U-shaped is a member of the FOG family of transcription factors, which interact with GATA proteins to control gene expression in various tissues. U-shaped has nine zinc fingers. The structure of the first finger of U-shaped (USF1) is very similar to that of a classical zinc finger, consisting of a short β-hairpin (yellow) and an α-helix. The central zinc atom (green) is ligated by 1 His and 3 Cys residues (blue), in contrast to classical fingers, which have a CCHH zinc ligation topology. While many classical fingers bind to DNA, USF1 has been shown to interact with the N-terminal zinc 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.