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RNA. 2017 Jun;23(6):952-967. doi: 10.1261/rna.058487.116. Epub 2017 Mar 21.

The sole LSm complex in Cyanidioschyzon merolae associates with pre-mRNA splicing and mRNA degradation factors.

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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.
Department of Biochemistry and Microbiology, University of Victoria, Victoria, BC, V8W 3P6, Canada.
Department of Biochemistry, University of Alberta, Edmonton, Alberta T6G 2H7, Canada.
Department of Oncology, University of Alberta, Edmonton, Alberta T6G 2H7, Canada.
Department of Biochemistry and Molecular Biology, The University of British Columbia, Vancouver, BC V6T 1Z3, Canada.
Kuroiwa Initiative Research Unit, College of Science, Rikkyo University, Toshima, Tokyo 171-8501, Japan.
Département de Biochimie, Université de Montréal, Montréal, QC H2W 1R7, Canada.


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 Cyanidioschyzon merolae 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 C. merolae 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.


Cyanidioschyzon merolae; LSm complex; Pat1; U6 snRNA; mRNA degradation; pre-mRNA splicing

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