HEME BINDS TO A SHORT SEQUENCE THAT SERVES A REGULATORY FUNCTION IN DIVERSE PROTEINS

Heme is a prosthetic group for numerous enzymes, cytochromes and globi ns, and it binds tightly, sometimes covalently, to these proteins. Int erestingly, heme also potentiates binding of the yeast transcriptional activator HAP1 to DNA and inhibits mitochondrial import of the mammal ian delta-aminolevulinate synthase (ALAS) and the catalytic activity o f the reticulocyte kinase, HRI. All three of these proteins contain a short sequence, the heme regulatory motif (HRM), that occurs six times adjacent to the HAP1 DNA binding domain, twice in the leader targetin g sequence of ALAS and twice near the catalytic domain of the HRI kina se, Here we show that a 10 amino acid peptide containing the HRM conse nsus binds to heme in the micromolar range, and shifts the heme absorp tion spectrum to a longer wavelength, a direction opposite to the chan ge caused by cytochromes or globins. Further, we show that a single FI RM regulates the acidic activation domains of HAP1 and GAL4 independen tly of regulation of DNA binding of the transcription factors. These f indings thus establish a novel heme binding sequence which is structur ally distinct from sequences in globins or cytochromes and which has a regulatory function.