snRNP

snRNP stands for small nuclear ribonucleoprotein, a molecular complex that combines a small nuclear RNA (snRNA) with associated proteins. The core features a ring of Sm proteins that binds a conserved Sm site on the snRNA, providing a scaffold for snRNP-specific proteins. The best-characterized examples are the five major spliceosomal snRNPs—U1, U2, U4, U5, and U6—which participate in pre-mRNA splicing, along with several minor snRNPs that function in specialized splicing pathways.

Each snRNP contains a distinctive snRNA that guides recognition of splicing signals and contributes to catalysis, together with protein factors that regulate assembly and function. SnRNPs are predominantly nuclear, residing in domains such as nuclear speckles and Cajal bodies, where they are prepared for participation in the spliceosome.

Biogenesis begins with transcription of snRNA genes (U1, U2, U4, U5 by RNA polymerase II; U6 by RNA polymerase III). The primary transcript is capped and processed, then exported to the cytoplasm, where the Sm core assembles onto the Sm site with the help of the SMN complex. After cap hypermethylation, the mature snRNP is imported back to the nucleus and trafficked to nuclear substructures before joining the spliceosome as needed.

Functionally, snRNPs assemble into the spliceosome to recognize 5' and 3' splice sites, the branch point, and exons. U1 binds the 5' splice site, U2 interacts with the branch point, and U4/U6 and U5 form the activated catalytic complex that catalyzes two transesterification reactions to remove introns and join exons. SnRNPs undergo dynamic remodeling during splicing. Defects in snRNP biogenesis or function are linked to diseases such as spinal muscular atrophy, where reduced SMN function impairs Sm-core assembly and snRNP maturation.