Gram-negative bacteria use specific heme uptake systems, relying on outer membrane
receptors and excreted heme-binding proteins (hemophores) to scavenge and actively
transport heme. To unravel the unknown molecular details involved, we present 3 structures
of the Serratia marcescens receptor HasR in complex with its hemophore HasA. The transfer
of heme over a distance of 9 Å from its high-affinity site in HasA into a site of lower affinity in
HasR is coupled with the exergonic complex formation of the 2 proteins. Upon docking to the …

Results The wild-type ternary complex was solved by MAD using anomalous scattering from
8 selenomethionines and the 2 other structures by difference Fourier methods. The final
models consist of 752 residues from HasR (the N-tcrminal extension involved in signal
transduction and transcriptional autoregulation, as well as the [ollowing TonB box, are not
seen in the structure), 161 from HasA and, in case of the ternary complexes, 1 additional b-
type he me (see