Rheological spectroscopy of concentrated colloidal dispersions

A new instrument, the virtual gap rheometer (VGR) for the determination of dynamic shear moduli (and mechanical relaxation spectra therefrom) at frequencies above 100 Hz is described. Several features of the VGR which make it particularly suited to the study of concentrated viscoelastic colloidal dispersions are discussed. The underlying theory is presented and the instrument's operation illustrated with results obtained on aqueous dispersions of a model clay colloid

SecYEG assembles into a tetramer to form the active protein translocation channel

Translocase mediates preprotein translocation across the Escherichia coli inner membrane. It consists of the SecYEG integral membrane protein complex and the peripheral ATPase SecA. Here we show by functional assays, negative-stain electron microscopy and mass measurements with the scanning transmission microscope that SecA recruits SecYEG complexes to form the active translocation channel. The active assembly of SecYEG has a side length of 10.5 nm and exhibits an ∼5 nm central cavity. The mass and structure of this SecYEG as well as the subunit stoichiometry of SecA and SecY in a soluble translocase–precursor complex reveal that translocase consists of the SecA homodimer and four SecYEG complexes