The N-terminal zinc finger (ZnF) from GATA transcription factors mediates interactions with FOG family proteins. In FOG proteins, the interacting domains are also ZnFs; these domains are related to classical CCHH fingers but have an His --> Cys substitution at the final zinc-ligating position. Here we demonstrate that different CCHC fingers in the FOG family protein U-shaped contact the N-terminal ZnF of GATA-1 in the same fashion although with different affinities. We also show that these interactions are of moderate affinity, which is interesting given the presumed low concentrations of these proteins in the nucleus. Furthermore, we demonstrate that the variant CCHC topology enhances binding affinity, although the His --> Cys change is not essential for the formation of a stably folded domain. To ascertain the structural basis for the contribution of the CCHC arrangement, we have determined the structure of a CCHH mutant of finger nine from U-shaped. The structure is very similar overall to the wild-type domain, with subtle differences at the C terminus that result in loss of the interaction in vivo. Taken together, these results suggest that the CCHC zinc binding topology is required for the integrity of GATA-FOG interactions and that weak interactions can play important roles in vivo.