Pumping iron

Bacteria have to resort to extreme measures to obtain iron, because it exists as ferric oxyhydroxide, a virtually insoluble form, in an oxidizing atmosphere. They secrete chelator molecules ? siderophores ? which bind ferric iron, and these complexes then bind to specific active transporter proteins in the outer membrane of Gram-negative bacteria. FecA from Escherichia coli is one of the most complex active iron transporters, and relies on the cytoplasmic membrane protein TonB to provide energy for this transport. In Science, Deisenhofer and colleagues now describe the crystallographic structure of FecA with and without bound ligand (in this case ferric citrate) at 2.5- and 2.0-Å resolution, respectively.

FecA is composed of three domains: a β-barrel domain that traverses the outer membrane, a plug domain located inside the barrel, and a periplasmic NH2-terminal extension. The authors found that FecA seems to have two gating mechanisms. One gating mechanism, previously seen in other active bacterial iron transporters, is provided by the plug domain, which prevents the direct passage of ferric citrate across the outer membrane. The second gating mechanism is provided mainly by the seventh and eighth extracellular loops of the β-barrel, which, after ligand binding, block the external pocket and ligand-binding site off from the external medium. These structural data have provided new insights into the gating mechanisms of TonB-dependent outer membrane proteins, and also enabled the authors to propose a mechanism for the energy-dependent transport of siderophores. REFERENCE Ferguson, A. D. et al. Structural basis of gating by the outer membrane transporter FecA. Science 295, 1715?1719 (2002)

Death machinery

Assembly of the apoptosome ? a multiprotein 'death' complex ? is a crucial step in the mitochondrial cell-death pathway, which requires apoptotic protease-activating factor 1 (Apaf-1), cytochrome c and dATP/ATP. The assembled apoptosome then binds to, and activates, procaspase-9, which can then activate executioner caspases such as caspase-3. The precise size, structure and number of subunits of the apoptosome are not known, and neither is the mechanism of procaspase-9 activation. However, in Molecular Cell, Akey and co-workers now provide the first three-dimensional structure of the apoptosome at 27-Å resolution, which was obtained using electron cryomicroscopy and molecular modelling techniques.

The structure shows a wheel-like particle with seven spokes radiating from a central hub. Each spoke is made up of a bent arm and a Y domain, with the latter having two lobes connected by a bridge. Using molecular modelling, Akey and colleagues were able to assign probable positions for Apaf-1 and cytochrome c within this structure, and to propose a model for apoptosome assembly, including a plausible role for cytochrome c. The authors also determined the structure of the apoptosome bound to a non-cleavable mutant of procaspase-9, which showed a dome-like feature on the central hub. This complex efficiently activated procaspase-3, indicating that procaspase-9 cleavage is not required to form an active cell-death complex. REFERENCE Acehan, D. et al. Three-dimensional structure of the apoptosome: implications for assembly, procaspase-9 binding, and activation. Mol. Cell 9, 423?432 (2002)