The endonuclease subdomain, found in the prokaryotic protein ribonucleotide reductase, assumes an alpha-beta-beta-alpha-beta-beta-alpha-alpha topology. The four stranded beta-sheet forms a saddle-shaped surface and assembles together through an interface made of alpha-helices. The presence of 14 basic residues on the surface of the beta-sheets suggests that this large groove may be involved in DNA binding [1].

1: Crystal structure of an archaeal intein-encoded homing endonuclease PI-PfuI. Ichiyanagi K, Ishino Y, Ariyoshi M, Komori K, Morikawa K; J Mol Biol. 2000;300:889-901. PMID:10891276