An Sm protein is a member of a related group of RNA-associated proteins best known for forming the core ring of several spliceosomal small nuclear ribonucleoproteins. In human U1 snRNP, seven different Sm subunits assemble around a segment of U1 small nuclear RNA. This core helps organize the particle that participates in RNA splicing.[ref]
Crystal structure of human spliceosomal U1 snRNP at 5.5 Å resolution by Daniel A. Pomeranz Krummel and others. Introduction and Figure1, seven-subunit Sm core
Seven subunits does not mean seven copies of one protein: the canonical ring contains B/B′, D1, D2, D3, E, F and G types. In a reconstituted U1-core experiment, heavy-atom labels associated with particular proteins identified their positions in an X-ray crystallography map. The assignment established an ordered arrangement of different subunits, beyond merely detecting their presence in one sample.[ref]
Crystal structure of human spliceosomal U1 snRNP at 5.5 Å resolution by Daniel A. Pomeranz Krummel and others. Structure of the U1 snRNP core domain and Figures1–2, labeling and subunit placement
For an original counting example, two complete canonical Sm rings contain14 subunits but only seven subunit types. Recovering14 protein molecules from a sample would not prove that two complete rings had assembled: the same count could include free components or incomplete particles. Composition, organization and RNA splicing activity consequently require different experimental evidence.
The Sm ring is also only part of the small nuclear ribonucleoprotein. Particle-specific proteins and the RNA contribute further interactions; the ring alone should not be equated with an intact spliceosome.
New to topics? Read the docs here!