The essential role of phosphatidylglycerol in photosynthesis

Since the first identification of phosphatidylglycerol in Scenedesmus by Benson and Maruo in 1958, researchers have studied many biological functions of this phospholipid. Genetic, biochemical, and structural studies of photosynthetic organisms have revealed that phosphatidylglycerol is crucial to the photosynthetic transport of electrons, the development of chloroplasts, and tolerance to chilling. In this review, we summarize our present understanding of the biochemical and physiological functions of phosphatidylglycerol in cyanobacteria and higher plants. Submitted to the special issue in honor of Andrew A. Benson     

Thylakoid membrane landscape in the sixties: a tribute to Andrew Benson

Prior to the 1960s, the model for the molecular structure of cell membranes consisted of a lipid bilayer held in place by a thin film of electrostatically-associated protein stretched over the bilayer surface: (the Danielli–Davson–Robertson “unit membrane” model). Andrew Benson, an expert in the lipids of chloroplast thylakoid membranes, questioned the relevance of the unit membrane model for biological membranes, especially for thylakoid membranes, instead of emphasizing evidence in favour of hydrophobic interactions of membrane lipids within complementary hydrophobic regions of membrane-spanning proteins. With Elliot Weier, Benson postulated a remarkable subunit lipoprotein monolayer model for thylakoids. Following the advent of freeze fracture microscopy and the fluid lipid-protein mosaic model by Singer and Nicolson, the subunits, membrane-spanning integral proteins, span a dynamic lipid bilayer. Now that high resolution X-ray structures of photosystems I and II are being revealed, the seminal contribution of Andrew Benson can be appreciated.     

Forest Succession in Tropical Hardwood Hammocks of the Florida Keys: Effects of Direct Mortality from Hurricane Andrew1

A tree species replacement sequence for dry broadleaved forests (tropical hardwood hammocks) in the upper Florida Keys was inferred from species abundances in stands abandoned from agriculture or other anthropogenic acitivities at different times in the past. Stands were sampled soon after Hurricane Andrew, with live and hurricane‐killed trees recorded separately; thus it was also possible to assess the immediate effect of Hurricane Andrew on stand successional status. We used weighted averaging regression to calculate successional age optima and tolerances for all species, based on the species composition of the pre‐hurricane stands. Then we used weighted averaging calibration to calculate and compare inferred successional ages for stands based on (1) the species composition of the pre‐hurricane stands and (2) the hurricane‐killed species assemblages. Species characteristic of the earliest stages of post‐agricultural stand development remains a significant component of the forest for many years, but are gradually replaced by taxa not present, even as seedlings, during the first few decades. This compositional sequence of a century or more is characterized by the replacement of deciduous by evergreen species, which is hypothesized to be driven by increasing moisture storage capacity in the young organic soils. Mortality from Hurricane Andrew was concentrated among early‐successional species, thus tending to amplify the long‐term trend in species composition.     

Antiprogestational agents. The synthesis of 7-alkyl steroidal ketones with anti-implantational and antidecidual activity

A series of 7α- and 7β- alkyl derivatives of steroidal 4-en- and 5-en-3-ones were prepared by 1,6-conjugate addition of organocopper reagents to various steroidal 4,6-dien-3-ones of the androstane, estrane and gonane series. Biological study of these and related compounds revealed that 17β-hydroxy-7α-methyl-5-androsten-3-one (2), 17β-hydroxy-7α-methyl-5-estren-3-one acetate and 17β-hydroxy-7α-methyl-4-estren-3-one acetate had significant anti-implantational and antidecidual activities. The contragestative effects were associated with the latter antihormonal properties, and not with the androgenicity of these compounds.     

Sensitivity of pigmented mucosa and skin to freezing injury.

Pigmented areas of canine skin and oral mucosa were subjected to freezing in situ at various temperatures for the purpose of investigating possible differences in the sensitivity of epidermal cells to cold injury. Destruction of melanocytes occurred in the range of ?4 to ?7 °C, while squamous cells resisted injury even at ?20 °C. Replacement of the lost pigmented cells occurred from the normal tissue at the periphery of the injured area. The experiments suggest that selective destruction of pigmented lesions in clinical conditions may be achieved by freezing tissue to ?4 to ?7 °C or colder, but not to exceed ?20 °C in order to avoid destruction of squamous cells. The experiments also support wider use of cryosurgery for pigmented lesions of the skin and oral cavity.     

Cryptic Disorder Out of Disorder: Encounter between Conditionally Disordered CP12 and Glyceraldehyde-3-Phosphate Dehydrogenase

Among intrinsically disordered proteins, conditionally disordered proteins undergo dramatic structural disorder rearrangements upon environmental changes and/or post-translational modifications that directly modulate their function. Quantifying the dynamics of these fluctuating proteins is extremely challenging but paramount to understanding the regulation of their function. The chloroplast protein CP12 is a model of such proteins and acts as a redox switch by formation/disruption of its two disulfide bridges. It regulates the Calvin cycle by forming, in oxidized conditions, a supramolecular complex with glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and then phosphoribulokinase. In this complex, both enzymes are inactive. The highly dynamic nature of CP12 has so far hindered structural characterization explaining its mode of action. Thanks to a synergistic combination of small-angle X-ray scattering, nuclear magnetic resonance and circular dichroism that drove the molecular modeling of structural ensembles, we deciphered the structural behavior of Chlamydomonas reinhardtii oxidized CP12 alone and in the presence of GAPDH. Contrary to sequence-based structural predictions, the N-terminal region is unstable, oscillates at the ms timescale between helical and random conformations, and is connected through a disordered linker to its C-terminus, which forms a stable helical turn. Upon binding to GAPDH, oxidized CP12 undergoes an induced unfolding of its N-terminus. This phenomenon called cryptic disorder contributes to decrease the entropy cost and explains CP12 unusual high affinity for its partners.     

