Solution Structures of Two CCHC Zinc Fingers from the FOG Family Protein U-Shaped that Mediate Protein–Protein Interactions

Abstract

Background: Zinc finger domains have traditionally been regarded as sequence-specific DNA binding motifs. However, recent evidence indicates that many zinc fingers mediate specific protein–protein interactions. For instance, several zinc fingers from FOG family proteins have been shown to interact with the N-terminal zinc finger of GATA-1. Results: We have used NMR spectroscopy to determine the first structures of two FOG family zinc fingers that are involved in protein–protein interactions: fingers 1 and 9 from U-shaped. These fingers resemble classical TFIIIA-like zinc fingers, with the exception of an unusual extended portion of the polypeptide backbone prior to the fourth zinc ligand. [ 15N, 1H]-HSQC titrations have been used to define the GATA binding surface of USH-F1, and comparison with other FOG family proteins indicates that the recognition mechanism is conserved across species. The surface of FOG-type fingers that interacts with GATA-1 overlaps substantially with the surface through which classical fingers typically recognize DNA. This suggests that these fingers could not contact both GATA and DNA simultaneously. In addition, results from NMR, gel filtration, and sedimentation equilibrium experiments suggest that the interactions are of moderate affinity. Conclusions: Our results demonstrate unequivocally that zinc fingers comprising the classical ββα fold are capable of mediating specific contacts between proteins. The existence of this alternative function has implications for the prediction of protein function from sequence data and for the evolution of protein function.