Magainin 2 and PGLa in bacterial membrane mimics III : membrane fusion and disruption

We previously speculated that the synergistically enhanced antimicrobial activity of Magainin 2 and PGLa is related to membrane adhesion, fusion, and further membrane remodelling. Here, we combined computer simulations with time-resolved in vitro fluorescence microscopy, cryogenic electron microscopy (cryo-EM), and small-angle X-ray scattering (SAXS) to interrogate such morphological and topological changes of vesicles at nanoscopic and microscopic length scales in real time. Coarse-grained simulations revealed the formation of an elongated and bent fusion zone between vesicles in the presence of equimolar peptide mixtures. Vesicle adhesion and fusion was observed to occur within few seconds by cryo-EM and corroborated by SAXS measurements. The latter experiments further indicated continued and time-extended structural remodelling also for individual peptides or chemically-linked peptide heterodimers, but with different kinetics. Fluorescence microscopy further captured peptide-dependent adhesion, fusion, and occasional bursting of giant unilamellar vesicles already few seconds after peptide addition. The synergistic interactions between the peptides shorten the time response of vesicles and enhance membrane fusogenic and disrupting properties of the equimolar mixture compared to the individual peptides

Reaction center of photosystem II with no peripheral pigments in D2 allows secondary electron transfer in D1

El articulo está disponible en: http://pubs.acs.org/cgi-bin/article.cgi/bichaw/2007/46/i51/pdf/bi701440z.pdf. La editorial no permite la inclusión del articulo en un repositorio institucional, pero está disponible on-line para quien tenga acceso a la suscripción.A pigment-deficient reaction center of photosystem II (PSII)-with all the core pigments (two molecules of chlorophyll a and one of pheophytin a in each D protein) but with only one molecule each of peripheral chlorophyll a (Chlz) and beta-carotene (Car)-has been investigated by pump-probe spectroscopy. The data imply that Car and Chlz are both bound to D1. The absence of Car and Chlz in D2 allows the unprecedented observation of secondary electron transfer in D1 of PSII reaction centers at room temperature. The absorption band of the Car cation in D1 (Car(D1)(+·)) peaks around 910 nm (as against 990 nm for Car(D2)(+·)), and its positive hole is shared by ChlzD1, whereas Car(D2)(+·) can disappear by capturing an electron from ChlzD2.This work was funded by the Research Council of Norway, the Spanish Ministry of Science and Education (Grant No. BFU2004-04914-C02-02), the Czech Ministry of Education, Youth and Sports (Grant Nos. MSM6007665808 and AV0Z50510513), and the Spanish and Czech bilateral collaboration program (Grant No. CZ2004001).Peer reviewe

Trolox, a Water-Soluble Analogue of α-Tocopherol, Photoprotects the Surface-Exposed Regions of the Photosystem II Reaction Center in Vitro. Is This Physiologically Relevant?

Can Trolox, a water-soluble analogue of α-tocopherol and a scavenger of singlet oxygen (1O2), provide photoprotection, under high irradiance, to the isolated photosystem II (PSII) reaction center (RC)? To answer the question, we studied the endogenous production of 1O2 in preparations of the five-chlorophyll PSII RC (RC5) containing only one β-carotene molecule. The temporal profile of 1O2 emission at 1270 nm photogenerated by RC5 in D2O followed the expected biexponential behavior, with a rise time, unaffected by Trolox, of 13 ± 1 μs and decay times of 54 ± 2 μs (without Trolox) and 38 ± 2 μs (in the presence of 25 μM Trolox). The ratio between the total (kt) and chemical (kr) bimolecular rate constants for the scavenging of 1O2 by Trolox in aqueous buffer was calculated to be 1.3, with a kt of (2.4 ± 0.2) × 108 M–1 s–1 and a kr of (1.8 ± 0.2) × 108 M–1 s–1, indicating that most of the 1O2 photosensitized by methylene blue chemically reacts with Trolox in the assay buffer. The photoinduced oxygen consumption in the oxygen electrode, when RC5 and Trolox were mixed, revealed that Trolox was a better 1O2 scavenger than histidine and furfuryl alcohol at low concentrations (i.e., <1 mM). After its incorporation into detergent micelles in unbuffered solutions, Trolox was able to photoprotect the surface-exposed regions of the D1-D2 heterodimer, but not the RC5 pigments, which were oxidized, together with the membrane region of the protein matrix of the PSII RC, by 1O2. These results are discussed and compared with those of studies dealing with the physiological role of tocopherol molecules as a 1O2 scavenger in thylakoid membranes of photosynthetic organisms.This work was funded by the Research Council of Norway (Project 191102), the Spanish Ministry of Science and Innovation (Grant BFU2007-68107-C02-02/BMC), and the Czech Ministry of Education (Project AV0Z50510513). J.B.A. is very grateful to the CSIC mobility program for funding his short-term visit to the Norwegian University of Science and Technology (PA1002668).Peer reviewe