Isolation and Characterization of Monomeric Human RAD51: A Novel Tool for Investigating Homologous Recombination in Cancer

DNA repair protein RAD51 is a key player in the homologous recombination pathway. Upon DNA damage, RAD51 is transported into the nucleus by BRCA2, where it can repair DNA double-strand breaks. Due to the structural complexity and dynamics, researchers have not yet clarified the mechanistic details of every step of RAD51 recruitment and DNA repair. RAD51 possesses an intrinsic tendency to form oligomeric structures, which make it challenging to conduct biochemical and biophysical investigations. Here, for the first time, we report on the isolation and characterization of a human monomeric RAD51 recombinant form, obtained through a double mutation, which preserves the protein's integrity and functionality. We investigated different buffers to identify the most suitable condition needed to definitively stabilize the monomer. The monomer of human RAD51 provides the community with a unique biological tool for investigating RAD51-mediated homologous recombination, and paves the way for more reliable structural, mechanistic, and drug discovery studies.

In Vitro Assays for DNA Branch Migration

Homologous recombination is a high-fidelity DNA double-strand break repair pathway that uses a homologous template to repair the break. Recombinases are the central enzymes that facilitate the strand invasion step of homologous recombination, which forms a DNA joint molecule. These DNA joint molecules can be moved through branch migration activity. In this chapter, we describe two assays to determine the branch migration activity and directionality of an enzyme. Monitoring the branch migration activity of an enzyme can provide insight into the roles of these factors in homologous recombination.

Homologous recombination properties of OsRad51, a recombinase from rice

cDNA corresponding to OsRad51 protein was isolated from cDNA library of rice flowers (Oryza sativa, Indica cultivar group) and cloned in to pET28a expression vector. The protein was over expressed in E. coli BL21 (DE3) and purified. Purified OsRad51 could bind single and double stranded DNA, however it showed higher affinity for single stranded DNA. Transmission Electron Microscopy (TEM) studies of OsRad51-DNA complexes showed that this protein formed ring like structures and bound DNA forming filaments. OsRad51 protein promoted renaturation of complementary single strands in to duplex DNA molecules and also showed ATPase activity, which was stimulated by single strand DNA. Fluorescence resonance energy transfer (FRET) assays revealed that OsRad51 promoted homology dependent renaturation as well as strand exchange reactions. Renaturation activity was ATP dependent; however strand exchange activity was ATP independent. This is the first report on in vitro characterization of Rad51 protein from crop plants.

Rad51 and Rad54 ATPase activities are both required to modulate Rad51-dsDNA filament dynamics

Rad51 and Rad54 are key proteins that collaborate during homologous recombination. Rad51 forms a presynaptic filament with ATP and ssDNA active in homology search and DNA strand exchange, but the precise role of its ATPase activity is poorly understood. Rad54 is an ATP-dependent dsDNA motor protein that can dissociate Rad51 from dsDNA, the product complex of DNA strand exchange. Kinetic analysis of the budding yeast proteins revealed that the catalytic efficiency of the Rad54 ATPase was stimulated by partial filaments of wild-type and Rad51-K191R mutant protein on dsDNA, unambiguously demonstrating that the Rad54 ATPase activity is stimulated under these conditions. Experiments with Rad51-K191R as well as with wild-type Rad51-dsDNA filaments formed in the presence of ATP, ADP or ATP-γ-S showed that efficient Rad51 turnover from dsDNA requires both the Rad51 ATPase and the Rad54 ATPase activities. The results with Rad51-K191R mutant protein also revealed an unexpected defect in binding to DNA. Once formed, Rad51-K191R-DNA filaments appeared normal upon electron microscopic inspection, but displayed significantly increased stability. These biochemical defects in the Rad51-K191R protein could lead to deficiencies in presynapsis (filament formation) and postsynapsis (filament disassembly) in vivo.

Purification and assays of Saccharomyces cerevisiae homologous recombination proteins

Homologous recombination is an important means of eliminating DNA double strand breaks from chromosomes. The homologous recombination reaction is mediated by the Rad51 recombinase, which requires a number of ancillary factors for maximal efficiency. The development of purification procedures and biochemical assays for yeast Rad51 and other yeast recombination proteins has allowed investigators to begin dissecting the hierarchy of physical and functional interactions among these protein factors that govern the integrity of the homologous recombination machinery. The biochemical studies done with yeast recombination factors have helped formulate conceptual frameworks to guide similar endeavors in other eukaryotes, including humans. Continuing efforts with reconstituted systems that comprise yeast factors will undoubtedly continue to provide insights into the mechanistic intricacy of the homologous recombination machinery.