Zinc binding to a regulatory zinc-sensing domain monitored in vivo by using FRET.

We have generated probes of metal binding to zinc fingers (ZFs) that provide tools to study zinc trafficking in vivo. In this study, we used these probes to examine zinc binding by the Zap1 transcription factor of Saccharomyces cerevisiae. Zap1 contains two zinc-regulated activation domains (ADs), AD1 and AD2. AD2 is located within two C2H2 ZFs, ZF1 and ZF2. Studies have indicated that apoAD2 activates transcription and zinc binding to ZF1 and that ZF2 forms an interacting-finger-pair structure that is necessary to inhibit AD function. A related structural finger pair, ZF3 and ZF4, is found in the Zap1 DNA binding domain. In vitro studies indicated that, although the ZF1/2 and ZF3/4 finger pairs bind zinc with similar affinities, zinc that was bound to ZF1/2 was much more labile. We examined the properties of Zap1 ZFs in vivo by FRET. ZF pairs were flanked by enhanced yellow fluorescent protein and enhanced cyan fluorescent protein, allowing detection of zinc-induced conformation changes by FRET. By using these reporters, we found that ZF1/2 and ZF3/4 showed similar responses to zinc under steady-state conditions in vivo. In contrast, ZF1/2 zinc binding was significantly more labile than was ZF3/4. Also, ZF1/2 accumulated in an apo form that could rapidly bind zinc, whereas the ZF3/4 pair did not. Last, we show that these properties are evolutionarily conserved indicating their importance to Zap1 function. These results indicate that the kinetic lability of ZF1/2 in vivo is a key component of Zap1 zinc responsiveness.