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Participation of Sm proteins in post-transcriptional regulation of gene expression in the cytoplasm

Post-transcriptional regulation of gene expression in Eukaryota is a complex and multistep process, comprising a cascade of events in the nucleus and cytoplasm. Several cytoplasmic post-transcriptional mRNA processing steps occur in highly specialized, evolutionary conserved microdomains called cytoplasmic bodies. Cytoplasmic bodies function as specific centres of balance coordination between mRNA translation and degradation, which results in gene expression on a proper level.

Our research revealed cyclic occurrence of large, spherical cytoplasmic bodies enriched in Sm proteins and polyadenylated RNA (poly(A)RNA) in larch microsporocytes. Sm proteins play a fundamental role in pre-mRNA splicing as being the core component of snRNPs. To date, however, the literature lacks information concerning the interaction between mRNA and Sm proteins in processes other than splicing in plants. Given this, we find our observations particularly interesting since larch Sm-rich accumulations lack other elements of spliceosomal units (UsnRNAs, U2B”, m3G cap), which indicates a novel role of Sm in the mRNA cell cycle in plants.

In order to identify which mRNAs are accumulated within Sm-rich cytoplasmic bodies, we applied the RIP-seq approach of Sm-RNA complexes from the cytoplasmic fraction of larch anthers. Transcriptomic analysis revealed hundreds of unique mRNAs. The identified transcripts could be divided into three major classes: those encoding ribosome/translation-related proteins and mitochondrial and chloroplastic proteins. Finally, in situ localization of selected transcripts confirmed its accumulation within Sm-rich cytoplasmic bodies (Hyjek et al. 2019Hyjek-Składanowska et al. 2020).

Based on our results, we propose that evolutionarily conserved Sm proteins have been adapted to perform several functions related to post-transcriptional regulation of gene expression in Eukaryota, which presumably enabled them to coordinate the interdependent processes of splicing and further mRNA maturation. Our observations are the first to report the participation of Sm proteins in post-transcriptional regulation of gene expression in the cytoplasm in plant cells.