Eukaryotic Pre-mRNA Processing

5' (m7G) Cap

A modified guanosine cap added to 5' end of emerging pre-mRNA after 20–25 nucleotides are transcribed to protect transcript and help export.

CBC (Cap-Binding Complex)

A nuclear protein complex that binds to 5' m7G cap to protect from degradation and assist in nuclear export.

Spliceosome

A multi-subunit ribonucleoprotein complex composed of snRNPs (U1, U2, U4, U5, U6 snRNAs bound to proteins) that recognizes intron boundaries to excise introns and ligate coding exons.

5' Splice Donor Site

A conserved cis sequence at 5' beginning of intron (typically GU dinucleotide) recognized by U1 snRNP

Branchpoint Adenosine

A conserved adenine residue located inside the intron sequence near the 3' end that performs a nucleophilic attack on the 5' splice donor site during splicing reaction.

3' Splice Acceptor Site

A conserved cis-acting sequence at 3' end of an intron (typically AG dinucleotide) attacked by 5'-exon intermediate during splicing reaction.

U1 & U2 snRNPs

Small nuclear ribonucleoproteins that position splice donor and acceptor sites in close physical proximity (U1 binds 5' donor site; U2 binds branchpoint adenosine).

U6 snRNA

catalytic core/ribozyme of the spliceosome that drives both chemical reactions (also replaces U1 at the 5' splice site)

Polyadenylation Signal Sequence (AAUAAA)

A conserved cis RNA sequence recognized by CTD processing factors to direct transcript cleavage and 3' tailing.

CPSF (Cleavage and Polyadenylation Specificity Factor)

A processing factor on CTD tail that binds to AAUAAA signal, catalyzes endonucleolytic cleavage 10–35 bp downstream, and activates Poly(A) Polymerase.

CstF (Cleavage Stimulation Factor)

A processing factor riding the phosphorylated CTD tail that binds to downstream G/U-rich elements to assist in cleavage complex assembly.

Symplekin

A scaffold protein that acts as glue to join CPSF and CstF together with cleavage factors CF1 and CF2 at the cleavage site.

CF1 & CF2 (Cleavage Factors 1 & 2)

Trans factors recruited by Symplekin to execute pre-mRNA endonucleolytic cleavage at CA dinucleotide region.

PAP (Poly(A) Polymerase)

An enzyme activated by contact with CPSF that adds adenine residues (no template) to free 3'-OH end of cleaved pre-mRNA.

PABPN1 (Nuclear Poly(A) Binding Protein)

A nuclear protein that binds to the expanding poly(A) tail (~1 per 11 adenines) to further stimulate PAP activity until the tail reaches ~250 nucleotides.