The crystal structure of HasA, a hemophore secreted by Serratia marcescens

Free iron availability is strongly limited in vertebrate hosts, making the iron acquisition by siderophores inappropriate. Pathogenic bacteria have de veloped various ways to use the host's iron from iron-containing proteins. Serratia marcescens can use the iron from hemoglobin through the secretion of a hemophore called HasA, which takes up the heme from hemoglobin and shu ttles it to the receptor HasR, which in turn, releases heme into the bacter ium. We report here the first crystal structure of such a hemophore, bound to a heme group at two different pH values and at a resolution of 1.9 Angst rom. The structure reveals a new original fold and suggests a hypothetical mechanism for both heme uptake and release.