RNA Polymerase II transcribes beyond what later becomes the 3′
end of a mature messenger RNA (mRNA). The formation of most mRNA 3′ ends
results from pre-mRNA cleavage followed by polyadenylation. In vitro studies
have shown that low concentrations of ATP stimulate the 3′ cleavage
reaction while high concentrations inhibit it, but the origin of these ATP
effects is unknown. ATP might enable a cleavage factor kinase or activate a
cleavage factor directly. To distinguish between these possibilities, we tested
several ATP structural analogs in a pre-mRNA 3′ cleavage reaction
reconstituted from DEAE-fractionated cleavage factors. We found that adenosine
5′-(β,γ-methylene)triphosphate (AMP-PCP) is an effective
in vitro 3′ cleavage inhibitor with an IC50 of ~300
μM, but that most other ATP analogs, including adenosine
5′-(β,γ-imido)triphosphate, which cannot serve as a
protein kinase substrate, promoted 3′ cleavage but less efficiently than
ATP. In combination with previous literature data, our results do not support
ATP stimulation of 3′ cleavage through cleavage factor phosphorylation
in vitro. Instead, the more likely mechanism is that ATP stimulates cleavage
factor activity through direct cleavage factor binding. The mammalian 3′
cleavage factors known to bind ATP include the cleavage factor II (CF
IIm) Clp1 subunit, the CF Im25 subunit and poly(A) polymerase
alpha (PAP). The yeast homolog of the CF IIm complex also binds ATP
through yClp1. To investigate the mammalian complex, we used a cell-line
expressing FLAG-tagged Clp1 to co-immunoprecipitate Pcf11 as a function of ATP
concentration. FLAG-Clp1 co-precipitated Pcf11 with or without ATP and the
complex was not affected by AMP-PCP. Diadenosine tetraphosphate (Ap4A), an ATP
analog that binds the Nudix domain of the CF Im25 subunit with higher affinity
than ATP, neither stimulated 3′ cleavage in place of ATP nor antagonized
ATP-stimulated 3′ cleavage. The ATP-binding site of PAP was disrupted by
site directed mutagenesis but a reconstituted 3′ cleavage reaction
containing a mutant PAP unable to bind ATP nevertheless underwent ATP-stimulated
3′ cleavage. Fluctuating ATP levels might contribute to the regulation
of pre-mRNA 3′ cleavage, but the three subunits investigated here do not
appear to be responsible for the ATP-stimulation of pre-mRNA cleavage.