Core spliceosomal Sm proteins as constituents of cytoplasmic mRNPs in plants

Abstract

In recent years, research has increasingly focused on the key role of posttranscriptional regulation of mRNP function and turnover. Due to their complexity and dynamic nature, the full composition of a single mRNP complex remains unrevealed and mRNPs are poorly described in plants. Here we identify canonical Sm proteins as part of the cytoplasmic mRNP complex indicating their function in the posttranscriptional regulation of gene expression in plants. Sm proteins comprise an evolutionarily ancient family of small RNA‐binding proteins involved in pre‐mRNA splicing. The latest research indicates that Sm could also impact on mRNA at subsequent stages of its life cycle. In this work we show that in the larch microsporocyte cytoplasm, Sm proteins accumulate within distinct cytoplasmic bodies, also containing polyadenylated RNA (poly(A) RNA). To date, several types of cytoplasmic bodies involved in the posttranscriptional regulation of gene expression have been described, mainly in animal cells. However, their role and molecular composition in plants remain less established. 222 mRNA transcripts have been identified as cytoplasmic partners for Sm proteins. The specific colocalization of these mRNAs with Sm proteins within cytoplasmic bodies has been confirmed via microscopic analysis. The results from this work support the hypothesis, that evolutionarily conserved Sm proteins have been adapted to perform a whole repertoire of functions related to the posttranscriptional regulation of gene expression in Eukaryota. This adaptation presumably enabled them to coordinate the interdependent processes of splicing element assembly, mRNA maturation and processing, and mRNA translation regulation, and its degradation.