Caulobacter crescentus bacteriophage phiCbK: structure and in vitro self-assembly of the tail

Electron micrographs of negatively stained and platinum-shadowed bacteriophage φCbK have been analyzed by optical diffraction and computer Fourier transformation. The results show that the phage tail is a helical “stacked disc” structure with an annular repeat of about 38 Å and with 3-fold rotational symmetry about the helix axis. Phage tails exhibited lateral and rotational flexibility and were found to possess variable helical parameters. The smaller angle of rotation about the helix axis between equivalent asymmetric units on adjacent discs measured from a number of tail images was found to have an average value of 41.5±0.9 °. Cross-sectional views of short tail fragments were obtained after sonication at 0 °C. These views confirmed the 3-fold symmetry of the 38 Å annular unit, which most probably consists of three identical subunits of the major tail protein. Formation of extended tail polymers, both linear and circular, was found to take place spontaneously in vitro after sonication. On the basis of these results, a low-resolution model for the tail helix is presented. The questions of head-tail symmetry mismatch in the phage and of tail length regulation are discussed.     

Bemerkungen zu Koenigs Schlufskritik

Ohne Zusammenfassung     

Subunit size and paracrystal structure of avian liver acetyl-CoA carboxylase.

The subunit molecular weight of chicken liver acetyl-CoA carboxylase has been redetermined by immunoprecipitation and sodium dodecyl sulfate gel electrophoresis. In the presence of parotid trypsin inhibitor, the immunoprecipitate gave a single band corresponding to a molecular weight of 230,000, which was also found to contain bound biotin. From the biotin content of the protomer (1.0 prosthetic group per 480,000 daltons) it appears that it consists of two non identical subunits, both with molecular weights of approximately 230,000.Electron microscopy has been carried out on the active filamentous form of the enzyme and on paracrystals formed under high-salt conditions. These indicate that the filaments are readily distortable helical ribbons, with an approximate axial repeat of 1100 Å, containing eight protomers. The paracrystals are made up of a staggered lateral packing of filaments.     

Berichtigungen zum vorigen Heft

Ohne Zusammenfassung     

Ueber Raben- und Nebelkr?he

Ohne Zusammenfassung     

Effect of filipin on liposomes prepared with different types of steroids

The action of the polyene antibiotic filipin on the permeability of liposomes prepared with lecithin and several plant and other steroids was studied. The effect of filipin was found to be dependent upon the type of steroid incorporated into the membrane. The interaction of filipin with steroids was found to be related both to the functional group at the 3-position and the aliphatic chain of the steroid. Complex formation of the steroid with filipin in aqueous solutions, as detected by ultraviolet spectrophotometry, correlated with the ability of filipin to increase the permeability of the liposomes.     

Outer membrane protein A of E. coli folds into detergent micelles, but not in the presence of monomeric detergent.

Outer membrane protein A (OmpA) of Escherichia coli is a beta-barrel membrane protein that unfolds in 8 M urea to a random coil. OmpA refolds upon urea dilution in the presence of certain detergents or lipids. To examine the minimal requirements for secondary and tertiary structure formation in beta-barrel membrane proteins, folding of OmpA was studied as a function of the hydrophobic chain length, the chemical structure of the polar headgroup, and the concentration of a large array of amphiphiles. OmpA folded in the presence of detergents only above a critical minimal chain length of the apolar chain as determined by circular dichroism spectroscopy and a SDS-PAGE assay that measures tertiary structure formation. Details of the chemical structure of the polar headgroup were unimportant for folding. The minimal chain length required for folding correlated with the critical micelle concentration in each detergent series. Therefore, OmpA requires preformed detergent micelles for folding and does not adsorb monomeric detergent to its perimeter after folding. Formation of secondary and tertiary structure is thermodynamically coupled and strictly dependent on the interaction with aggregated amphiphiles.