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Membrane-bound progesterone receptors contain a cytochrome b5-like ligand-binding domain

William Mifsud12 and Alex Bateman1*

Author Affiliations

1 The Wellcome Trust Sanger Institute, Wellcome Trust Genome Campus, Hinxton CB10 1SA, UK

2 University of Malta Medical School, Saint Luke's Hospital, Guardamangia Hill, Guardamangia MSD 09, Malta

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Genome Biology 2002, 3:research0068-research0068.5  doi:10.1186/gb-2002-3-12-research0068

Published: 12 November 2002



Membrane-associated progesterone receptors (MAPRs) are thought to mediate a number of rapid cellular effects not involving changes in gene expression. They do not show sequence similarity to any of the classical steroid receptors. We were interested in identifying distant homologs of MAPR better to understand their biological roles.


We have identified MAPRs as distant homologs of cytochrome b5. We have also found regions homologous to cytochrome b5 in the mammalian HERC2 ubiquitin transferase proteins and a number of fungal chitin synthases.


In view of these findings, we propose that the heme-binding cytochrome b5 domain served as a template for the evolution of membrane-associated binding pockets for non-heme ligands.