mRNA splicing performed by snRNPs

mRNAs need splicing. Splicing is the function that cuts the non-coding region of a mRNA (intron), and connects the coding region of the mRNA (exon). The bases of introns always starts at GU, and ends in AG. Splicing is performed by some small RNAs, snRNAs (small nuclear RNAs). snRNAs act as snRNPs that are the complex of snRNAs and some proteins. At first, U1 snRNP binds to the GU element of the 5’ of intron. Next, U2AF binds to the lower area of a branch region, which is the special element in introns, and subsequently U2 snRNP binds to the branch region. Then U4 snRNP, U5 snRNP and U6 snRNP gather to the intron. After releasing of U1 snRNP from GU element, U5 snRNP and U6 snRNP gather to the GU element. When U4 snRNP is released from the intron, U6 snRNP interacts to U2 snRNP and binds to GU element, and then cleaves the mRNA in the GU element. The G in the cleaved point hybridizes with the branch region, then a RNA lariat structure is formed. The exon-side of the cleaved mRNA is maintained binding to U5 snRNP. U5 snRNP also maintains binding to the AG element of the 3’ of intron. Here, the exon connects to the AG element, then the lariat-formed intron is cut off.