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Mol Cell Biol, July 1998, p. 3819-3828, Vol. 18, No. 7
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Molecular Mechanism Governing Heme Signaling in Yeast: a Higher-Order Complex Mediates Heme Regulation of the Transcriptional Activator HAP1

Li Zhang,* Angela Hach, and Cheng Wang

Department of Biochemistry, NYU Medical Center, New York, New York 10016

Received 19 December 1997/Returned for modification 26 January 1998/Accepted 10 April 1998

Apart from serving as a prosthetic group in globins and enzymes, heme is a key regulator controlling a wide range of molecular and cellular processes involved in oxygen sensing and utilization. To gain insights into molecular mechanisms of heme signaling and oxygen sensing in eukaryotes, we investigated the yeast heme-responsive transcriptional activator HAP1. HAP1 activity is regulated precisely and tightly by heme. Here we show that in the absence of heme, HAP1 forms a biochemically distinctive higher-order complex. Our data suggest that this complex contains HAP1 and four other cellular proteins including Hsp82 and Ydj1. The formation of this complex is directly correlated with HAP1 repression in the absence of heme, and mutational or heme disruption of the complex correlates with HAP1 activation, suggesting that this complex is responsible for heme regulation of HAP1 activity. Further, we determined HAP1 domains required for heme regulation: three domains---the dimerization domain, the heme domain, and the HRM7 (heme-responsive motif 7) domain---cooperate to form the higher-order complex and mediate heme regulation. Strikingly, we uncovered a novel function for the HAP1 dimerization domain: it not only allows dimerization but also provides critical functions in heme regulation and transcriptional activation. Our studies provide significant insights into the molecular events leading to heme activation of HAP1 and may shed light on molecular mechanisms of various heme-controlled biological processes in diverse organisms.

* Corresponding author. Mailing address: Department of Biochemistry, NYU Medical Center, 550 First Ave., New York, NY 10016. Phone: (212) 263-8506. Fax: (212) 263-8166. E-mail: zhangl02{at}mcrcr.med.nyu.edu.

Mol Cell Biol, July 1998, p. 3819-3828, Vol. 18, No. 7
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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Copyright © 1998 by the American Society for Microbiology. All rights reserved.