HAP1 is nuclear but is bound to a cellular factor in the absence of heme.

The activity of the yeast transcription activator HAP1 is controlled by heme. Previously, it has been shown that a heme-responsive domain containing multiple repeats of a conserved motif blocks DNA binding in the absence of heme. In this report, we show that HAP1 is sequestered in a high molecular weight complex in the absence of heme. Titration of the high molecular weight complex by addition of a non-DNA-binding form of HAP1 allows the protein to form dimeric complexes in the absence of heme in vitro and acquires partial transcriptional activity in vivo. The results indicate that one or more cellular factor(s) complexes with HAP1 and represses its activity in the absence of heme. Deletion of the heme domain prevents sequestration of HAP1 in the high molecular weight complex. We discuss these findings in a model that postulates that the heme domain of HAP1 can interact with other cellular factors to regulate HAP1.