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Lindsay-Huggins

Page history last edited by LHuggins 14 years, 11 months ago

Proliferating cell nuclear antigen

-PCNA- 

 

 

     Proliferating cell nuclear antigen (PCNA) is a dynamic protein that was originally distinguished as the processivity factor for DNA polymerase delta and as an essential component for eukaryotic chromosomal DNA replication. Further studies have shown this sliding clamps ability to work and cooperate with many different proteins involved in several different metabolic pathways, which include DNA repair, translesion DNA synthesis, DNA methylation, chromatin assembly/remodeling and involved in the regulation of the cell cycle (Naryzhny 2008).  DNA sliding clamps are ring shaped proteins that encircles DNA forming a topological link not an actual sequence specific link to DNA guiding it along in it's different pathways. (Moldovan et al, 2007).

 

 

Replication                

     PCNA has no enzymatic activity, but is a central and essential factor for DNA replication acting as a moving platform to strengthen the process. At the start of replication, DNA unwinds and splits into two strands resembling a fork (replication fork). One strand goes from 5'-3' called the leading strand; the other strand goes 3'-5' and is called the lagging strand.  The ring shaped clamp encircles DNA and secures polymerases firmly to DNA. Stabilized by the PCNA sliding clamp, DNA polymerase delta or epsilon replicate the leading strand continuously in an error free manner. The lagging strand however, is replicated in a discontinuous fashion, copying small sections (Okazaki fragments). DNA ligase-I interacts with PCNA to seal the nicks between these fragments. Chilkova et al, 2007.  The PCNA ring like structure allows itself to wrap around DNA and slide along the DNA double-helical structure freely in both directions (Bell and Dutta, 2002)

 

 

Repair

     DNA repair is a process where many details work concurrent with PCNA involvement.  Different DNA repair pathways such as nucleotide excision repair (NER), base excision repair (BER), mismatch repair (MMR) and double-strand break repair (DSBR) all involve a DNA synthesis step requiring polymerase, which further indicates a function for PCNA. During NER it is suggested PCNA may be responsible for recruitting the essential protein to the site of the lesion. After the incision has been made, PCNA can promote the transition to the next steps by binding polymerase for re-synthesis of the complementary strand and also recruiting ligase-I for the final ligation step (Moldovan et al, 2007).

 


 

 

 

Functions:

 

     The function of PCNA involve heavy interaction with different protein partners.  Most of the PCNA binding proteins contain the consensus PCNA binding motif termed PIP (PCNA interacting protein) box (Vivona and Kelman, 2003).  The sequence "Qxxhxxaa" aligns with other proteins invoking interaction.  In the sequence 'x' represents any amino acid, 'h' represents residues with hydrophobic side chains, 'a' represents a highly hydrophobic side chain and 'x' is any residue (Vivona and Kelman, 2003). The presence of this binding-sequence has been found on many unrelated proteins explaining the presence of PCNA involved in various functions. In fact this shows a single protein can be used in various pathways to activate different functions (Hingorani and O'Donnell 2000)

 

 

-This is a table that gives examples of how extent protein interactions exists with PCNA. Retrieved from Vivona and  Kelman, 2003

 


 

Structure:

 

                                                    A front and side view of PCNA. Also included a model of a double trimer PCNA (far right).  Image found in Naryzhny, 2008.

 

 

 

.    PCNA clamps are bound onto DNA by other proteins, in an ATP dependent process. The structure is highly symmetrical, containing three monomers with almost identical topology (Krishna et al, 1994). PCNA has an overall negative charge, the inner surface is found to be positively charged due to the presence of many lysine and arginine residues (Naryzhny 2008).  This helps explain how the negatively charged DNA can pass through the ring without repulsions (Hingorani and O'Donnell, 2000) . The PCNA has a distinct front and back side . PCNA has been found to only interact with different proteins using the front side (see image above) (Naryzhny 2008).

     The DNA polymerase delta and epsilon consist of three components: an enzymatic core that carries out polymerization and proof-reading, a DNA clamp (Beta subunit in prokaryotics and PCNA in eukaryotic cells), and a set of ATP- dependent proteins that are required to load PCNA onto DNA (RFC complex in eukaryotic cells)(Krishna et al, 1994). This RFC complex is an arc-shaped structure that joins with PCNA to allow the loading onto the template-primer 3' end (Moldovan et al, 2007). The direction of loading is an important feature to avoid mistakes leading to carcenogenic products.

 

 

 


 

 

Human Disease:

 

     It only makes sense for a protein such as PCNA, which is involved in several aspects of the cell cycle and cell life to inevitably in some way be associated with cancer. As cells age, the PCNA levels don't show a drastic change in numbers.  But it has been found that cancer often correlates with an enhanced level of PCNA (Naryzhny, 2008).  PCNA levels have also been used as marker towards cancers.  Naryzhny, 2008 showed that with a cell culture analysis of cancerous cells, there were significant finding of higher levels of PCNA than in normal cells. Malkas et al, 2006 have found two isoforms of PCNA in breast cancer cells; one in which they term caPCNA could serve as malignant breast cancer marker. Malkas et al, 2006 also determined that the malignant forming caPCNA is not linked to a genetic modification of the actual sliding clamp ring, but as a potranscriptional alteration.

     A recent study also declared that PCNA may be involved in transcription of the Eukaryotic cell; which is associated with the interaction and regulation of remodeling factors (Naryzhny, 2008).  If PCNA is involved with this process, then Williams Syndrome, a syndrome caused by the deletion of a set of chromosomes, could ultimatley be a result of PCNA and it's different protein interactions Naryzhny 2008.

 

 

 

 

 

 

 

 

 

 

Comments (1)

Christopher Korey said

at 10:07 pm on Apr 6, 2009

NIce outline so far. I would include the second of the two fork images and get rid of the first. I might also just include the clamp and not the image with the loader and the clamp. When translating the outline to the text--make short concise paragraphs that have link outs to more detailed information. You want the user to look for the more detailed stuff if they want it. Subdivide each section with an inserted horizontal bar. Also make sure to reference in the text with footnotes, and if you are going to link out the references, use the insert link tool to make the reference a link rather than pasting in the url to the references.

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