteins widely found in plant kingdom conserved LOTUS domain followed by the IDR despite their diverse domain structures, suggesting the functional conservation of these domains among plant species. This study highlights the role of MEL2 granule dynamics and its impact on meiotic transition and progression.

Cotranslational Mechanisms of Protein Biogenesis and Complex Assembly in Eukaryotes

The formation of protein complexes is crucial to most biological functions. The cellular mechanisms governing protein complex biogenesis are not yet well understood, but some principles of cotranslational and posttranslational assembly are beginning to emerge. In bacteria, this process is favored by operons encoding subunits of protein complexes. Eukaryotic cells do not have polycistronic mRNAs, raising the question of how they orchestrate the encounter of unassembled subunits. Here we review the constraints and mechanisms governing eukaryotic co- and posttranslational protein folding and assembly, including the influence of elongation rate on nascent chain targeting, folding, and chaperone interactions. Recent evidence shows that mRNAs encoding subunits of oligomeric assemblies can undergo localized translation and form cytoplasmic condensates that might facilitate the assembly of protein complexes. Understanding the interplay between localized mRNA translation and cotranslational proteostasis will be critical to defining protein complex assembly in vivo.

Functional nuclear retention of pre-mRNA involving Cajal bodies during meiotic prophase in European larch (Larix decidua)

Gene regulation ensures that the appropriate genes are expressed at the proper time. Nuclear retention of incompletely spliced or mature mRNAs is emerging as a novel, previously underappreciated layer of posttranscriptional regulation. Studies on this phenomenon indicated that it exerts a significant influence on the regulation of gene expression by regulating export and translation delay, which allows the synthesis of specific proteins in response to a stimulus or at strictly controlled time points, for example, during cell differentiation or development. Here, we show that transcription in microsporocytes of European larch (Larix decidua) occurs in a pulsatile manner during prophase of the first meiotic division. Transcriptional activity was then silenced after each pulse. However, the transcripts synthesized were not exported immediately to the cytoplasm but were retained in the nucleoplasm and Cajal bodies (CBs). In contrast to the nucleoplasm, we did not detect mature transcripts in CBs, which only stored nonfully spliced transcripts with retained introns. Notably, the retained introns were spliced at precisely defined times, and fully mature mRNAs were released into the cytoplasm for translation. As similar processes have been observed during spermatogenesis in animals, our results illustrate an evolutionarily conserved mechanism of gene expression regulation during generative cells development in Eukaryota.