The puzzle of PCNA's many partners
Emma Warbrick
Summary
The identification of proteins that interact with proliferat-
ing cell nuclear antigen (PCNA) has recently been a
rapidly expanding field of discovery. PCNA is involved in
many aspects of DNA replication and processing, form-
ing a sliding platform that can mediate the interaction of
proteins with DNA. It is striking that many proteins bind to
PCNA through a small region containing a conserved
motif; these include proteins involved in cell cycle
regulation as well as those involved in DNA processing.
Sequential and regulated binding of motif-containing
proteins to PCNA may contribute to the ordering of events
during DNA replication and repair. Results from bacterio-
phages and archaea show that the structural basis for the
interaction of this motif with PCNA is extremely ancient.
The analysis of how such functional motifs have been
recruited to proteins in present day organisms helps us to
understand how these complex systems arose from
ancestral organisms. BioEssays 22:997±1006, 2000.
ß 2000 John Wiley & Sons, Inc.
Introduction
PCNA (proliferating cell nuclear antigen) was originally dis-
covered as an antigen found only in the nucleus of dividing
from which it derives its name.
(1)
It was independently iden-
tified as a protein with elevated levels during S phase of the cell
cycle.
(2)
A few years later, as a result of work to reconstitute
SV40-dependent replication in vitro, PCNA was identified as
an essential factor in DNA replication.
(3)
PCNA is now known
to be essential for the processivity of the DNA polymerase
complex, and also for the correct co-ordination of leading and
lagging strand synthesis.
(4)
Study of the crystal structure of
PCNA reveals a ring-shaped trimeric complex with striking
sixfold symmetry, which can encircle double-stranded DNA
and slide freely along it.
(5,6)
This structural information was
the vital clue to how PCNA could stably associate with DNA
without binding directly to it, and how it could link the poly-
merase complex to the DNA strand in a processive manner. It
is now known that, although PCNA has no endogenous enzy-
matic activity, it forms a sliding platform that can mediate the
interaction of several proteins with DNA in a non-sequence-
specific manner.
(7±9)
So-called ``sliding clamps'' are highly
conserved through evolution: homologues of PCNA are found
in eukaryotes, archaea, bacteriophages and some viruses.
The b-subunit of E. coli DNA polymerase III shares a very
similar three-dimensional structure with eukaryotic PCNA,
despite forming a dimeric, rather than trimeric, complex.
In the last few years, it has been shown that PCNA is
essential not only for DNA replication, but also for several
forms of DNA repair, including nucleotide excision repair
(NER), base excision repair (BER) and mismatch repair
(MMR).
(7,8)
More recently, a role for PCNA has been demon-
strated in various aspects of post-replicative processing, such
as cytosine methylation and chromatin assembly, although the
exact function of PCNA in these processes is not clear.
The identification of proteins that interact with PCNA has
been extensively studied recently.
(10)
As predicted, many
PCNA-interacting proteins are involved in various aspects of
DNA replication and processing, and hence these proteins are
probably using PCNA's ``sliding clamp'' properties to mediate
their interaction with DNA. It is intriguing, however, that cell
cycle regulatory proteins have also been found to interact with
PCNA. Such proteins, which include p21
WAF1/Cip1
, p57, cyclin
D, Gadd45 and Myd118 play multiple and, in many cases, not
completely understood, roles in the regulation of proliferation
and cell cycle progression. Although there is no clear evidence
yet, it seems likely that protein interactions with PCNA may be
a mechanism to co-ordinate DNA replication and repair with
the cell cycle.
One of the most interesting observations that has emerged
from the study of PCNA-binding proteins is that many contain
a conserved PCNA-binding motif (Table 1; Figure 1).
(11)
The PCNA-binding motif present in the regulatory protein p21
(also known as WAF1, Cip1, Sdi1 etc.) has been extensively
characterised, and its interactions with PCNA have been
mapped at the molecular level.
(6,12)
Biochemical evidence has
shown that other motif-containing proteins share a common
binding site on PCNA.
(13±17)
Although each PCNA trimer
theoretically has the capacity to interact with three such
proteins, the large number of motif-containing proteins far
exceeds this capacity, suggesting that the interactions of these
proteins with PCNA must be regulated. As described above,
this motif is also found in regulatory molecules, such as p21,
which have roles in regulating proliferation and in DNA
BioEssays 22:997±1006, ß 2000 John Wiley & Sons, Inc. BioEssays 22.11 997
Department of Surgery and Molecular Oncology, University of
Dundee, Ninewells Hospital and Medical School, Dundee DD1 9SY,
UK. E-mail: e.warbrick@dundee.ac.uk
Funding agency: The Association for International Cancer Research.
Abbreviations: ATP, adenosine triphosphate; BER, base excision
repair; CDK, cyclin-dependent kinase; CKII, casein kinase II; DNA
Mtase, DNA cytosine 5-methyltransferase; MMR, mismatch repair;
NER, nucleotide excision repair; PCNA, proliferating cell nuclear
antigen; RFC, replication factor C; RPA, replication protein A; UDG,
uracil DNA glycosylase.
Review articles
剩余9页未读，继续阅读
资源评论
