Phospholipid chain immobilization and steriod rotational immobilization in acetylcholine receptor-rich membranes from torpedo marmorata

1. The ESR spectra of both phosphatidylcholine and phosphatidylethanolamine spin labels reveal an immobilized lipid component (τ_R ? 50 ns), in addition to a fluid component (τ_R ～ 1 ns), in acetylcholine receptorrich membranes prepared from Torpedo marmorata electroplax according to the method of Cohen et al. (Cohen, J.B., Weber, M., Huchet, M. and Changeux, J.P. (1972) FEBS Lett. 26, 43–47). 2. The ESR spectra of the androstanol spin label display a component corresponding to molecules which are immobilized with respect to rotation about the long molecular axis (

), in addition to the fluid lipid bilayer component in which the molecules are rotating rapidly about their long axes (

). This immobilized component is observed throughout the temperature range 2–22°C, at an approximately constant relative intensity of approx. 45% of the total, which is quantitatively the same as previously observed with fatty acid spin labels.     

Interaction of a phosphorylated site of muscle phosphofructokinase with the activation of the enzyme by NH4+ and K+.1

The crystal structure of the cyclic peptide disulfide

has been determined by X-ray diffraction. The peptide crystallizes in the space group P2₁2₁2₁, with a = 8.646(1), b = 18.462(2), c = 19.678(3)Å and Z = 4. The molecules adopt a highly folded compact conformation, stabilized by two intramolecular 4→ 1 hydrogen bonds between the Cys (1) and Pro (2) CO groups and the Cys (4) and methylamide NH groups, respectively. The backbone conformational angles for the peptide lie very close to those expected for a 3₁₀ helix. The S-S bridge adopts a right handed twist with a dihedral angle of 82°. The structure illustrates the role of stereochemically constrained residues, in generating novel peptide conformations.     

The sulfhydryl groups of the thiol-dependent cytolytic toxin from Bacillus alvei evidence for one essential sulfhydryl group

Alveolysin, an extracellular protein toxin (M_r ? 63,000) excreted by Bacillus alvei and purified to homogeneity was shown to contain four cysteine residues. All thiol groups of the hemolytically active toxin preparation were free as found by direct titration by 5,5′-dithiobis (2-nitrobenzoic acid) and confirmed by the absence of disulfide bond. Toxin alkylation with tosyl lysine chloromethyl ketone resulted in the complete loss of hemolytic activity and the disappearance of only one thiol group with no modification of histidine residues. These results support the conclusion that one essential thiol group is implicated in the membrane-disrupting activity of alveolysin.     

A respiration-dependent primary sodium extrusion system functioning at alkaline pH in the marine bacterium Vibrioalginolyticus

The membrane potential generated at pH 8.5 by K⁺-depleted and Na⁺-loaded $\text{Vibrio}\text{alginolyticus}$ is not collapsed by proton conductors which, instead, induce the accumulation of protons in equilibrium with the membrane potential. The generation of such a membrane potential and the accumulation of protons are specific to Na⁺-loaded cells at alkaline pH and are dependent on respiration. Extrusion of Na⁺ at pH 8.5 occurs in the presence of proton conductors unless respiration is inhibited while it is abolished by proton conductors at acidic pH. The uptake of α-aminoisobutyric acid, which is driven by the Na⁺-electrochemical gradient, is observed even in the presence of proton conductors at pH 8.5 but not at acidic pH. We conclude that a respiration-dependent primary electrogenic Na⁺ extrusion system is functioning at alkaline pH to generate the proton conductor-insensitive membrane potential and Na⁺ chemical gradient.     

Aggregation of human lymphoblastoid cells by tumor-promoting phorbol esters and dihydroteleocidin B

Human lymphoblastoid cells transformed by Epstein-Barr virus aggregated rapidly in the presence of tumor-promoting phorbol esters and dihydroteleocidin B. Cell aggregation was almost complete after incubation for 6 hours. In amounts of a few ng, they induced significant aggregation. Their abilities to aggregate cells could be measured quantitatively and correlated well with their effects in promoting skin tumors.