The role of non-bilayer lipids and non-lamellar lipid phases in biological membranes is an enigmatic problem of membrane biology. Non-bilayer lipids are present in large amounts in all membranes; in energy-converting membranes they constitute about half of their total lipid content-yet their functional state is a bilayer. In vitro experiments revealed that the functioning of the water-soluble violaxanthin de-epoxidase (VDE) enzyme of plant thylakoids requires the presence of a non-bilayer lipid phase. 31P-NMR spectroscopy has provided evidence on lipid polymorphism in functional thylakoid membranes. Here we reveal reversible pH- and temperature-dependent changes of the lipid-phase behaviour, particularly the flexibility of isotropic non-lamellar phases, of isolated spinach thylakoids. These reorganizations are accompanied by changes in the permeability and thermodynamic parameters of the membranes and appear to control the activity of VDE and the photoprotective mechanism of non-photochemical quenching of chlorophyll-a fluorescence. The data demonstrate, for the first time in native membranes, the modulation of the activity of a water-soluble enzyme by a non-bilayer lipid phase.

• MeSH
• diferenciální skenovací kalorimetrie MeSH
• epoxidové sloučeniny metabolismus   MeSH
• kinetika MeSH
• koncentrace vodíkových iontů MeSH
• lipidové dvojvrstvy chemie   MeSH
• lipidy chemie   MeSH
• magnetická rezonanční spektroskopie MeSH
• oxidoreduktasy metabolismus   MeSH
• rozpustnost MeSH
• Spinacia oleracea metabolismus   MeSH
• světlo MeSH
• teplota MeSH
• tylakoidy chemie   MeSH
• voda chemie   MeSH
• xanthofyly metabolismus   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• epoxidové sloučeniny MeSH
• lipidové dvojvrstvy MeSH
• lipidy MeSH
• oxidoreduktasy MeSH
• violaxanthin de-epoxidase MeSH Prohlížeč
• violaxanthin MeSH Prohlížeč
• voda MeSH
• xanthofyly MeSH

Build-up of the energized state of thylakoid membranes and the synthesis of ATP are warranted by organizing their bulk lipids into a bilayer. However, the major lipid species of these membranes, monogalactosyldiacylglycerol, is a non-bilayer lipid. It has also been documented that fully functional thylakoid membranes, in addition to the bilayer, contain an inverted hexagonal (HII) phase and two isotropic phases. To shed light on the origin of these non-lamellar phases, we performed 31P-NMR spectroscopy experiments on sub-chloroplast particles of spinach: stacked, granum and unstacked, stroma thylakoid membranes. These membranes exhibited similar lipid polymorphism as the whole thylakoids. Saturation transfer experiments, applying saturating pulses at characteristic frequencies at 5 °C, provided evidence for distinct lipid phases-with component spectra very similar to those derived from mathematical deconvolution of the 31P-NMR spectra. Wheat-germ lipase treatment of samples selectively eliminated the phases exhibiting sharp isotropic peaks, suggesting easier accessibility of these lipids compared to the bilayer and the HII phases. Gradually increasing lipid exchanges were observed between the bilayer and the two isotropic phases upon gradually elevating the temperature from 5 to 35 °C, suggesting close connections between these lipid phases. Data concerning the identity and structural and functional roles of different lipid phases will be presented in the accompanying paper.

• Klíčová slova
• 31P-NMR, DEM—dynamic exchange model, HII phase, bilayer membrane, grana, isotropic phase, non-bilayer lipids, non-lamellar lipid phases, structural flexibility, thylakoid membranes,
• MeSH
• chloroplasty chemie   MeSH
• galaktolipidy chemie   MeSH
• magnetická rezonanční spektroskopie metody   MeSH
• membránové lipidy chemie   MeSH
• teplota MeSH
• tylakoidy chemie   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• galaktolipidy MeSH
• membránové lipidy MeSH
• monogalactosyldiacylglycerol MeSH Prohlížeč

It has been thoroughly documented, by using 31P-NMR spectroscopy, that plant thylakoid membranes (TMs), in addition to the bilayer (or lamellar, L) phase, contain at least two isotropic (I) lipid phases and an inverted hexagonal (HII) phase. However, our knowledge concerning the structural and functional roles of the non-bilayer phases is still rudimentary. The objective of the present study is to elucidate the origin of I phases which have been hypothesized to arise, in part, from the fusion of TMs (Garab et al. 2022 Progr Lipid Res 101,163). We take advantage of the selectivity of wheat germ lipase (WGL) in eliminating the I phases of TMs (Dlouhý et al. 2022 Cells 11: 2681), and the tendency of the so-called BBY particles, stacked photosystem II (PSII) enriched membrane pairs of 300-500 nm in diameter, to form large laterally fused sheets (Dunahay et al. 1984 BBA 764: 179). Our 31P-NMR spectroscopy data show that BBY membranes contain L and I phases. Similar to TMs, WGL selectively eliminated the I phases, which at the same time exerted no effect on the molecular organization and functional activity of PSII membranes. As revealed by sucrose-density centrifugation, magnetic linear dichroism spectroscopy and scanning electron microscopy, WGL disassembled the large laterally fused sheets. These data provide direct experimental evidence on the involvement of I phase(s) in the fusion of stacked PSII membrane pairs, and strongly suggest the role of non-bilayer lipids in the self-assembly of the TM system.

