ily. The A-subunit of PP2A is a HEAT repeat protein (36). The
FYF motif in PR70 resembles the FG amino acid repeats (FXFG
and GLFG) within the nucleoporin family of nuclear pore pro-
teins. The nucleoporins interact with nuclear transport factors,
including importin-
, which are also HEAT repeat proteins.
The FG repeats of nucleoporins bind to shallow hydrophobic
pockets in importin-
 (37, 38). The A-subunit of PP2A contains
exposed hydrophobic surfaces, predicted to play a role in inter-
action with the regulatory subunits (36), that are possible sites
of interaction with the FYF motif of PR70.
The requirement for the N-terminal region of the R3 domain
of PR70 for binding to the A-subunit is distinct from results
observed with the PR72 protein. A fragment of PR72 consisting
of amino acids 219 – 473 (corresponding to residues 257–509 of
PR70) interacts with the A-subunit in the yeast two-hybrid
assay (27). This fragment of PR72 is missing the N-terminal
region of the R3 domain. Two fragments of PR72 containing
putative A-subunit binding domains prepared by in vitro trans-
lation (corresponding to residues 234 –339 and 378 – 436 of
PR70) interacted with the A-subunit in vitro using GST pull-
down assays (39). These PR72 fragments also do not contain the
conserved N-terminal region of the R3 domain that was neces-
sary for interaction of PR70 with the AC core dimer in our
assays. These observations indicate that additional regions of
PR70, beyond those required in PR72, are required for binding
to the A-subunit, or that the differences observed are due to
different assays employed.
In summary, the present study shows that the PR70 regula-
tory subunit targets PP2A to Cdc6 and that PP2A is likely to be
a physiological Cdc6 phosphatase. The targeting of PP2A to
Cdc6 is enhanced by binding of calcium to PR70 raising the
possibility that changes in intracellular calcium can influence
formation of pre-replicative complexes through regulation of
Cdc6 dephosphorylation.
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PR70 Targets PP2A to Cdc6
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JOURNAL OF BIOLOGICAL CHEMISTRY
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