SnRNPs

snRNPs, or small nuclear ribonucleoproteins, are essential components of the eukaryotic spliceosome. They are ribonucleoprotein particles composed of one small nuclear RNA (snRNA) molecule and a set of associated proteins. The snRNA provides the base-pairing and structural framework for recognizing splice sites in pre-mRNA, while the proteins contribute to assembly, stability, and catalytic activity.

Most snRNPs share a common core of Sm or LSm proteins that surround the snRNA. The major spliceosome snRNPs include U1, U2, U4, U5, and U6, which participate in recognition of the 5' splice site, branch point, and 3' splice site. A separate set of minor (or U12-type) snRNPs—such as U11, U12, U4atac, and U6atac—together with the shared U5 snRNP, participate in splicing of a subset of introns with alternative recognition motifs.

snRNP biogenesis begins with transcription of snRNA genes. Most snRNAs are transcribed by RNA polymerase II, with some by RNA polymerase III. The snRNA is capped and exported to the cytoplasm for assembly of the Sm ring by the SMN complex, after which the mature snRNP is re-imported into the nucleus to participate in spliceosome assembly. Within the nucleus, snRNPs, in concert with numerous proteins, form the dynamic spliceosome that performs the two transesterification reactions essential for intron removal and exon joining.

Mutations or deficiencies in snRNP components or their assembly factors, such as SMN in spinal muscular atrophy, can disrupt splicing fidelity and cellular viability. Understanding snRNPs illuminates the broader mechanism of pre-mRNA processing and gene expression.