The human Rad52 protein exists as a heptameric ring

The RAD52 epistasis group was identified in yeast as a group of genes requi red to repair DNA damaged by ionizing radiation [1]. Genetic evidence indic ates that Rad52 functions in Rad51-dependent and Rad51-independent recombin ation pathways [2-4]. Consistent with this, purified yeast and human Rad52 proteins have been shown to promote single-strand DNA annealing [5-7] and t o stimulate Rad51-mediated homologous pairing [8-11]. Electron microscopic examinations of the yeast [12] and human [13] Rad52 proteins have revealed their assembly into ring-like structures in vitro. Using both conventional transmission electron microscopy and scanning transmission electron microsc opy (STEM), we found that the human Rad52 protein forms heptameric rings. A three-dimensional (3D) reconstruction revealed that the heptamer has a lar ge central channel. Like the hexameric helicases such as Escherichia coli D naB [14,15], bacteriophage T7 gp4b [16,17], simian virus 40 (SV40) large T antigen [18] and papilloma virus E1 [19], the Rad52 rings show a distinctly chiral arrangement of subunits. Thus, the structures formed by the hexamer ic helicases may be a more general property of other proteins involved in D NA metabolism, including those, such as Rad52, that do not bind and hydroly se ATP.