= Small nuclear ribonucleoprotein
A \b[small nuclear ribonucleoprotein] is a <ribonucleoprotein> particle containing a <small nuclear RNA> assembled with <proteins>. Several such particles cooperate in the <spliceosome>, which removes <introns> from precursor <messenger RNA>. <U1 snRNP> is a concrete example: its <U1 small nuclear RNA> helps recognize a <splice donor site>, while its proteins stabilize and organize the particle.<Crystal structure of human spliceosomal U1 snRNP at 5.5 Å resolution by Daniel A. Pomeranz Krummel and others>[Introduction, spliceosomal particles and U1 composition]{ref}
The distinction between component and assembly matters. An isolated <small nuclear RNA> is not the entire <small nuclear ribonucleoprotein>. Human U1 contains an RNA and ten proteins, including seven different <Sm proteins>. Nor is one U1 particle by itself the complete <spliceosome>.<Crystal structure of human spliceosomal U1 snRNP at 5.5 Å resolution by Daniel A. Pomeranz Krummel and others>[Abstract and Introduction, U1 RNA, seven Sm proteins and three U1-specific proteins]{ref}
A structural experiment rebuilt a functional U1 core from separately prepared components and determined a $5.5\,\mathrm{\mathring A}$ <X-ray crystallography> map. Labels containing heavy atoms helped identify which protein occupied which density. The crystallized particle was engineered and lacked one native U1 protein; modeling a complete particle required additional structural information.<Crystal structure of human spliceosomal U1 snRNP at 5.5 Å resolution by Daniel A. Pomeranz Krummel and others>[Structure determination, Figures1–4 and model in Figure5]{ref}
For an original completeness example, detecting an RNA together with one of its expected proteins establishes co-recovery of those components. It does not establish that all ten proteins occupy each recovered particle. Comparing component inventories with a functional <RNA splicing> assay tests composition and activity separately.
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