Dissection of the xeroderma pigmentosum group C protein function by site-directed mutagenesis

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Xeroderma pigmentosum group C (XPC) protein functions as a key sensor in the global genome repair (GGR) pathway.
• XPC detects helix-distorting DNA lesions by recognizing the single-stranded character of bases opposite the damage.
• Structural studies of its yeast homologue (Rad4) identified critical residues involved in nucleotide interaction.

Purpose of the Study:

• To investigate the functional roles of conserved residues (N754, F756, F797, F799) in human XPC protein.
• To elucidate the dynamic interplay between XPC and DNA during the initiation of GGR.
• To understand the mechanism of XPC's intranuclear trafficking and DNA damage search.

Main Methods:

• Site-directed mutagenesis of conserved residues in human XPC.
• Assays for DNA-binding activity and lesion recognition.
• Analysis of XPC accumulation in DNA lesion tracks and foci.
• Measurement of nuclear protein mobility and GGR induction.

Main Results:

• Mutagenesis revealed that XPC's association with one displaced nucleotide initiates lesion site encounter.
• A subsequent flipping-out of an adjacent nucleotide is required for handing over the damaged site to downstream repair factors.
• Intranuclear trafficking of XPC relies on constitutive interactions with undamaged DNA, suggesting facilitated diffusion for damage searching.

Conclusions:

• The study reveals a dynamic, multi-step DNA interaction model for XPC in GGR initiation.
• XPC's facilitated diffusion mechanism allows efficient searching for DNA base damage within the nucleus.
• Conserved residues are critical for both DNA lesion recognition and the dynamic cellular localization of XPC.