Transport and membrane integration of polypeptides is carried out by specific protein complexes in the membranes of all living cells. The Sec transport path provides an essential and ubiquitous route for protein translocation. In the bacterial cytoplasmic membrane, the channel is formed by oligomers of a heterotrimeric membrane protein complex consisting of subunits SecY, SecE and SecG. In the endoplasmic reticulum membrane, the channel is formed from the related Sec61 complex. Here we report the structure of the Escherichia coli SecYEG assembly at an in-plane resolution of 8 A. The three-dimensional map, calculated from two-dimensional SecYEG crystals, reveals a sandwich of two membranes interacting through the extensive cytoplasmic domains. Each membrane is composed of dimers of SecYEG. The monomeric complex contains 15 transmembrane helices. In the centre of the dimer we observe a 16 x 25 A cavity closed on the periplasmic side by two highly tilted transmembrane helices. This may represent the closed state of the protein-conducting channel.

Absorption of light in excess of the capacity for photosynthetic electron transport is damaging to photosynthetic organisms. Several mechanisms exist to avoid photodamage, which are collectively referred to as nonphotochemical quenching. This term comprises at least two major processes. State transitions (qT) represent changes in the relative antenna sizes of photosystems II and I. High energy quenching (qE) is the increased thermal dissipation of light energy triggered by lumen acidification. To investigate the respective roles of qE and qT in photoprotection, a mutant (npq4 stt7-9) was generated in Chlamydomonas reinhardtii by crossing the state transition-deficient mutant (stt7-9) with a strain having a largely reduced qE capacity (npq4). The comparative phenotypic analysis of the wild type, single mutants, and double mutants reveals that both state transitions and qE are induced by high light. Moreover, the double mutant exhibits an increased photosensitivity with respect to the single mutants and the wild type. Therefore, we suggest that besides qE, state transitions also play a photoprotective role during high light acclimation of the cells, most likely by decreasing hydrogen peroxide production. These results are discussed in terms of the relative photoprotective benefit related to thermal dissipation of excess light and/ or to the physical displacement of antennas from photosystem II.

A protocol has been developed for the purification of the cytochrome b 6 f complex from the unicellular alga Chlamydomonas reinhardtii. It is based on the use of the neutral detergent Hecameg (6-O-(N-heptylcarbamoyl)-methyl-␣-D-glycopyranoside) and comprises only three steps: selective solubilization from thylakoid membranes, sucrose gradient sedimentation, and hydroxylapatite chromatography. The purified complex contains two b hemes (␣ bands, 564 nm; E m,8 ‫؍‬ ؊84 and ؊158 mV) and one chlorophyll a ( max ‫؍‬ 667-668 nm) per cytochrome f (␣ band, 554 nm; E m,8 ‫؍‬ ؉330 mV). It is highly active in transferring electrons from decylplastoquinol to oxidized plastocyanin (turnover number 250 -300 s ؊1 ). The purified complex contains seven subunits, whose identity has been established by N-terminal sequencing and/or peptide-specific immunolabeling, namely four high molecular weight subunits (cytochrome f, Rieske iron-sulfur protein, cytochrome b 6 , and subunit IV) and three ϳ4-kDa miniproteins (PetG, PetL, and PetX). Stoichiometry measurements are consistent with every subunit being present as two copies per b 6 f dimer.The photosynthetic electron transfer chain, the device that provides essentially all of the free energy dissipated by living beings, comprises two membrane protein complexes that are homologous to complexes of the respiratory chain, the CF 1 CF 0 (respectively, F 1 F 0 ) proton ATP-synthase and the b 6 f (respectively, bc 1 ) oxidoreductase. The bf/bc complexes transfer electrons from a liposoluble quinol to a hydrosoluble protein, plastocyanin or cytochrome c, and couple the resulting electron free energy drop to setting up a transmembrane proton electrochemical potential. Neither the structure of the b 6 f nor that of the bc 1 complex is known in any detail, and the existence of an unique versus alternative mechanism(s) of electron transfer remains a matter of discussion (see, e.g., Refs.

