Reimer Kirsten A, Stark Martha R, Aguilar Lisbeth-Carolina, Stark Sierra R, Burke Robert D, Moore Jack, Fahlman Richard P, Yip Calvin K, Kuroiwa Haruko, Oeffinger Marlene, Rader Stephen D
Department of Chemistry, University of Northern British Columbia, Prince George, BC V2N 4Z9, Canada.
Laboratory of RNP Biochemistry, Institut de Recherches Cliniques de Montréal (IRCM), Faculty of Medicine, McGill University, Montreal, QC H3A 0G4, Canada.
RNA. 2017 Jun;23(6):952-967. doi: 10.1261/rna.058487.116. Epub 2017 Mar 21.
Proteins of the Sm and Sm-like (LSm) families, referred to collectively as (L)Sm proteins, are found in all three domains of life and are known to promote a variety of RNA processes such as base-pair formation, unwinding, RNA degradation, and RNA stabilization. In eukaryotes, (L)Sm proteins have been studied, inter alia, for their role in pre-mRNA splicing. In many organisms, the LSm proteins form two distinct complexes, one consisting of LSm1-7 that is involved in mRNA degradation in the cytoplasm, and the other consisting of LSm2-8 that binds spliceosomal U6 snRNA in the nucleus. We recently characterized the splicing proteins from the red alga and found that it has only seven LSm proteins. The identities of CmLSm2-CmLSm7 were unambiguous, but the seventh protein was similar to LSm1 and LSm8. Here, we use in vitro binding measurements, microscopy, and affinity purification-mass spectrometry to demonstrate a canonical splicing function for the LSm complex and experimentally validate our bioinformatic predictions of a reduced spliceosome in this organism. Copurification of Pat1 and its associated mRNA degradation proteins with the LSm proteins, along with evidence of a cytoplasmic fraction of CmLSm complexes, argues that this complex is involved in both splicing and cytoplasmic mRNA degradation. Intriguingly, the Pat1 complex also copurifies with all four snRNAs, suggesting the possibility of a spliceosome-associated pre-mRNA degradation complex in the nucleus.
Sm和Sm样（LSm）家族的蛋白质，统称为（L）Sm蛋白质，存在于生命的所有三个域中，已知可促进多种RNA过程，如碱基对形成、解旋、RNA降解和RNA稳定。在真核生物中，（L）Sm蛋白质尤其因其在mRNA前体剪接中的作用而受到研究。在许多生物体中，LSm蛋白质形成两种不同的复合物，一种由参与细胞质中mRNA降解的LSm1 - 7组成，另一种由在细胞核中结合剪接体U6 snRNA的LSm2 - 8组成。我们最近对红藻中的剪接蛋白进行了表征，发现它只有七种LSm蛋白质。CmLSm2 - CmLSm7的身份明确，但第七种蛋白质与LSm1和LSm8相似。在这里，我们使用体外结合测量、显微镜检查和亲和纯化 - 质谱来证明LSm复合物具有典型的剪接功能，并通过实验验证了我们对该生物体中简化剪接体的生物信息学预测。Pat1及其相关的mRNA降解蛋白与LSm蛋白共纯化，以及CmLSm复合物存在细胞质部分的证据，表明该复合物参与剪接和细胞质mRNA降解。有趣的是，Pat1复合物也与所有四种snRNA共纯化，这表明在细胞核中可能存在与剪接体相关的mRNA前体降解复合物。
