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

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Abstract
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.