BEGIN:VCALENDAR VERSION:2.0 PRODID:-//CERN//INDICO//EN BEGIN:VEVENT SUMMARY:The structure and flexibility of the photosynthetic machinery in p lants and algal cells DTSTART;VALUE=DATE-TIME:20110714T120000Z DTEND;VALUE=DATE-TIME:20110714T123000Z DTSTAMP;VALUE=DATE-TIME:20260811T055222Z UID:indico-contribution-4891@events.saip.org.za DESCRIPTION:Speakers: Gyözö Garab (Biological Research Center\, Hung. Ac ad. Sci.)\nIn order to increase the efficiency of light capturing\, all ph otosynthetic organisms capable of oxygen evolution have evolved multilamel lar membrane systems of the thylakoid membranes\, flat closed bilayer lipi d vesicles\, which accommodate the densely packed protein complexes in ord ered\, often semi-crystalline arrays. This highly organized system with su bstantial stability exhibit remarkable structural and functional flexibili ty at different levels of complexity\, which enables these organisms to ca rry out rapid adaptations in response to changes in the environmental cond itions. In this talk\, my attention will be focused on the macro-organizat ion of the protein complexes - and their role in determining the multilame llar membrane ultrastructure\, and will show mechanisms that allow well id entifiable reversible reorganizations in the internal order of the complex es and in the membrane system. \nWe have shown that the main chlorophyll a /b light harvesting complexes of photosystem II (LHCII) form chirally orga nized macrodomains both in vivo and in vitro. These macrodomains\, togethe r with stacking\, play important roles in the lateral segregation (sorting ) of the two photosystems between the granum and stroma membranes\, and th us in the assembly and stabilization of the membrane ultrastructure [Must árdy and Garab\, 2003\, TIPS 8: 117]\, which has been determined by elect ron tomography [Mustárdy et al.\, 2008\, Plant Cell 20: 2552]. The macrod omains also possess a remarkable structural flexibility\, being capable of undergoing light-induced reversible reorganizations\, that are largely in dependent of the photochemical activity of thylakoids\, and are approximat ely linearly proportional to the light intensity above the saturation of p hotosynthesis – an important\, unique feature with respect to protection of plants against excess excitation [Barzda et al.\, 1996\, Biochemistry 35: 8981]. This type of reorganizations include (i) unstacking of membrane s\, (ii) a lateral desorganization of the macrodomains\, and (iii) monomer ization of the LHCII trimers [Dobrikova et al.\, 2003\, Biochemistry 42: 1 12726]. Isolated\, lipid-enriched\, loosely stacked lamellar aggregates of LHCII also possess the ability to undergo similar reorganizations\, accom panied by fluorescence quenching transients. These structural transitions are accounted for by a biological thermo-optic mechanism: fast thermal tra nsients\, arising from dissipated excitation energy\, which can lead to el ementary structural transitions in the close vicinity of the site of dissi pation due to the presence of ‘built-in’ thermal structure-instabiliti es [Cseh et al.\, 2005\, Photosynth Res 86: 263]. They lend local structur al flexibility to molecular (macro)assemblies of high stability\, and appe ar to be involved in important enzymatic reactions\, as revealed in other laboratories [Zer et al.\, 1999\, PNAS 96: 8277\, Yang et al.\, 2000\, FEB S Lett 466: 385]. The lipid content of the membranes is self regulated by non-bilayer lipids\, via their segregation capability. By this means they safe-guard the high protein content of the thylakoid membranes and\, at th e same time\, they contribute to the structural flexibility of the membran e system [Garab et al.\, 2000 Trends Plant Sci. 5:489\; Krumova et al.\, 2 008\, Biochim. Biophys. Acta\, Biomembranes 1778: 997].\nIn order to chara cterize the multilamellar membrane system\, we determined characteristic r epeat distances of the photosynthetic membranes in living cyanobacterial a nd eukaryotic algal cells and in intact thylakoid membranes isolated from higher plants with time-resolved small-angle neutron scattering (SANS). It has been shown how the different organization of multilamellar membrane s ystem can be correlated with different compositions and protein macro-orga nizations in different organisms. SANS also revealed small (~10 Å) but we ll identifiable light-induced reversible changes in these organisms\, obse rved for the first time in living cyanobacteria and diatom cells. These re organizations\, which could be recorded with time resolutions of several s econds and minutes\, appear to be associated with functional changes in vi vo [Nagy et al.\, 2011\, Biochem J. 436: 225].\n\nhttps://events.saip.org. za/event/7/contributions/4891/ LOCATION: Acro3 URL:https://events.saip.org.za/event/7/contributions/4891/ END:VEVENT END:VCALENDAR