Resistance of human nucleotide excision repair synthesis in vitro to p21Cdn1

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• p21Cdn1 (cip1/waf1/sdi1) protein inhibits cell proliferation by targeting cyclin-Cdk complexes.
• p21 also inhibits DNA replication by binding to PCNA, a key factor in DNA synthesis.
• DNA nucleotide excision repair (NER) is PCNA-dependent but continues despite p21 induction in damaged cells.

Purpose of the Study:

• Investigate the mechanisms of p21-resistant DNA repair, specifically NER.
• Determine how p21 interacts with PCNA during the NER pathway.
• Characterize the inhibitory effects of p21 on DNA repair synthesis in human cell extracts.

Main Methods:

• Utilized a direct end-labeling assay to quantify NER excision of damaged oligonucleotides.
• Assessed the impact of p21-PCNA binding on DNA gap filling by purified polymerases and cell extracts.
• Employed a synthetic p21 peptide mimicking PCNA binding to evaluate inhibition of NER synthesis.

Main Results:

• p21 did not inhibit the excision step of NER.
• Pre-binding p21 to PCNA inhibited gap filling by DNA polymerases delta and epsilon, particularly when uncoupled from incision.
• Inhibition of repair synthesis by p21 in cell extracts was reversible over time, suggesting dissociation from PCNA.
• p21 blocks polymerase association with PCNA but not PCNA loading onto DNA, enabling rapid repair synthesis upon p21 dissociation.

Conclusions:

• p21's inhibition of NER gap filling is dependent on its interaction with PCNA.
• The transient nature of p21-PCNA interaction in cell extracts allows for efficient DNA repair synthesis.
• A synthetic p21 peptide effectively inhibits NER synthesis, highlighting the importance of the p21-PCNA interaction in this process.