The plant ATP-binding cassette (ABC) transporter ABCB25 from Arabidopsis thaliana is a mitochondrial inner-membrane protein homologous to yeast Sc Atm1 and human ABCB7. ABCB25 and Sc Atm1 are essential for plant and yeast viability, respectively, while ABCB7 is associated with X-Linked sideroblastic anemia and erebellar ataxia (XLSA). Prior studies have shown that these transporters are required for the proper function of cytosolic Fe/S proteins involved in vital downstream cellular processes; however, their physiological transported substrate(s) remain unknown. ABC transporters hydrolyze ATP in the absence of their substrate (basal activity), and this rate typically increases in the presence of the transported substrate. Among the three homologs, Sc Atm1 is the most studied and has previously been shown to exhibit increased ATPase activity in response to thiol-containing compounds (such as cysteine, glutathione, DTT, among others) and peptides, as well as proven to transport glutathione in liposomes. For the plant homolog ABCB25, stimulation of ATPase activity and transport has only been reported with the physiological thiol antioxidant glutathione. Here, we reconstituted ABCB25 into MSP2N2 nanodiscs to measure ATPase activity in the absence and presence of different sulfur-containing molecules. Our data shows for the first time that ABCB25’s basal activity is stimulated by sulfur-containing molecules (such as cysteine, DTT, and β-mercaptoethanol) other than glutathione. Our preliminary results suggest that ABCB25 and Sc Atm1 may transport a wide variety of substrates, and that the physiological substrate(s) for these proteins is likely to be a sulfur-containing molecule that might be conserved across yeast, plants, and potentially humans. These findings provide insights into conserved molecular mechanisms across kingdoms of life and open the door for possible therapeutic applications. R01GM145938.
Barreto et al. (Sun,) studied this question.