Solution structures of two CCHC zinc fingers from the FOG family protein U-Shaped that mediate protein-protein interactions

Background: Zinc finger domains have traditionally been regarded as sequenc e-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 w ith 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 interacti ons: fingers 1 and 9 from U-shaped. These fingers resemble classical TFIIIA -like zinc fingers, with the exception of an unusual extended portion of th e polypeptide backbone prior to the fourth zinc ligand. [N-15,H-1]-HSQC tit rations have been used to define the GATA binding surface of USH-F1, and co mparison with other FOG family proteins indicates that the recognition mech anism is conserved across species. The surface of FOG-type fingers that int eracts with GATA-1 overlaps substantially with the surface through which cl assical fingers typically recognize DNA. This suggests that these fingers c ould not contact both GATA and DNA simultaneously. In addition, results fro m NMR, gel filtration, and sedimentation equilibrium experiments suggest th at the interactions are of moderate affinity. Conclusions: Our results demonstrate unequivocally that zinc fingers compri sing the classical ppa fold are capable of mediating specific contacts betw een proteins. The existence of this alternative function has implications f or the prediction of protein function from sequence data and for the evolut ion of protein function.