Letter to the Editor: Hypothetical protein At2g24940.1 from Arabidopsis thaliana has a cytochrome b5 like fold*

Abstract

Progesterone is believed to exert rapid non-genomic actions through its interaction with membrane associated progesterone receptors (MAPRs) (Bramley, 2003; Li and O’Malley, 2003). BLAST sequence searches (Altieri et al., 1995) for mammalian MAPRs and putative MAPRs from plants have identified that these proteins all contain a cytochrome b5-like ligandbinding domain (Mifsud and Bateman, 2002). Interestingly, unlike cytochrome b5 itself, these MAPRs domains appear not to bind heme and not to be involved in redox reactions. Their distinct biological functions suggest that these steroid receptors adopt the cytochrome b5 domain as a template in order to build their own ligand-binding pockets (Mifsud and Bateman, 2002). The Center for Eukaryotic Structural Genomics is engaged in determining the three-dimensional structures of novel proteins from eukaryotic gene families. Its target selection algorithm selected Arabidopsis thaliana putative protein At2g24940.1 for structure determination. The biochemical function of At2g24940.1 currently is unknown. Its ∼40% sequence identity with mammalian MAPR suggests that At2g24940.1 may act as a steroid binding protein. In addition, its sequence is distantly similar to that of cytochrome b5 (Figure 1). Here we describe the threedimensional structure of At2g24940.1 as determined by NMR spectroscopy. At present, no structure of a MAPR is available from the Protein Data Bank. Thus, the structure of At2g24940.1 may provide clues to the function of a class of steroid binding proteins in plants.