The three-dimensional structure of the globin domain of globin-coupled sensor from Geobacter sulfurreducens

The recently discovered globin-coupled sensors (GCS) are heme-containing two domain transducers, that mediate the cellular responses to metabolic and environmental stimuli, such as NO, CO and O2. In particular, the GCSs identified in the strict anaerobic -Proteobacteria Geobacter sulfurreducens and Geobacter metalloreducens may be involved in sulfate/sulfur reduction. They are constituted by 300 amino acid residues and characterised by a N-terminal globin domain (152 amino acids) and by a bundle of four transmembrane helices at C-terminal (148 amino acids). Here, we present the X-ray three-dimensional structure of globin domain of GCS from Geobacter sulfurreducens (GeoHb), at atomic resolution (1.4 Å). The crystal structure of GeoHb was solved by MAD techniques (data collected at the synchrotron beamline ID23, ESRF, Grenoble, France), based on the heme Fe atom anomalous scattering. We collected three data set, one at the absorption edge (E=7.131 keV), one at the inflection point (E=7.125 keV) and one at the remote point (E=13 keV), respectively. The crystals belong to the orthorhombic space group P212121, with unit cell constants: a = 56.6 Å, b = 73.8 Å, c = 89.9 Å, two molecules per asymmetric unit (2 x 152 amino acids). The 3D structure was solved using the program SOLVE and the wARP package, for MAD phases calculation, their refinement and autotracing of almost the all protein backbone and side chains. The quality of the electron density allowed to recognize automatically the whole amino acid sequence. The 3D structure is still under refinement. The GeoHb fold is a variant of the classical globin fold of mioglobin and hemoglobin, with helices A through H, and an additional Z helix at the N-terminus. GeoHb displays a six-coordinated heme, both in the ferrous and ferric forms, the axial heme Fe ligands being identified as the proximal and distal His residue.