Assembly Of Photosynthetic Units Research Articles

Structural and Functional Proteomics of Intracytoplasmic Membrane Assembly in Rhodobacter sphaeroides

The results of a detailed structural and functional proteomic analysis of intracytoplasmic membrane (ICM) assembly in the model purple phototrophic bacterium Rhodobacter sphaeroides are reviewed in this report. Proteomics approaches have focused upon identification of membrane proteins temporally expressed during ICM development and spatially localized within the internal cell membranes, together with their structural and functional correlates. For the examination of temporal protein expression, procedures were established for the induction of ICM formation at low oxygen tension and for ICM remodeling in cells adapting to low intensity illumination, which permitted isolation by rate-zone sedimentation of ICM growth initiation sites (CM invaginations) in an upper-pigmented band (UPB), together with more mature ICM vesicles (chromatophores) as the main band. Nondenaturing clear native gel electrophoresis of the chromatophore fraction gave rise to four pigmented bands: the top and bottom bands contained the reaction center-light-harvesting 1 (RC-LH1) core complex and the LH2 peripheral antenna, respectively, while two bands of intermediate migration exhibited distinct associations of LH2 and core complexes. Proteomic analysis of the gel bands revealed developmental changes including increasing levels of LH2 polypeptides relative to those of core complexes as ICM development proceeded, as well as a large array of other associated proteins including high spectral counts for the F₁F_O-ATP synthase subunits, and the cytochrome bc₁ complex. High counts were also observed for RSP6124, a protein of unknown function, that were correlated with increasing LH2 levels. RC-LH1-containing clear native electrophoresis gel bands from the UPB were enriched in cytoplasmic membrane (CM) markers, including electron transfer and transport proteins, as well as general membrane assembly factors (viz., preprotein translocases YidC, YajC and SecY, bacterial type 1 signal peptidase and twin arg translocation subunit TatA), thereby confirming the origin of the UPB from both peripheral respiratory membrane and sites of active CM invagination in which preferential assembly of the RC-LH1 complex occurs. Functional aspects of the photosynthetic unit assembly process were monitored by fluorescence induction/relaxation measurements of the variable fluorescence arising from LH-bacteriochlorophyll a. Slowing of the rate of RC electron transfer turnover (τ_QA), as assessed from the relaxation phase, was correlated with the growth of the functional absorption cross section (σ) and LH2/LH1 molar ratios. This is thought to arise from the imposition of constraints upon free diffusion of ubiquinone (UQ) redox species between the RC and cytochrome bc₁ complex as the ICM bilayer becomes densely packed with LH2 rings. Such LH2 packing was confirmed in a comparison by high-resolution atomic force microscopy of ICM patches from cells grown at high and low light intensity [Adams and Hunter: Biochim Biophys Acta 2012;1817:1616-1627], in which the increasing LH2 levels form densely packed LH2-only domains, representing the light-responsive antenna complement arising under low illumination. In contrast, LH2 is initially dispersed in rows and small cluster-separating linear arrays of largely dimeric RC-LH1 core complexes, which become filled with LH2 during acclimation to reduced light intensity. In phototrophically grown cells that were transferred to oxic conditions in the dark, fluorescence induction/relaxation measurements showed that despite a growth burst independent of photosynthetic pathways, functional photosynthetic units were maintained for up to 24 h after the transition. The τ_QA was accelerated from ∼1 to 0.5 ms by 8 h, reflecting the decrease in LH2 levels, facilitating more rapid UQ redox species diffusion in the membrane bilayer as crowding by LH2 is overcome. Under these circumstances, UPB levels were elevated with significant increases in LH1/LH2 molar ratio. These changes indicate that vesiculation of CM growth initiation sites to form vesicular ICM was arrested under oxic conditions.

The Accumulation of the Light-Harvesting 2 Complex during Remodeling of the Rhodobacter sphaeroides Intracytoplasmic Membrane Results in a Slowing of the Electron Transfer Turnover Rate of Photochemical Reaction Centers

A functional proteomic analysis of the intracytoplasmic membrane (ICM) development process was performed in Rhodobacter sphaeroides during adaptation from high-intensity illumination to indirect diffuse light. This initiated an accelerated synthesis of the peripheral light-harvesting 2 (LH2) complex relative to that of LH1-reaction center (RC) core particles. After 11 days, ICM vesicles (chromatophores) and membrane invagination sites were isolated by rate-zone sedimentation and subjected to clear native gel electrophoresis. Proteomic analysis of gel bands containing the RC-LH1 and -LH2 complexes from digitonin-solubilized chromatophores revealed high levels of comigrating electron transfer enzymes, transport proteins, and membrane assembly factors relative to their equivalent gel bands from cells undergoing adaptation to direct low-level illumination. The GroEL chaperonin accounted for >65% of the spectral counts in the RC-LH1 band from membrane invagination sites, which together with the appearance of a universal stress protein suggested that the viability of these cells was challenged by light limitation. Functional aspects of the photosynthetic unit assembly process were monitored by near-IR fast repetition rate analysis of variable fluorescence arising from LH-bacteriochlorophyll a components. The quantum yield of the primary charge separation during the early stages of adaptation showed a gradual increase (variable/maximal fluorescence = 0.78-0.83 between 0 and 4 h), while the initial value of ~70 for the functional absorption cross section (σ) gradually increased to 130 over 4 days. These dramatic σ increases showed a direct relation to gradual slowing of the RC electron transport turnover rate (τ(QA)) from ~1.6 to 6.4 ms and an ~3-fold slowing of the rate of reoxidation of the ubiquinone pool. These slowed rates are not due to changes in UQ pool size, suggesting that the relation between increasing σ and τ(QA) reflects the imposition of constraints upon free diffusion of ubiquinone redox species between the RC and cytochrome bc(1) complex as the membrane bilayer becomes densely packed with LH2 rings.

