Crystal Structure of Hemoglobin Protease, a Heme Binding Autotransporter Protein from Pathogenic Escherichia coli*

The acquisition of iron is essential for the survival of pathogenic bacteria, which have consequently evolved a wide variety of uptake systems to extract iron and heme from host proteins such as hemoglobin. Hemoglobin pro-tease (Hbp) was discovered as a factor involved in the symbiosis of pathogenic Escherichia coli and Bacte-roides fragilis, which cause intra-abdominal abscesses. Released from E. coli, this serine protease autotrans-porter degrades hemoglobin and delivers heme to both bacterial species. The crystal structure of the complete passenger domain of Hbp (110 kDa) is presented, which is the first structure from this class of serine proteases and the largest parallel -helical structure yet solved. Autotransporters (ATs)1 are virulence proteins produced by a variety of pathogenic bacteria, including several types o