Human RAD51 protein (HsRad51)-promoted DNA strand exchange, a crucial step in homologous recombination, is regulated by proteins and calcium ions. The activator protein Swi5-Sfr1 and Ca2+ ions stimulate different reaction steps and induce a perpendicular orientation of DNA bases in the presynaptic complex. To investigate the importance of base orientation in the strand exchange reaction, we examined the Ca2+ concentration dependence of strand exchange activities and structural changes in the presynaptic complex. Our results show that optimal D-loop formation (strand exchange with closed circular DNA) requires Ca2+ concentrations greater than 5 mM, while 1 mM is sufficient for strand exchange between two oligonucleotides. The structural change, which is evidenced by an increase in fluorescence intensity of poly(dεA) (a poly(dA) analog), reaches a plateau at 1 mM Ca2+. Meanwhile, the linear dichroism signal intensity at 260 nm, which is indicative of rigid perpendicular DNA base orientation, requires >2 mM Ca2+ for saturation and thus correlates with the stimulation of D-loop formation. Therefore, Ca2+ exerts two different effects. Thermal stability measurements suggest that HsRad51 binds two Ca2+ ions with KD values of 0.3 mM and 2.5 mM, implying that one step is stimulated by one Ca2+ bond and the other by two Ca2+ bonds. We further discuss the parallels between Mg2+ activation of RecA and Ca2+ activation of HsRad51.

Most pivotal clinical trials in advanced non-small cell lung cancer (NSCLC) have excluded patients with poor performance status (PS), and data on the efficacy and safety of pharmacotherapy have not been fully accumulated. For NSCLC patients with PS 2 and without druggable genetic alterations, monotherapy with cytotoxic agents or carboplatin-based combination therapy is usually administered based on the results of several randomized trials. However, the evidence of cytotoxic chemotherapy for patients with PS 2 is insufficient, with limited efficacy and toxicity concerns. Immune checkpoint inhibitors (ICIs) are a promising treatment for patients with PS 2 because of lower incidence of severe toxicity compared to cytotoxic chemotherapy. Meanwhile, several reports suggest that anti-PD-1 antibodies monotherapy is less effective for patients with PS 2, especially for those with PS 2 caused by disease burden. Although the combination therapy of nivolumab and ipilimumab is a promising treatment option, there is a divergence in efficacy data between clinical trials. The standard of care for advanced NSCLC with PS 2 has not been established, and future therapeutic strategies should take into account the heterogeneity of the PS 2 population.