Synthesis of undecagold cluster molecules as biochemical labeling reagents. 2. Bromoacetyl and maleimido undecagold clusters

Cytochrome c oxidase: structural studies by electron microscopy of two-dimensional crystals

Cytochrome c oxidase is a complex integral membrane protein consisting of 13 different polypeptide chains and four metal centers having a total molecular weight of approximately 200,000 daltons. It can be isolated in two 2-dimensional crystalline forms differing in aggregation state of the enzyme. One crystal form consists of cytochrome oxidase dimers (approximately 400,000 daltons) embedded unidirectionally in the lipid bilayer of a collapsed vesicle while the other form consists of crystalline sheets of cytochrome oxidase monomers. Both crystal forms have been studied by electron microscopy during the past two decades, and this paper summarizes the results of early structural studies as well as more recent results applying techniques of cryoelectron microscopy and digital image processing. The structure of frozen-hydrated cytochrome oxidase dimers at 20 A resolution is discussed as well as the packing of monomers within dimers and the site of cytochrome c binding.

Undecagold cluster modified tRNA(Phe) from Escherichia coli and its activity in the protein elongation cycle

An undecagold cluster (Au11) of molecular mass 6200Da was attached to the 3-(3-amino-3-carboxypropyl)uridine at position 47 of tRNA(Phe) from Escherichia coli. This modified tRNA can be enzymically aminoacylated with phenylalanine in the reaction catalyzed by phenylalanyl-tRNA synthetase. Au11-labeled Phe-tRNA(Phe) forms a ternary complex with the elongation factor Tu.GTP and is active in poly(U)-dependent poly(phe) synthesis. The Au11 modification does not hinder the specific binding of tRNA to distinct ribosomal binding sites or the precise positioning of the aminoacyl and peptidyl residues in the peptidyltransferase center, and does not impair the translocation. The modified tRNA is suitable for the identification of ribosomal binding sites by scanning transmission electron microscopy and for crystallographic studies of the 70S ribosome at different states of the protein-elongation cycle.

Visibility and stability of a 12-tungsten atom complex in the scanning transmission electron microscope

A complex consisting of 12 tungsten atoms has been studied in terms of signal-to-noise (S/N) and dose response in the scanning transmission electron microscope (STEM), to evaluate its suitability for use as a approximately 1 nm resolution biological label. Molecular weight of the complex was measured as a function of radius of integration, and results were in agreement with the calculated formula weight. S/N was highest at the lowest radius of integration (0.25 nm), and decreased monotonically with increasing radius. The complex was clearly visible at a dose of 4 X 10(3) e/nm2, and exhibited negligible mass loss (approximately 8%) after an accumulated dose of 1.28 X 10(5) e/nm2. Beam-induced motion was small, 0.46 nm rms after 4 X 10(4) e/nm2. Some intensity fluctuations were observed between successive scans of the same clusters, for which a diffraction-based explanation is advanced. Upon suitable functionalization, the tungsten complex is expected to complement the undecagold cluster already in use for site-specific labeling.

Covalent labelling of histones with aurothiomalate. Gold-labelled H2A-H2B dimers assemble to octamers, which form crystalline helical tubes

Histone octamers were covalently labelled with aurothiomalate at amino groups by the method of carbodiimide activation. The labelling procedure was demonstrated to result in the specific covalent coupling through a single bond of the heavy metal atom label to protein amino groups. Such octamers were dissociated to yield soluble H2A-H2B dimers containing three gold atoms per dimer. The dimers were reconstituted with native H3-H4 tetramers to form labelled octamers, which were crystallized to form helical tubes. This strongly suggests that this procedure resulted in minimal changes of protein conformation.

Characterization of single crystals of the large ribosomal particles from a mutant of Bacillus stearothermophilus

Single, three-dimensional crystals of 50 S ribosomal subunits, from a mutant of Bacillus stearothermophilus that lacks the protein L11, have been characterized using a synchrotron X-ray source. The crystals of the mutated particles grow under the same conditions and are isomorphous to those of the wild type of the same bacteria. They are orthorhombic, contain at least one 2-fold screw axis, and have unit cell dimensions of a = 350(+/- 10) A, b = 670(+/- 10) A, and c = 910(+/- 10) A. They diffract to 15 to 18 A resolution at 4 degrees C and are stable in the synchrotron beam for several hours.

Undecagold clusters for site-specific labeling of biological macromolecules: simplified preparation and model applications

We report simple and rapid procedures for the synthesis of a variety of stable, water-soluble undecagold cluster, and model applications of a thiol-reactive gold cluster for the specific labeling of cysteine residues in proteins.