The maintenance of genomic stability is crucial
in a cell and during DNA replication the Proliferating Cell Nuclear Antigen
(PCNA) has an important role in this maintenance.
PCNA is a sliding clamp protein located in the
nucleus and is a ring-shaped molecule that encircles DNA acting as a polymerase
clamp and a sliding platform for the recruitment of other proteins, like DNA
helicase, nuclease and more. This protein is also required on the lagging
strand where PCNA interacts with DNA polymerase to synthesize the Okazaki
fragments where PCNA must be loaded repeatedly to continue DNA replication.
The loading of PCNA depends on the ATP-dependent
Replication Factor C (RFC). The RFC is responsible for the loading and unloading of PCNA on double-stranded DNA by nicking a 3′ primer
template junction and it is involved in DNA replication, genome integrity,
homologous recombination-mediated repair and is required for PCNA unloading
during DNA replication. The RFC is composed by 5 subunits, Rfc1-5 but the Rfc1
subunit can be substituted by Elg1 in yeast or by ATAD5, the homolog in humans,
forming an RFC-like complex (RLCs) (Fox et.
al., 2012; Kubota et. al., 2013).
It is known that one of the possible functions
of Elg1-RLC is the unloading of PCNA during DNA replication, a function that
appears to be conserved in humans. ATAD5 is also necessary for the removal of
PCNA from chromatin in Gallus gallus and for the maintenance of a correct
regulation of DNA replication. In mice, ATAD5 inactivation results in embryonic
lethality and it is reported that ATAD5 knockdown extends DNA replication
lifespan and it can cause genomic instability and predisposition to cancer (Lee
Usually, mutations in different genes involved in
the ubiquitylation and deubiquitylation are associated with many types of
cancer, neurodegeneration and metabolic disorders (Choo et al., 2009). The PCNA is an example of a protein that has
posttranslational modifications like ubiquitylation that are fundamental for
its function. PCNA monoubiquitylation is involved in DNA damage bypass by
activating DNA damage repair pathway like the translesion synthesis (TLS) by
recruiting TLS polymerases able to surpass an error during DNA replication.
When the PCNA is polyubiquitylated it activates another pathway called template
switching, an error-free pathway. In parallel, PCNA deubiquitylation is also
important to control the levels of mutagenesis in a cell. The UAS1/USP1 complex
is responsible for the deubiquitylation of monoubiquitinated PCNA controlling
the recruitment of the error-prone polymerases in the TLS mechanism. In the
literature, it is also found that ATAD5 interacts with the UAS1/USP1 complex
indicating that, perhaps, ATAD5 has a role in PCNA deubiquitylation. The PCNA
can also be SUMOylated, another posttranslational modification, during S phase
in the absence of DNA damage (Fox et. al., 2012). Therefore, we would not investigate the SUMOylated
PCNA and in the literature, is also described that unloading of SUMOylated PCNA
does not interact with the RLCs composed of ATAD5, but only the ubiquitinated
PCNA, the form related with DNA damage repair pathways (Kubota et. al., 2013). The process of PCNA unloading is not fully known.
Therefore, in this report, we are proposing that the role of ATAD5 protein and
its ability of unloading PCNA from the DNA and a probable function in DNA
damage repair happens by affecting the UAF1-USP1 complex.