Gigantism in unique biogenic magnetite at the Paleocene-Eocene Thermal Maximum

We report the discovery of exceptionally large biogenic magnetite crystals in clay-rich sediments spanning the Paleocene-Eocene Thermal Maximum (PETM) in a borehole at Ancora, New Jersey. Aside from previously-described abundant bacterial magnetofossils, electron microscopy reveals novel spearhead-like and spindle-like magnetite up to 4 μm long and hexaoctahedral prisms up to 1.4 μm long. Similar to magnetite produced by magnetotactic bacteria, these single-crystal particles exhibit chemical composition, lattice perfection, and oxygen isotopes consistent with an aquatic origin. Electron holography indicates single-domain magnetization despite their large crystal size. We suggest that the development of a thick suboxic zone with high iron bioavailability – a product of dramatic changes in weathering and sedimentation patterns driven by severe global warming – drove diversification of magnetite-forming organisms, likely including eukaryotes

The Structure of the CF1 Part of the ATP-Synthase Complex from Chloroplasts

The proton ATP synthase consists of a membrane integrated part, Fo, and a hydrophilic part, F1. F1 is composed of five different subunits: α, β, γ, δ and ε. This chapter focuses on the chloroplast F1 (CF1) structure and discusses the overall shape and dimensions of CF1, shape and size of the various subunits, subunit interactions and conformational changes in the subunit positions related to catalysis. Structural data originate mainly from X-ray diffraction and electron microscopy. Recently, the structure of F1 from beef heart mitochondria has been determined at 2.8 Å and this structure, although not fully identical to CF1, can be taken as a blueprint for models of the CF1 structure.

Electron cryomicroscopy of two-dimensional crystals of the H+-ATPase from chloroplasts

The H+-ATPase from spinach chloroplasts was isolated and purified. Two-dimensional crystals were obtained from the protein/lipid/detergent micelles by treatment with phospholipase and simultaneous removal of detergent and fatty acids by Biobeads. The resulting two-dimensionally ordered arrays were investigated by electron cryomicroscopy. The ordered arrays showed top view projections of CF0F1. The images were analysed by correlation averaging. In this view CF0F1 has dimensions of 11.4 × 9 nm. The average view shows a strongly asymmetric molecule, in contrast to the rather hexagonal features of CF1, previously analyzed from two-dimensional arrays. It is concluded that this is due either to an asymmetric structure and positioning of CF0 relative to CF1 or to a rearrangement of CF1 subunits induced by binding of CF0 to CF1.

Structural model of phospholipid-reconstituted human transferrin receptor derived by electron microscopy

AbstractBackground: The transferrin receptor (TfR) regulates the cellular uptake of serum iron. Although the TfR serves as a model system for endocytosis receptors, neither crystal structure analysis nor electron microscopy has yet revealed the molecular dimensions of the TfR. To derive the first molecular model, we analyzed purified, lipid-reconstituted human TfR by high-resolution electron microscopy.Results: A structural model of phospholipid-reconstituted TfR was derived from 72 cryo-electron microscopic images. The TfR dimer consists of a large extracellular globular domain (6.4 × 7.5 × 10.5 nm) separated from the membrane by a thin molecular stalk (2.9 nm). A comparative protein sequence analysis suggests that the stalk corresponds to amino acid residues 89–126. Under phospholipid-reconstitution conditions, the human TfR not only integrates into vesicles, but also forms rosette-like structures called proteoparticles. Scanning transmission electron microscopy revealed an overall diameter of 31.5 nm and a molecular mass of 1669 ± 26 kDa for the proteoparticles, corresponding to nine TfR dimers. The average mass of a single receptor dimer was determined as being 186 ± 4 kDa.Conclusions: Proteoparticles resemble TfR exosomes that are expelled by sheep reticulocytes upon maturation. The structure of proteoparticles in vitro is thus interpreted as being the result of the TfR's strong self-association potential, which might facilitate the endosomal sequestration of the TfR away from other membrane proteins and its subsequent return to the cell surface within tubular structures. The stalk is assumed to facilitate the tight packing of receptor molecules in coated pits and recycling tubuli