Symplekin (SYMPK) is a scaffolding protein that mediates the interaction between CPSF and CstF.
In mammalian CPSF, both cleavage factor I (CFI) and cleavage and polyadenylation specificity factor (CPSF) are required for cleavage and polyadenylation whereas cleavage stimulation factor (CstF) is only essential for the cleavage step. CPSF and CstF travel along with RNA polymerase II (RNAP II) during nascent gene transcription in search of the PAS.
Cleavage factor I (CFI) is made of 25 (CPSF5), 59 (CPSF7), and 68 (CPSF6) kDa proteins. Cleavage factor II (CFII) is made of Pcf11, Clp1, and cleavage stimulation factor (CstF). CFII binds to the RNAP II C-terminal domain and other CpA factors.
Cleavage stimulation factor (CstF) has three subunits: CstF77 (CstF3), CstF50 (CstF1), and CstF64 (CstF2 and CstF2T). CstF recognizes the PAS that is 20 nucleotides downstream the signaling region of the cleavage site, which is a GU-rich sequence motif followed by U-rich sequences. CstF contributes to the selection of the cleavage site, as well as alternative polyadenylation.