Sequential assembly of photosynthetic units in Rhodobacter sphaeroides as revealed by fast repetition rate analysis of variable bacteriochlorophyll a fluorescence

The development of functional photosynthetic units in Rhodobacter sphaeroides was followed by near infra-red fast repetition rate (IRFRR) fluorescence measurements that were correlated to absorption spectroscopy, electron microscopy and pigment analyses. To induce the formation of intracytoplasmic membranes (ICM) (greening), cells grown aerobically both in batch culture and in a carbon-limited chemostat were transferred to semiaerobic conditions. In both aerobic cultures, a low level of photosynthetic complexes was observed, which were composed of the reaction center and the LH1 core antenna. Interestingly, in the batch cultures the reaction centers were essentially inactive in forward electron transfer and exhibited low photochemical yields F V/ F M, whereas the chemostat culture displayed functional reaction centers with a rather rapid (1–2 ms) electron transfer turnover, as well as a high F V/ F M of ∼0.8. In both cases, the transfer to semiaerobiosis resulted in rapid induction of bacteriochlorophyll a synthesis that was reflected by both an increase in the number of LH1–reaction center and peripheral LH2 antenna complexes. These studies establish that photosynthetic units are assembled in a sequential manner, where the appearance of the LH1–reaction center cores is followed by the activation of functional electron transfer, and finally by the accumulation of the LH2 complexes.

Transiently expressed early light-inducible thylakoid proteins share transmembrane domains with light-harvesting chlorophyll binding proteins

The early light-inducible proteins (ELIPs) of barley chloroplasts are encoded in two multigene families yielding end products of different molecular mass. Sequencing of complete cDNA clones showed that the low and high molecular mass proteins differ by the presence or absence of a 65 amino acid peptide in the amino-terminal part of the mature proteins. Two domains of the ELIPs reveal striking similarity in amino acid sequence with two transmembrane domains of all known light-harvesting chlorophyll a/b-binding proteins from photosystem I and II and may be of importance in anchoring the polypeptides in the membrane. The cDNA sequences of two low molecular mass ELIPs differ by an insert of 5 codons in the putative transit peptide. By in vitro transcription and translation of the cloned DNA and subsequent transport of the products into chloroplasts it could be established that the precursors are processed into products of identical apparent molecular mass. In vitro translated ELIPs were incorporated into thylakoid membranes both as precursors and mature polypeptides. It is suggested that ELIPs are pigment-free substitutes for light-harvesting polypeptides in the assembly of photosynthetic units during early development of thylakoids.

Role of apparent membrane growth initiation sites during photosynthetic membrane development in synchronously dividing Rhodopseudomonas sphaeroides.

Sites of intracytoplasmic membrane growth and temporal relations in the assembly of photosynthetic units were examined in synchronously dividing Rhodopseudomonas sphaeroides cells. After rate-zone sedimentation of cell-free extracts, apparent sites of initiation of intracytoplasmic membrane growth formed an upper pigmented band that sedimented more slowly than the intracytoplasmic membrane-derived chromatophore fraction. Throughout the cell cycle, the levels of the peripheral B800-850 light-harvesting pigment-protein complex relative to those of the core B875 complex in the upper pigmented fraction were only about half those of chromatophores. Pulse-labeling studies with L-[35S]methionine indicated that the rates of assembly of proteins in the upper pigmented fraction were much higher than those of chromatophores throughout the cell cycle; rates for the reaction center polypeptides were estimated to be approximately 3.5-fold higher than in chromatophores when the two membrane fractions were equalized on a protein basis. In pulse-chase studies, radioactivity of the reaction center and B875 polypeptides increased significantly in chromatophores and decreased in the upper pigmented band during cell division. These data suggest that the B875 reaction center cores of the photosynthetic units are inserted preferentially into sites of membrane growth initiation isolated in the upper pigmented band and that the incomplete photosynthetic units are transferred from their sites of assembly into the intracytoplasmic membrane during cell division. These results suggested further that B800-850 is added directly to the intracytoplasmic membrane throughout the cell cycle.

Linear dichroism and fluorescence emission of antenna complexes during photosynthetic unit assembly in Rhodopseudomonas sphaeroides

Photosynthetic membrane formation was induced in a nonpigmented, aerobic culture of Rhodopseudomonas sphaeroides. During the course of the experiment, the bacteriochlorophyll content of the cells increased approx. 40-fold [1]. Membranes prepared from pigmenting cells were analysed by fluorescence emission and excitation spectroscopy at 4 K; at all stages efficient energy transfer was observed between B850 and B875 bacteriochlorophylls within the light-harvesting system [2]. A light-harvesting domain containing several interconnected traps (reaction centres) and about 200 B875 bacteriochlorophylls was present in membranes from cells grown under high aeration. Over the course of the experiment the size of this domain increases 5-fold in intracytoplasmic membranes, with the result that the total light-harvesting network is composed of 3000 bacteriochlorophyll molecules; these surround and interconnect approx. 30 reaction centres. We estimate the average reaction centre-reaction centre distance in mature intracystoplasmic membranes to be about 15 nm and the average bacteriochlorophyll-bacteriochlorophyll distance to be about 1.5 nm [3]. An examination of the linear dichroism properties of stretched pigmenting membranes demonstrated that all orientations of pigments are maintained during development, the orientation of the B875 absorbance band being strongly parallel to the stretch axis. The expansion of the photosynthetic unit during development appears to involve the addition of B800–850 antenna to form increasingly random aggregates around highly ordered B875 antenna complexes.