DNA binding mechanism of O6-alkylguanine-DNA alkyltransferase: stoichiometry and effects of DNA base composition and secondary structure on complex stability
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M G Fried1, S Kanugula, J L Bromberg
1Department of Biochemistry and Molecular Biology, Pennsylvania State University College of Medicine, Hershey 17033, USA. mfried@cor-mail.biochem.hmc.psu.edu
Abstract:
O6-Alkylguanine-DNA alkyltransferase (AGT) is an important cellular defense against the mutagenic effects of DNA alkylating agents. In humans this defense can contribute to the ability of some tumors to resist the effects of chemotherapeutic agents that act through DNA alkylation. We report here studies that characterize the interaction of AGT with DNA. We show that although AGT sediments as a monomer in the absence of DNA, it binds cooperatively to single stranded deoxyribonucleotides. The stoichiometries of complexes formed with 16-, 30-, and 80-base oligodeoxyribonucleotides are 3.8 +/- 0.3, 5.3 +/- 0.2, and 8.9 +/- 0.2, respectively; the binding density decreasing from approximately 4 nt/monomer to approximately 9 nt/monomer as DNA length increases over this range. Binding competition assays show that DNA affinities depend only weakly on base composition or secondary structure, although in general G + C-rich sequences are bound with greater affinity than are A + T-rich ones and single-stranded DNA is bound with greater affinity than duplex forms. These results suggest mechanisms by which AGT may search for alkylated sites and interact with them to effect DNA repair.
Insights
O6-Alkylguanine-DNA alkyltransferase (AGT) binds cooperatively to single-stranded DNA, aiding in DNA repair. This interaction is crucial for cellular defense against DNA alkylating agents and impacts tumor resistance to chemotherapy.
Area of Science:
- Molecular Biology
- Biochemistry
- Genetics
Background:
- O6-Alkylguanine-DNA alkyltransferase (AGT) is a key protein for cellular defense against DNA alkylating agents.
- AGT's role in DNA repair is critical, but its interaction with DNA is not fully understood.
- Tumor resistance to DNA alkylating chemotherapy can be mediated by AGT activity.
Purpose of the Study:
- To characterize the interaction between AGT and DNA.
- To elucidate the binding mechanisms and affinities of AGT to various DNA structures.
- To understand how AGT's DNA binding influences its DNA repair function.
Main Methods:
- Sedimentation analysis to determine AGT's quaternary structure in the absence of DNA.
- Electrophoretic mobility shift assays (EMSAs) or similar techniques to study AGT-DNA complex formation.
- Binding competition assays to assess DNA binding affinities based on sequence and structure.
Main Results:
- AGT binds cooperatively to single-stranded deoxyribonucleotides, forming complexes with varying stoichiometries.
- Binding density changes with DNA length, decreasing from approximately 4 nt/monomer to 9 nt/monomer for longer strands.
- DNA binding affinity is weakly dependent on base composition and secondary structure, favoring G+C-rich and single-stranded DNA.
Conclusions:
- AGT exhibits specific binding characteristics to single-stranded DNA, suggesting a mechanism for searching and interacting with alkylated DNA sites.
- The cooperative binding and affinity variations provide insights into AGT's DNA repair pathway.
- Understanding AGT-DNA interactions is vital for developing strategies to overcome chemotherapy resistance in tumors.