Coiled-coil packing in spermine-induced tropomyosin crystals. A comparative study of three forms

Characterization of Cep135, a novel coiled-coil centrosomal protein involved in microtubule organization in mammalian cells

By using monoclonal antibodies raised against isolated clam centrosomes, we have identified a novel 135-kD centrosomal protein (Cep135), present in a wide range of organisms. Cep135 is located at the centrosome throughout the cell cycle, and localization is independent of the microtubule network. It distributes throughout the centrosomal area in association with the electron-dense material surrounding centrioles. Sequence analysis of cDNA isolated from CHO cells predicted a protein of 1,145-amino acid residues with extensive alpha-helical domains. Expression of a series of deletion constructs revealed the presence of three independent centrosome-targeting domains. Overexpression of Cep135 resulted in the accumulation of unique whorl-like particles in both the centrosome and the cytoplasm. Although their size, shape, and number varied according to the level of protein expression, these whorls were composed of parallel dense lines arranged in a 6-nm space. Altered levels of Cep135 by protein overexpression and/or suppression of endogenous Cep135 by RNA interference caused disorganization of interphase and mitotic spindle microtubules. Thus, Cep135 may play an important role in the centrosomal function of organizing microtubules in mammalian cells.

Molecular polarity in tropomyosin-troponin T co-crystals

New features of the structure and interactions of troponin T and tropomyosin have been revealed by electron microscopy of so-called double-diamond co-crystals. These co-crystals were formed using rabbit alpha2 tropomyosin complexed with troponin T from either skeletal or cardiac muscle, which have different lengths in the amino-terminal region, as well as a bacterially expressed skeletal muscle troponin T fragment of 190 residues that lacks the amino-terminal region. Differences in the images of the co-crystals have allowed us to establish the polarities of both the troponin T subunit and tropomyosin in the projected lattice. Moreover, in agreement with their sequences, the amino-terminal region of a bovine cardiac muscle troponin T isoform appears to be longer than that from the rabbit skeletal muscle troponin T isoform and to span more of the amino terminus of tropomyosin at the head-to-tail filament joints. Images of crystals tilted relative to the electron beam also reveal the supercoiling of the tropomyosin filaments in this lattice. Based on these results, a three-dimensional model of the double-diamond lattice has been constructed.

Reconstitution of bacteriorhodopsin from a mixture of a proteinase V8 fragment and two synthetic peptides

The peptide and retinal mixture of bacteriorhodopsin, composed of two synthetic peptides corresponding to helices F (160-197) and G (202-237) and a proteinase V8-derived fragment V1 (1-166), generated the characteristic features of bacteriorhodopsin with absorbance maximum at 550 nm and fluorescence quenching as in two synthetic peptides corresponded to helix A (sequence 7-31) and B (41-65) and a chymotryptic fragment (72-248). The recovery of reconstitution estimated from the absorption and the fluorescence quenching of these mixture was 16-19% and 25-32% of the native purple membrane, respectively, whereas mixtures lacking any one of the peptides exhibited no absorption recovery Circular dichroism of each peptide fragment showed complete formation of alpha-helical structure in a membrane-mimetic medium of sodium dodecyl sulfate. These results indicate that the specific interactions or mutual recognitions between alpha-helices in lipid bilayers are essential for correct bundling of the seven helices and formation of the retinal binding pocket.