Proliferating cell nuclear antigen

Proliferating cell nuclear antigen (PCNA) is a is_associated_with::DNA clamp that acts as a is_associated_with::processivity factor for DNA polymerase δ in is_associated_with::eukaryotic cells and is essential for replication. PCNA is a homotrimer and achieves its processivity by encircling the DNA, where it acts as a scaffold to recruit proteins involved in DNA replication, DNA repair, chromatin remodeling and epigenetics.

Many proteins interact with PCNA via the two known PCNA-interacting motifs PCNA-interacting peptide (PIP) box and AlkB homologue 2 PCNA interacting motif (APIM). Proteins binding to PCNA via the PIP-box are mainly involved in DNA replication whereas proteins binding to PCNA via APIM are mainly important in the context of genotoxic stress.

Function
The protein encoded by this gene is found in the nucleus and is a cofactor of DNA polymerase delta. The encoded protein acts as a homotrimer and helps increase the processivity of leading strand synthesis during DNA replication. In response to DNA damage, this protein is is_associated_with::ubiquitinated and is involved in the RAD6-dependent DNA repair pathway. Two transcript variants encoding the same protein have been found for this gene. is_associated_with::Pseudogenes of this gene have been described on chromosome 4 and on the X chromosome.

Expression in the nucleus during DNA synthesis
PCNA was originally identified as an is_associated_with::antigen that is expressed in the nuclei of cells during the DNA synthesis phase of the is_associated_with::cell cycle. Part of the protein was sequenced and that sequence was used to allow isolation of a is_associated_with::cDNA clone. PCNA helps hold is_associated_with::DNA polymerase epsilon (Pol ε) to DNA. PCNA is clamped to is_associated_with::DNA through the action of is_associated_with::replication factor C (RFC), which is a heteropentameric member of the is_associated_with::AAA+ class of ATPases. Expression of PCNA is under the control of is_associated_with::E2F is_associated_with::transcription factor-containing complexes.

Role in DNA repair
Since DNA polymerase epsilon is involved in resynthesis of excised damaged DNA strands during is_associated_with::DNA repair, PCNA is important for both DNA synthesis and DNA repair.

PCNA is also involved in the DNA damage tolerance pathway known as post-replication repair (PRR). In PRR, there are two sub-pathways: (1) a translesion pathway, which is carried out by specialised DNA polymerases that are able to incorporate damaged DNA bases into their active sites (unlike the normal replicative polymerase, which stall), and hence bypass the damage, and (2) a proposed "template switch" pathway that is thought to involve damage bypass by recruitment of the homologous recombination machinery. PCNA is pivotal to the activation of these pathways and the choice as to which pathway is utilised by the cell. PCNA becomes post-translationally modified by is_associated_with::ubiquitin. Mono-ubiquitin of lysine number 164 on PCNA activates the translesion synthesis pathway. Extension of this mono-ubiquitin by a non-canonical lysine-63-linked poly-ubiquitin chain on PCNA is thought to activate the template switch pathway. Furthermore, sumoylation (by small ubiquitin-like modifier, SUMO) of PCNA lysine-164 (and to a lesser extent, lysine-127) inhibits the template switch pathway. This antagonistic effect occurs because sumoylated PCNA recruits a DNA helicase called Srs2, which has a role in disrupting Rad51 nucleoprotein filaments fundamental for initiation of homologous recombination.

PCNA-binding proteins
• TCP protein domain  •  NKp44 Receptor  •  procaspases  •  is_associated_with::DNA polymerases  •  Clamp loader  •  is_associated_with::Flap endonuclease  •  is_associated_with::DNA ligase  •  is_associated_with::Topoisomerase  •  is_associated_with::Replication licensing factor  •  E3 is_associated_with::ubiquitin ligases  •  E2 SUMO-conjugating enzyme  •  is_associated_with::Helicases, is_associated_with::ATPases  •  is_associated_with::Mismatch repair enzymes  •  is_associated_with::Base excision repair enzymes  •  is_associated_with::Nucleotide excision repair enzyme  •  is_associated_with::Poly ADP ribose polymerase  •  is_associated_with::Histone chaperone  •  Chromatin remodeling factor  •  is_associated_with::Histone acetyltransferase  •  is_associated_with::Histone deacetyltransferase  •  is_associated_with::DNA methyltransferase  •  Sister-chromatid cohesion factors  •  is_associated_with::Protein kinases  •  Cell-cycle regulators  •  Apoptotic factors

for details see

Interactions
PCNA has been shown to interact with:


 * is_associated_with::Annexin A2,
 * is_associated_with::CDC25C,
 * is_associated_with::CHTF18,
 * is_associated_with::Cyclin D1,
 * is_associated_with::Cyclin O,
 * is_associated_with::Cyclin-dependent kinase 4,
 * is_associated_with::Cyclin-dependent kinase inhibitor 1C,
 * is_associated_with::DNMT1,
 * is_associated_with::EP300,
 * is_associated_with::Flap structure-specific endonuclease 1,
 * is_associated_with::GADD45A,
 * is_associated_with::GADD45G,
 * is_associated_with::HDAC1,
 * is_associated_with::HUS1,
 * is_associated_with::ING1,
 * is_associated_with::KCTD13,
 * is_associated_with::KIAA0101,
 * is_associated_with::Ku70,
 * is_associated_with::Ku80,
 * is_associated_with::MCL1,
 * is_associated_with::MSH3,
 * is_associated_with::MSH6,
 * is_associated_with::MUTYH,
 * is_associated_with::P21,
 * is_associated_with::POLD2,
 * is_associated_with::POLD3,
 * is_associated_with::POLDIP2,
 * is_associated_with::POLH,
 * is_associated_with::POLL,
 * is_associated_with::RFC1,
 * is_associated_with::RFC2,
 * is_associated_with::RFC3,
 * is_associated_with::RFC4,
 * is_associated_with::RFC5,
 * is_associated_with::Ubiquitin C
 * is_associated_with::Werner syndrome ATP-dependent helicase,
 * is_associated_with::XRCC1, and
 * is_associated_with::Y box binding protein 1.

Proteins interacting with PCNA via APIM include human AlkB homologue 2, TFIIS-L, TFII-I, Rad51B, XPA, ZRANB3, and FBH1.

Uses
Antibodies against proliferating cell nuclear antigen (PCNA) or is_associated_with::monoclonal antibody termed Ki-67 can be used for grading of different is_associated_with::neoplasms, e.g. is_associated_with::astrocytoma. They can be of diagnostic and prognostic value. Imaging of the nuclear distribution of PCNA (via antibody labeling) can be used to distinguish between early, mid and late is_associated_with::S phase of the cell cycle. However, an important limitation of antibodies is that cells need to be fixed leading to potential artifacts.

On the other hand, the study of the dynamics of replication and repair in living cells can be done introducing translational fusions of PCNA. To obviate the need for transfection and bypass the problem of difficult to transfect and/or short lived cells, cell permeable replication and/or repair markers can be used. These peptides offer the distinct advantage that can be used in situ in living tissue and even distinguish cells undergoing replication from cells undergoing repair.

PCNA is a potential therapeutic target in cancer therapy.