• Klíčová slova
• 31P-NMR spectroscopy; BBY membrane, Linear dichroism spectroscopy, Membrane fusion; non-bilayer lipids, Wheat germ lipase,
• MeSH
• fotosystém II - proteinový komplex * metabolismus   MeSH
• fúze membrán fyziologie   MeSH
• lipidy chemie   MeSH
• magnetická rezonanční spektroskopie MeSH
• tylakoidy * metabolismus   MeSH
• Publikační typ
• časopisecké články MeSH
• Názvy látek
• fotosystém II - proteinový komplex * MeSH
• lipidy MeSH

In Part I, by using 31P-NMR spectroscopy, we have shown that isolated granum and stroma thylakoid membranes (TMs), in addition to the bilayer, display two isotropic phases and an inverted hexagonal (HII) phase; saturation transfer experiments and selective effects of lipase and thermal treatments have shown that these phases arise from distinct, yet interconnectable structural entities. To obtain information on the functional roles and origin of the different lipid phases, here we performed spectroscopic measurements and inspected the ultrastructure of these TM fragments. Circular dichroism, 77 K fluorescence emission spectroscopy, and variable chlorophyll-a fluorescence measurements revealed only minor lipase- or thermally induced changes in the photosynthetic machinery. Electrochromic absorbance transients showed that the TM fragments were re-sealed, and the vesicles largely retained their impermeabilities after lipase treatments-in line with the low susceptibility of the bilayer against the same treatment, as reflected by our 31P-NMR spectroscopy. Signatures of HII-phase could not be discerned with small-angle X-ray scattering-but traces of HII structures, without long-range order, were found by freeze-fracture electron microscopy (FF-EM) and cryo-electron tomography (CET). EM and CET images also revealed the presence of small vesicles and fusion of membrane particles, which might account for one of the isotropic phases. Interaction of VDE (violaxanthin de-epoxidase, detected by Western blot technique in both membrane fragments) with TM lipids might account for the other isotropic phase. In general, non-bilayer lipids are proposed to play role in the self-assembly of the highly organized yet dynamic TM network in chloroplasts.

• Klíčová slova
• SAXS, bilayer, chlorophyll fluorescence, cryo-electron-tomography, electron microscopy, membrane energization, membrane networks, non-bilayer lipid phases, violaxanthin de-epoxidase,
• MeSH
• cirkulární dichroismus metody   MeSH
• elektronová mikroskopie metody   MeSH
• fotosyntéza genetika   MeSH
• lipidy genetika   MeSH
• magnetická rezonanční spektroskopie metody   MeSH
• tylakoidy genetika   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• lipidy MeSH

It is well established that plant thylakoid membranes (TMs), in addition to a bilayer, contain two isotropic lipid phases and an inverted hexagonal (HII) phase. To elucidate the origin of non-bilayer lipid phases, we recorded the 31P-NMR spectra of isolated spinach plastoglobuli and TMs and tested their susceptibilities to lipases and proteases; the structural and functional characteristics of TMs were monitored using biophysical techniques and CN-PAGE. Phospholipase-A1 gradually destroyed all 31P-NMR-detectable lipid phases of isolated TMs, but the weak signal of isolated plastoglobuli was not affected. Parallel with the destabilization of their lamellar phase, TMs lost their impermeability; other effects, mainly on Photosystem-II, lagged behind the destruction of the original phases. Wheat-germ lipase selectively eliminated the isotropic phases but exerted little or no effect on the structural and functional parameters of TMs-indicating that the isotropic phases are located outside the protein-rich regions and might be involved in membrane fusion. Trypsin and Proteinase K selectively suppressed the HII phase-suggesting that a large fraction of TM lipids encapsulate stroma-side proteins or polypeptides. We conclude that-in line with the Dynamic Exchange Model-the non-bilayer lipid phases of TMs are found in subdomains separated from but interconnected with the bilayer accommodating the main components of the photosynthetic machinery.

• Klíčová slova
• 31P-NMR spectroscopy, lipid polymorphism, lipocalins, membrane fusion, membrane models, non-bilayer lipids, plastoglobuli, structural and functional plasticity, thylakoid membrane,
• MeSH
• lipasa metabolismus   MeSH
• lipidové dvojvrstvy * metabolismus   MeSH
• magnetická rezonanční spektroskopie MeSH
• proteasy metabolismus   MeSH
• tylakoidy * metabolismus   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• lipasa MeSH
• lipidové dvojvrstvy * MeSH
• proteasy MeSH

Chloroplast thylakoid membranes contain virtually all components of the energy-converting photosynthetic machinery. Their energized state, driving ATP synthesis, is enabled by the bilayer organization of the membrane. However, their most abundant lipid species is a non-bilayer-forming lipid, monogalactosyl-diacylglycerol; the role of lipid polymorphism in these membranes is poorly understood. Earlier 31P-NMR experiments revealed the coexistence of a bilayer and a non-bilayer, isotropic lipid phase in spinach thylakoids. Packing of lipid molecules, tested by fluorescence spectroscopy of the lipophilic dye, merocyanine-540 (MC540), also displayed heterogeneity. Now, our 31P-NMR experiments on spinach thylakoids uncover the presence of a bilayer and three non-bilayer lipid phases; time-resolved fluorescence spectroscopy of MC540 also reveals the presence of multiple lipidic environments. It is also shown by 31P-NMR that: (i) some lipid phases are sensitive to the osmolarity and ionic strength of the medium, (ii) a lipid phase can be modulated by catalytic hydrogenation of fatty acids and (iii) a marked increase of one of the non-bilayer phases upon lowering the pH of the medium is observed. These data provide additional experimental evidence for the polymorphism of lipid phases in thylakoids and suggest that non-bilayer phases play an active role in the structural dynamics of thylakoid membranes.

• MeSH
• fluorescenční spektrometrie * metody   MeSH
• izotopy fosforu * MeSH
• katalýza MeSH
• koncentrace vodíkových iontů MeSH
• lipidy chemie   MeSH
• magnetická rezonanční spektroskopie * metody   MeSH
• tylakoidy chemie   metabolismus   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• izotopy fosforu * MeSH
• lipidy MeSH