Combination Treatment with the GSK-3 Inhibitor 9-ING-41 and CCNU Cures Orthotopic Chemoresistant Glioblastoma in Patient-Derived Xenograft Models

Resistance to chemotherapy remains a major challenge in the treatment of human glioblastoma (GBM). Glycogen synthase kinase-3β (GSK-3β), a positive regulator of NF-κB–mediated survival and chemoresistance of cancer cells, has been identified as a potential therapeutic target in human GBM. Our objective was to determine the antitumor effect of GSK-3 inhibitor 9-ING-41 in combination with chemotherapy in patient-derived xenograft (PDX) models of human GBM. We utilized chemoresistant PDX models of GBM, GBM6 and GBM12, to study the effect of 9-ING-41 used alone and in combination with chemotherapy on tumor progression and survival. GBM6 and GBM12 were transfected by reporter constructs to enable bioluminescence imaging, which was used to stage animals prior to treatment and to follow intracranial GBM tumor growth. Immunohistochemical staining, apoptosis assay, and immunoblotting were used to assess the expression of GSK-3β and the effects of treatment in these models. We found that 9-ING-41 significantly enhanced 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) antitumor activity in staged orthotopic GBM12 (no response to CCNU) and GBM6 (partial response to CCNU) PDX models, as indicated by a decrease in tumor bioluminescence in mouse brain and a significant increase in overall survival. Treatment with the combination of CCNU and 9-ING-41 resulted in histologically confirmed cures in these studies. Our results demonstrate that the GSK-3 inhibitor 9-ING-41, a clinical candidate currently in Investigational New Drug (IND)-enabling development, significantly enhances the efficacy of CCNU therapy for human GBM and warrants consideration for clinical evaluation in this difficult-to-treat patient population.     

Mapping the carbon reduction cycle: a personal retrospective

The photosynthetic carbon reduction cycle was elucidated through the use of ¹⁴CO₂ during photosynthesis to label metabolic intermediates. Mapping and proof of the cycle required identification of labeled metabolites, observation of changes in levels of labeled metabolites during transitions from light to dark and from high to low CO₂ levels, determination of intramolecular distribution of ¹⁴C within the metabolites after a few seconds of photosynthesis with ¹⁴CO₂, and estimation of metabolite concentrations, used to calculate true free energy changes at each step in the cycle. This revised version was published online in August 2006 with corrections to the Cover Date.     

A generalized computational framework to streamline thermodynamics and kinetics analysis of metabolic pathways

Metabolic engineering is a critical biotechnological approach in addressing global energy and environment challenges. Most engineering efforts, however, consist of laborious and inefficient trial-and-error of target pathways, due in part to the lack of methodologies that can comprehensively assess pathway properties in thermodynamics and kinetics. Metabolic engineering can benefit from computational tools that evaluate feasibility, expense and stability of non-natural metabolic pathways. Such tools can also help us understand natural pathways and their regulation at systems level. Here we introduce a computational toolbox, PathParser, which, for the first time, integrates multiple important functions for pathway analysis including thermodynamics analysis, kinetics-based protein cost optimization and robustness analysis. Specifically, PathParser enables optimization of the driving force of a pathway by minimizing the Gibbs free energy of least thermodynamically favorable reaction. In addition, based on reaction thermodynamics and enzyme kinetics, it can compute the minimal enzyme protein cost that supports metabolic flux, and evaluate pathway stability and flux in response to enzyme concentration perturbations. In a demo analysis of the Calvin–Benson–Bassham cycle and photorespiration pathway in the model cyanobacterium Synechocystis PCC 6803, the computation results are corroborated by experimental proteomics data as well as metabolic engineering outcomes. This toolbox may have broad application in metabolic engineering and systems biology in other microbial systems.     

Photosynthesis in non‐foliar tissues: implications for yield

Photosynthesis is currently a focus for crop improvement. The majority of this work has taken place and been assessed in leaves, and limited consideration has been given to the contribution that other green tissues make to whole‐plant carbon assimilation. The major focus of this review is to evaluate the impact of non‐foliar photosynthesis on carbon‐use efficiency and total assimilation. Here we appraise and summarize past and current literature on the substantial contribution of different photosynthetically active organs and tissues to productivity in a variety of different plant types, with an emphasis on fruit and cereal crops. Previous studies provide evidence that non‐leaf photosynthesis could be an unexploited potential target for crop improvement. We also briefly examine the role of stomata in non‐foliar tissues, gas exchange, maintenance of optimal temperatures and thus photosynthesis. In the final section, we discuss possible opportunities to manipulate these processes and provide evidence that Triticum aestivum (wheat) plants genetically manipulated to increase leaf photosynthesis also displayed higher rates of ear assimilation, which translated to increased grain yield. By understanding these processes, we can start to provide insights into manipulating non‐foliar photosynthesis and stomatal behaviour to identify novel targets for exploitation in continuing breeding programmes.