The molecular weight of the cytochrome b 6 f complex purified from Chlamydomonas reinhardtii thylakoid membranes has been determined by combining velocity sedimentation measurements, molecular sieving analyses, and determination of its lipid and detergent content. The complex in its enzymatically active form is a dimer. Upon incubation in detergent solution, it converts irreversibly into an inactive, monomeric form that has lost the Rieske iron-sulfur protein, the b 6 f-associated chlorophyll, and, under certain conditions, the small 32-residue subunit PetL. The results are consistent with the view that the dimer is the predominant form of the b 6 f in situ while the monomer observed in detergent solution is a breakdown product. Indirect observations suggest that subunit PetL plays a role in stabilizing the dimeric state. Delipidation is shown to be a critical factor in detergent-induced monomerization.

In photosynthesis, electron transfer along the photosynthetic chain results in a vectorial transfer of protons from the stroma to the lumen space of the thylakoids.This promotes the generation of an electrochemical proton gradient (Δμ H + ), which comprises a gradient of electric potential (ΔΨ) and of proton concentration (ΔpH).The Δμ H + has a central role in the photosynthetic process, providing the energy source for ATP synthesis, and being involved in many regulatory mechanisms. The pH modulates the rate of electron transfer and triggers de excitation of excess light within the light harvesting complexes. The ΔΨ is required for metabolites and protein transport across the membranes. Its presence also induces a shift in the absorption spectra of some photosynthetic pigments, resulting in the so called Electro Chromic Shift (ECS). In this review, we discuss the characteristics features of the ECS, and illustrate possible applications of it for the study of photosynthetic processes in vivo.

The vast majority of phages, bacterial viruses, possess a tail ensuring host recognition, cell wall perforation and safe viral DNA transfer from the capsid to the host cytoplasm. Long flexible tails are formed from the tail tube protein (TTP) polymerised as hexameric rings around and stacked along the tape measure protein (TMP). Here, we report the crystal structure of T5 TTP pb6 at 2.2 Å resolution. Pb6 is unusual in forming a trimeric ring, although structure analysis reveals homology with all classical TTPs and related tube proteins of bacterial puncturing devices (type VI secretion system and R-pyocin). Structures of T5 tail tubes before and after interaction with the host receptor were determined by cryo-electron microscopy at 6 Å resolution. Comparison of these two structures reveals that host-binding information is not propagated to the capsid through conformational changes in the tail tube, suggesting a role of the TMP in this information transduction process.

Highly purified preparations of cytochrome b 6 f complex from the unicellar freshwater alga Chlamydomonas reinhardtii contain about 1 molecule of chlorophyll a/ cytochrome f. Several lines of evidence indicate that the chlorophyll is an authentic component of the complex rather than a contaminant. In particular, (i) the stoichiometry is constant; (ii) the chlorophyll is associated with the complex at a specific binding site, as evidenced by resonance Raman spectroscopy; (iii) it does not originate from free chlorophyll released from thylakoid membranes upon solubilization; and (iv) its rate of exchange with free, radioactive chlorophyll a is extremely slow (weeks

Adaptation of photosynthesis in marine environment has been examined in two strains of the green, picoeukaryote Ostreococcus: OTH95, a surface/high-light strain, and RCC809, a deep-sea/lowlight strain. Differences between the two strains include changes in the light-harvesting capacity, which is lower in OTH95, and in the photoprotection capacity, which is enhanced in OTH95. Furthermore, RCC809 has a reduced maximum rate of O2 evolution, which is limited by its decreased photosystem I (PSI) level, a possible adaptation to Fe limitation in the open oceans. This decrease is, however, accompanied by a substantial rerouting of the electron flow to establish an H2O-to-H2O cycle, involving PSII and a potential plastid plastoquinol terminal oxidase. This pathway bypasses electron transfer through the cytochrome b6f complex and allows the pumping of ''extra'' protons into the thylakoid lumen. By promoting the generation of a large ⌬pH, it facilitates ATP synthesis and nonphotochemical quenching when RCC809 cells are exposed to excess excitation energy. We propose that the diversion of electrons to oxygen downstream of PSII, but before PSI, reflects a common and compulsory strategy in marine phytoplankton to bypass the constraints imposed by light and/or nutrient limitation and allow successful colonization of the open-ocean marine environment. marine environment ͉ PTOX ͉ water cycle ͉ eletron flow ͉ photoprotection