Amyloids are implicated in neurodegenerative diseases. Fibrillar aggregates of the amyloid-β protein (Aβ) are the main component of the senile plaques found in brains of Alzheimer's disease patients. We present the structure of an Aβ(1-42) fibril composed of two intertwined protofilaments determined by cryo-electron microscopy (cryo-EM) to 4.0-angstrom resolution, complemented by solid-state nuclear magnetic resonance experiments. The backbone of all 42 residues and nearly all side chains are well resolved in the EM density map, including the entire N terminus, which is part of the cross-β structure resulting in an overall "LS"-shaped topology of individual subunits. The dimer interface protects the hydrophobic C termini from the solvent. The characteristic staggering of the nonplanar subunits results in markedly different fibril ends, termed "groove" and "ridge," leading to different binding pathways on both fibril ends, which has implications for fibril growth.

The homodimeric nickel-containing CO dehydrogenase from the anaerobic bacterium Carboxydothermus hydrogenoformans catalyzes the oxidation of CO to CO2. A crystal structure of the reduced enzyme has been solved at 1.6 angstrom resolution. This structure represents the prototype for Ni-containing CO dehydrogenases from anaerobic bacteria and archaea. It contains five metal clusters of which clusters B, B', and a subunit-bridging, surface-exposed cluster D are cubane-type [4Fe-4S] clusters. The active-site clusters C and C' are novel, asymmetric [Ni-4Fe-5S] clusters. Their integral Ni ion, which is the likely site of CO oxidation, is coordinated by four sulfur ligands with square planar geometry.

The CO dehydrogenase of the eubacterium Oligotropha carboxidovorans is a 277-kDa Mo-and Cu-containing iron-sulfur flavoprotein. Here, the enzyme's active site in the oxidized or reduced state, after inactivation with potassium cyanide or with n-butylisocyanide bound to the active site, has been reinvestigated by multiple wavelength anomalous dispersion measurements at atomic resolution, electron spin resonance spectroscopy, and chemical analyses. We present evidence for a dinuclear heterometal [CuSMo(AO)OH] cluster in the active site of the oxidized or reduced enzyme, which is prone to cyanolysis. C arbon monoxide dehydrogenases (CODHs) of aerobic or anaerobic Bacteria and Archaea, which represent the essential catalyst in the global biogeochemical cycle of atmospheric carbon monoxide (CO), catalyze the oxidation of CO to carbon dioxide (CO 2 ) or the reverse reaction [CO ϩ H 2 O 7 CO 2 ϩ 2 H ϩ ϩ 2 e Ϫ ] (1, 2). The annual removal of CO from the lower atmosphere and earth by microorganisms has been estimated to be Ϸ1 ϫ 10 8 tons (3). Thus, an important role of CODHs is to remove CO from the environment, helping to maintain the toxic gas at subhazardous concentrations.The Ni-containing CODHs from the anaerobic hydrogenogenic bacteria Carboxydothermus hydrogenoformans (4, 5) or Rhodospirillum rubrum (6) and the Mo-containing CODHs from the aerobic carboxidotrophic bacteria Oligotropha carboxidovorans (7-9) or Hydrogenophaga pseudoflava (10) have been structurally characterized. The homodimeric CODHs of C. hydrogenoformans or R. rubrum contain five metal clusters, of which clusters B, BЈ and a subunit-bridging, surface-exposed cluster D are cubane-type [4Fe-4S] clusters (5, 6). The active-site clusters C and CЈ of the C. hydrogenoformans CODH are asymmetric [Ni-4Fe-5S] clusters identified in the enzyme reduced with dithionite. Their integral Ni ion, which is the likely site of CO oxidation, is coordinated by four sulfur ligands with square planar geometry (5). Interestingly, the corresponding cluster of the CODH from R. rubrum has been described as an Fe mononuclear site in combination with an [NiFe 3 S 4 ] cubane (6).CODH from O. carboxidovorans consists of a dimer of LMS heterotrimers (7). Each heterotrimer is composed of a 17.8-kDa iron-sulfur protein (S), which carries two types of [2Fe-2S] clusters, a 30.2-kDa flavoprotein (M), which contains a noncovalently bound FAD cofactor, and an 88.7-kDa molybdoprotein (L), which harbors the active site of the enzyme. In a previous paper (7), a CODH preparation with a specific activity of 6.6 units͞mg was analyzed at a resolution of 2.2 Å. The enzyme's active site was modeled to contain Mo with three oxygen ligands, the molybdopterin cytosine dinucleotide (MCD) cofactor, and an SeH-group bound to the S␥ atom of Cys-388. In the present paper, we have applied multiple wavelength anomalous dispersion methods at up to 1.09-Å resolution to crystals containing fully functional CODH (23.2 units͞mg). The SeH-group could not be confirmed, and a Cu atom was identified instead, at ...

Lipidated Rho and Rab GTP-binding proteins are transported between membranes in complex with solubilizing factors called 'guanine nucleotide dissociation inhibitors' (GDIs). Unloading from GDIs using GDI displacement factors (GDFs) has been proposed but remains mechanistically elusive. PDEδ is a putative solubilizing factor for several prenylated Ras-subfamily proteins. Here we report the structure of fully modified farnesylated Rheb-GDP in complex with PDEδ. The structure explains the nucleotide-independent binding of Rheb to PDEδ and the relaxed specificity of PDEδ. We demonstrate that the G proteins Arl2 and Arl3 act in a GTP-dependent manner as allosteric release factors for farnesylated cargo. We thus describe a new transport system for farnesylated G proteins involving a GDI-like molecule and an unequivocal GDF. Considering the importance of PDEδ for proper Ras and Rheb signaling, this study is instrumental in developing a new target for anticancer therapy.