Structure of a biological oxygen sensor: A new mechanism for heme-driven signal transduction

The FixL proteins are biological oxygen sensors that restrict the expression of specific genes to hypoxic conditions. FixL’s oxygen-detecting domain is a heme binding region that controls the activity of an attached histidine kinase. The FixL switch is regulated by binding of oxygen and other strong-field ligands. In the absence of bound ligand, the heme domain permits kinase activity. In the presence of bound ligand, this domain turns off kinase activity. Comparison of the structures of two forms of the Bradyrhizobium japonicum FixL heme domain, one in the “on” state without bound ligand and one in the “off” state with bound cyanide, reveals a mechanism of regulation by a heme that is distinct from the classical hemoglobin models. The close structural resemblance of the FixL heme domain to the photoactive yellow protein confirms the existence of a PAS structural motif but reveals the presence of an alternative regulatory gateway.

Assembly of the neutrophil respiratory burst oxidase: A direct interaction between p67PHOX and cytochrome b558

Activation of the phagocyte NADPH oxidase complex requires the assembly of the cytosolic factors p47PHOX, p67PHOX, p40PHOX, and Rac1 or Rac2, with the membrane-bound cytochrome b558. Whereas the interaction of p47PHOX with cytochrome b558 is well established, an interaction between p67PHOX and cytochrome b558 has never been investigated. We report here a direct interaction between p67PHOX and cytochrome b558. First, labeled p67PHOX recognizes a 91-kDa band in specific granules from a normal patient but not from a cytochrome b558-deficient patient. Second, p67PHOX binds to cytochrome b558 that has been bound to nitrocellulose. Third, GTP-p67PHOX bound to glutathione agarose is able to pull down cytochrome b558. Rac1-GTP or Rac1-GDP increased the binding of p67PHOX to cytochrome b558, suggesting that at least one of the oxidase-related functions of Rac1 is to promote the interaction between p67PHOX and cytochrome b558.