The Mechanism of Polyadenylation
There are a number of factors that must bind to the pre-mRNA for 3' cleavage and polyadenylation. Poly A polymerase (PAP) catalyzes poly A synthesis. Uniquely amongst polymerases, it does not require a template for polymerization. Cleavage and Polyadenylation Specificity Factor (CPSF) binds to the CPSF binding site (characteristic sequence: AAUAAA). PAP attaches to CPSF. Cleavage Stimulation Factor (CstF) binds the CstF binding site (characteristic sequence: GUNU, where N can stand for any nitrogenous base). The pre-mRNA folds so that CPSF and CstF can interact. Cleavage Factor I and II (CFI/II) bind to the mRNA between the CPSF and CstF binding sites. Cleavage occurs at this site. Poly A binding proteins bind to polyadenylated regions after PAP synthesizes them.
Functions of Polyadenylation
Addition of the poly-A tail facilitates translation and transportation of mRNA out of the nucleus.