Staphylococcus aureus is the source of a large number of hospital-acquired infections, of which many are serious and can lead to death. Iron is critically important to the survival and growth of the bacterium, and complex, multistep mechanisms are present to fulfill the necessary iron requirement. Isd proteins located on the wall and membrane of S. aureus have been proposed to function in heme acquisition. We report characterization of the S. aureus heme-binding protein IsdE, the lipoprotein component of a membrane-localized ABC transporter that is believed key to receiving heme from cell wall-anchored Isd proteins. Magnetic circular dichroism (MCD) data, which greatly extend the results from our initial study of IsdE in bacterial cell lysates (Mack, J., Vermeiren, C., Heinrichs, D. E., and Stillman, M. J. (2004) Biochem. Biophys. Res. Commun. 320, 781-788), probe the ligand and redox properties of the bound heme. The MCD data show that IsdE, when overexpressed in E. coli, binds either ferric or ferrous heme but that the largest fraction is low spin ferrous heme. Studies of mutants allowed identification and characterization of the ligands in the fifth and sixth position on the heme iron as histidine, proximally, and methionine, distally. This histidine-methionine heme-iron ligation is unique to heme transport proteins. The smaller fraction of ferric heme in the protein is not bound by methionine, allowing for access by strong field ligands, such as cyanide. Electrospray ionization mass spectral data are reported for the first time and show that only one heme ligand binds per IsdE protein molecule. These data also show there is little change in the conformation of the protein between the heme-bound and heme-free species, indicating that the heme-free IsdE adopts a structure essentially independent of the heme. The mass spectral data clearly show that IsdE reversibly unwinds under denaturing conditions to form at least two distinct, heme-free conformations.