Abstract
It is increasingly apparent that the components involved in photosynthetic electron transport are distributed with a pronounced lateral assymettry within the thylakoid membrane system (Barber 1983). Photosystem two (PS2) complexes and the light harvesting chlorophyll a/b protein (LHCP) seem to be localized in the appressed thylakoid membranes whilst photosystem I complexes are restricted to the unappressed lamellae. The location of the cytochrome b6-f complex is less certain (Barber 1983 and in these proceedings). Due to this spatial separation diffusion of integral membrane components occurs to enable both electron transfer and the regulation of energy distribution between the photosystems. Such diffusional processes must of necessity depend on the physical state of the lipid matrix. In the thylakoid membrane, the lipids possess a high degree of unsaturation, having an average five to six double bonds per lipid molecule. In consequence the membrane is very fluid at physiological temperatures, as monitored by fluorescence anisotropy studies with 1,6-diphenyl-1,3,5-hexatriene (DPH) (Ford and Barber 1983). Further studies have indicated that the thylakoid fluidity expressed in response to growth temperature and the differential fluidity of the stromal and granal lamellae are specified not by changes in the lipid composition or unsaturation level but rather by the lipid to protein ratio within these membranes (Ford and Barber 1983, Chapman et al. 1983). In order to further investigate the factors controlling thylakoid fluidity, we have studied the fluorescence anisotropy decay of DPH in aqueous dispersions of isolated thylakoid lipids. Additionally preliminary investigations into the role of membrane protein in defining the membrane fluidity have been performed by monitoring the steady state anisotropy of DPH and its charged analog 1-(4-trinethylammonium phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) in lipid vesicles into which varying levels of coupling factor/ATPase (CF1-CF0) have been incorporated.
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