Hairpin coding end opening is mediated by RAG1 and RAG2 proteins

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Antigen receptor gene assembly is crucial for adaptive immunity.
• The process involves the opening of DNA hairpins, but the enzymes responsible were previously unknown.
• Understanding this mechanism is key to comprehending immune system development.

Purpose of the Study:

• To identify the molecular machinery that mediates DNA hairpin opening during V(D)J recombination.
• To elucidate the mechanism and regulation of hairpin opening by identified enzymes.
• To determine if hairpin opening is linked to other DNA cleavage reactions mediated by the same enzymes.

Main Methods:

• Biochemical assays to test the activity of purified RAG1 and RAG2 proteins.
• Site-directed mutagenesis of RAG1 and RAG2 to assess the impact on DNA cleavage and hairpin opening.
• Analysis of reaction products to determine the chemical mechanism involved.

Main Results:

• Recombination-activating genes (RAG1 and RAG2) were shown to directly catalyze DNA hairpin opening.
• Hairpin opening is not sequence-specific but requires a regulated cleavage complex in the presence of Mg2+.
• The chemical mechanism of hairpin opening is similar to RSS cleavage and 3' end processing, proceeding via alcoholysis.
• Mutations affecting RSS cleavage also impaired hairpin opening, indicating a shared active site.

Conclusions:

• RAG1 and RAG2 are the key enzymes responsible for DNA hairpin opening in V(D)J recombination.
• RAGs possess a single active site capable of catalyzing multiple DNA cleavage reactions, including hairpin opening.
• This finding provides a unified view of the RAG enzymatic function in adaptive immunity.