Regulatory Switching by Concerted Motions on the Microsecond Time Scale of the Oxygen Sensor Protein FixL

New insights into the signal transduction mechanism of O2-sensing FixL and other biological heme-based sensor proteins

Recent structural and biophysical studies of O2-sensing FixL, NO-sensing soluble guanylate cyclase, and other biological heme-based sensing proteins have begun to reveal the details of their molecular mechanisms and shed light on how nature regulates important biological processes such as nitrogen fixation, blood pressure, neurotransmission, photosynthesis and circadian rhythm. The O2-sensing FixL protein from S. meliloti, the eukaryotic NO-sensing protein sGC, and the CO-sensing CooA protein from R. rubrum transmit their biological signals through gas-binding to the heme domain of these proteins, which inhibits or activates the regulatory, enzymatic domain. These proteins appear to propagate their signal by specific structural changes in the heme sensor domain initiated by the appropriate gas binding to the heme, which is then propagated through a coiled-coil linker or other domain to the regulatory, enzymatic domain that sends out the biological signal. The current understanding of the signal transduction mechanisms of O2-sensing FixL, NO-sensing sGC, CO-sensing CooA and other biological heme-based gas sensing proteins and their mechanistic themes are discussed, with recommendations for future work to further understand this rapidly growing area of biological heme-based gas sensors.

Aim18p and Aim46p are chalcone isomerase domain-containing mitochondrial hemoproteins in Saccharomyces cerevisiae

Chalcone isomerases (CHIs) have well-established roles in the biosynthesis of plant flavonoid metabolites. Saccharomyces cerevisiae possesses two predicted CHI-like proteins, Aim18p (encoded by YHR198C) and Aim46p (YHR199C), but it lacks other enzymes of the flavonoid pathway, suggesting that Aim18p and Aim46p employ the CHI fold for distinct purposes. Here, we demonstrate using proteinase K protection assays, sodium carbonate extractions, and crystallography that Aim18p and Aim46p reside on the mitochondrial inner membrane and adopt CHI folds, but they lack select active site residues and possess an extra fungal-specific loop. Consistent with these differences, Aim18p and Aim46p lack CHI activity and also the fatty acid-binding capabilities of other CHI-like proteins, but instead bind heme. We further show that diverse fungal homologs also bind heme and that Aim18p and Aim46p possess structural homology to a bacterial hemoprotein. Collectively, our work reveals a distinct function and cellular localization for two CHI-like proteins, introduces a new variation of a hemoprotein fold, and suggests that ancestral CHI-like proteins were hemoproteins.