This is the large domain insertion of ORC subunit 3 (ORC3) located between the N-terminal AAA+-like subunit and the C-terminal winged- helix (WH) DNA-binding domain [1]. ORC3 is a component of the origin recognition complex (ORC) that binds origins of replication, required to assemble the pre-replication complex necessary to initiate DNA replication. This domain insertion forms a bi-lobed alpha-helical protuberance that engages a short alpha-helix formed at the ORC6 C-terminal, allowing its binding to ORC1-5 subunits [1], which constitute the ORC core [1,2].

1: Crystal structure of the eukaryotic origin recognition complex. Bleichert F, Botchan MR, Berger JM; Nature. 2015;519:321-326. PMID:25762138

2: Structure of the active form of human origin recognition complex and its ATPase motor module. Tocilj A, On KF, Yuan Z, Sun J, Elkayam E, Li H, Stillman B, Joshua-Tor L; Elife. 2017; [Epub ahead of print] PMID:28112645