Acetylation/Deacetylation Modulates the Stability of DNA Replication
Licensing Factor
Cdt1 |
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Authors: | Michele A Glozak and Edward Seto |
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Institution: | Department of Molecular Oncology, H. Lee Moffitt Cancer Center and Research Institute, Tampa, Florida 33612 |
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Abstract: | Proper expression of the replication licensing factor Cdt1 is primarily
regulated post-translationally by ubiquitylation and proteasome degradation.
In a screen to identify novel non-histone targets of histone deacetylases
(HDACs), we found Cdt1 as a binding partner for HDAC11. Cdt1 associates
specifically and directly with HDAC11. We show that Cdt1 undergoes acetylation
and is reversibly deacetylated by HDAC11. In vitro, Cdt1 can be
acetylated at its N terminus by the lysine acetyltransferases KAT2B and KAT3B.
Acetylation protects Cdt1 from ubiquitylation and subsequent proteasomal
degradation. These results extend the list of non-histone acetylated proteins
to include a critical DNA replication factor and provide an additional level
of complexity to the regulation of Cdt1.To maintain genomic integrity, DNA replication must be tightly controlled
to ensure that each portion of the genome replicates once and only once per
cell cycle (reviewed in Ref.
1). Replication licensing
begins by the formation of the prereplication complex at multiple potential
origins of replication. This is established sequentially, with the origin
recognition complex
(ORC)2 proteins
binding first, followed by the recruitment of Cdc6 and Cdt1, which in turn
recruit the MCM2–7 proteins. MCM proteins act as the replicative
helicase. The licensed replication origins are activated by cyclin-dependent
kinases at the start of S phase. Licensing occurs throughout the cell cycle
once S phase is complete.Cdt1 levels fluctuate throughout the cell cycle. It is destabilized at
G1/S transition, and then levels begin to climb again upon S phase
completion. To prevent licensing at inappropriate times, two separate
processes regulate the inactivation or destruction of Cdt1. First, geminin
negatively regulates Cdt1 function by prevention of the association of Cdt1
with MCM2–7 via steric hindrance
(2). Interestingly, geminin
also positively regulates Cdt1 by preventing its ubiquitylation, perhaps by
prevention of its interaction with an E3 ligase. This allows Cdt1 to
accumulate in G2 and M phases, to ensure adequate pools of Cdt1 to
license the next cycle of replication
(3). The ratio of geminin to
Cdt1 likely determines whether geminin positively or negatively regulates Cdt1
(4). Second, Cdt1 is targeted
for proteolysis by two distinct ubiquitin E3 ligases: the SCF-Skp2 complex and
the DDB1-Cul4 complex (5).
Phosphorylation by cyclin A/Cdk2 promotes interaction of Cdt1 with Skp2,
leading to Cdt1 degradation during S phase
(6–8).
In addition, DDB1-Cul4 utilizes proliferating cell nuclear antigen as a
binding platform to contact Cdt1, targeting the destruction of Cdt1 in S phase
or following DNA damage (9,
10). Ubiquitylation by either
of these E3 ligases promotes degradation of Cdt1 by the proteasome.Ubiquitylation occurs primarily (but not exclusively) on the ε-amino
group of lysine residues. Another prominent post-translational modification
that occurs on that residue is acetylation. Acetylation and, correspondingly,
deacetylation can modulate the function and activity of a variety of proteins
(see Ref. 11 for review).
Here, we report that Cdt1 physically interacts with HDAC11, a class IV histone
deacetylase (12,
13), as well as with several
lysine acetyltransferases (KATs). We show that Cdt1 is an acetylated protein
and further show that acetylation protects Cdt1 from ubiquitylation and
subsequent proteasomal degradation. This study uncovers yet another layer of
complexity to the regulation of the critical licensing factor Cdt1. |
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