= Sm protein
{c}
An \b[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>.<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]{ref}
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.<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]{ref}
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>.
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