Functional role of BLAP75 in BLM-topoisomerase IIIα-dependent Holliday junction processing

The BLAP75 protein combines with the BLM helicase and topoisomerase (Topo) III alpha to form an evolutionarily conserved complex, termed the BTB complex, that functions to regulate homologous recombination. BLAP75 binds DNA, associates with both BLM and Topo III alpha, and enhances the ability of the BLM-Topo III alpha pair to branch migrate the Holliday junction (HJ) or dissolve the double Holliday junction (dHJ) structure to yield non-crossover recombinants. Here we seek to understand the relevance of the biochemical attributes of BLAP75 in HJ processing. With the use of a series of BLAP75 protein fragments, we show that the evolutionarily conserved N-terminal third of BLAP75 mediates complex formation with BLM and Topo III alpha and that the DNA binding activity resides in the C-terminal third of this novel protein. Interestingly, the N-terminal third of BLAP75 is just as adept as the full-length protein in the promotion of dHJ dissolution and HJ unwinding by BLM-Topo III alpha. Thus, the BLAP75 DNA binding activity is dispensable for the ability of the BTB complex to process the HJ in vitro. Lastly, we show that a BLAP75 point mutant (K166A), defective in Topo III alpha interaction, is unable to promote dHJ dissolution and HJ unwinding by BLM-Topo III alpha. This result provides proof that the functional integrity of the BTB complex is contingent upon the interaction of BLAP75 with Topo III alpha.