USF9 has the same zinc ligation topology as USF1 but the two fingers share only 14% sequence identity outside of the zinc ligands (blue). However, USF9 adopts an almost identical fold to USF1, with a short β-hairpin (orange) and an α-helix. Furthermore, USF9 is also able to interact with the N-terminal finger of GATA-1.
This non-native structure corresponds to a fragment of the CH1 domain of CBP. Although the CH1 domain in its native form binds 3 zinc ions (Dames et al, PNAS 2002, 99, 5271-5276), this shorter domain binds only one zinc ion, through a different combination of zinc ligating residues. This novel zinc-binding fold is quite remarkable, given that a sequence, taken out of the context of a whole domain, is able to fold into a stable, yet different structure.