In mammals, CPSF is a protein complex, consisting of six subunits: CPSF-160 (CPSF1), CPSF-100 (CPSF2), CPSF-73 (CPSF3), and CPSF-30 (CPSF4) kDa subunits, WDR33 and Fip1 (FIP1L1).
The subunits form two components: mammalian polyadenylation specificity factors (mPSF) and mammalian cleavage factor (mCF). The mPSF is made up of CPSF-160, WDR33, CPSF-30, and Fip1. It is necessary for PAS recognition and polyadenylation. The mCF is made up of CPSF-73, CPSF-100, and symplekin. It catalyzes the cleavage reaction by recognizing the histone mRNA 3' processing site.
CPSF-73 is a zinc-dependent hydrolase which cleaves the mRNA precursor between a CA dinucleotide just downstream the polyadenylation signal sequence AAUAAA.
CPSF-100 contributes to the endonuclease activity of CPSF-73.
CPSF-160 (160 kDa) is the largest subunit of CPSF and directly binds to the AAUAAA polyadenylation signal. 160 kDa has three β-propeller domains and a C-terminal domain.
CPSF-30 (30 kDa) has five Cys-Cys-Cys-His (CCCH) zinc-finger motifs near the N terminus and a CCCH zinc knuckle at the C terminus. Two isoforms of CPSF-30 exist and can be found in CPSF complexes. The RNA binding activity of CPSF-30 is mediated by its zinc-fingers 2 and 3. WD repeat domain 33 (146 kDa) has a WD40 domain near the N terminus. The WD40 domain interacts with RNA. WDR33 and CPSF-30 recognize the polyadenylation signal (PAS) in pre-mRNA, which aids in defining the position of RNA cleavage. CPSF-30 recognizes the AU-rich hexamer region by a cooperative, metal-dependent binding mechanism.
Although CPSF-160 is the largest subunit of CPSF, a study conducted by Schönemann et al., debate that WDR33 is responsible for recognizing the PAS and not CPSF-160 as previously believed. The study concluded that the reason that CPSF-160 was believed to be responsible for recognizing the PAS was due to the fact that the WDR33 subunit had not been discovered at the time of the claim.
Fip1 binds to U-rich RNAs by its arginine-rich C-terminus. It binds to RNA sequences upstream of the AAUAAA hexamer region in vitro. Fip1 and CPSF-160 recruit poly(A) polymerase (PAP) to the 3' processing site. PAP is stimulated by Poly(A) binding protein nuclear one to add the poly(A) tail, a non-templated adenosine residues, at the cleavage site.
Only CPSF-160, CPSF-30, Fip1, and WDR33 are necessary and sufficient to form an active CPSF subcomplex in AAUAAA-dependent polyadenylation. CPSF-73 and CPSF-100 are disposable.
CPSF recruits proteins to the 3' region. Identified proteins that are coordinated by CPSF activity include: cleavage stimulatory factor and the two poorly understood cleavage factors. The binding of the polynucleotide adenylyltransferase responsible for actually synthesizing the tail is a necessary prerequisite for cleavage, thus ensuring that cleavage and polyadenylation are tightly coupled processes.