NMR structure verifies the eponymous degenerate zinc finger domain of transcription factor ZNF750

ZNF750 is a nuclear transcription factor that activates skin differentiation and has tumor suppressor roles in several cancers. Unusually, ZNF750 has only a single zinc-finger (ZNF) domain, Z*, with an amino acid sequence that differs markedly from the CCHH family consensus. Because of its sequence differences Z* is classified as degenerate, presumed to have lost the ability to bind the zinc ion required for folding. AlphaFold predicts an irregular structure for Z* with low confidence. Low confidence predictions are often inferred to be intrinsically disordered regions of proteins, which would be the case if Z* did not bind Zn2+. We use NMR and CD spectroscopy to show that a 25-51 segment of ZNF750, corresponding to the Z* domain, folds into a well-defined antiparallel {beta}{beta} tertiary structure with a pM dissociation constant for Zn2+, and a thermal stability >80 {degrees}C. Of three alternative Zn2+ ligand sets, Z* uses a CCHC rather than the expected CCHH motif. The switch in the last ligand maintains the folding topology and hydrophobic core of the classical ZNF motif. CCHC ZNFs are associated with protein-protein interactions but Z* binds DNA. Since the metal chelating site is on the other side of the molecule, it suggests functional preferences are a result of divergent evolution rather than physical constraints on the structure. The structure of Z* provides a context for understanding the domains DNA-binding properties and mutations associated with cancers. We expect other ZNFs currently classified as degenerate, are CCHC-type structures like Z*.