Structure and Function of Intact Photosystem 1 Monomers from the Cyanobacterium Thermosynechococcus elongatus

Plant Biochemistry, Ruhr University Bochum, 44780 Bochum, Germany.
Biochemistry (Impact Factor: 3.02). 04/2010; 49(23):4740-51. DOI: 10.1021/bi901807p
Source: PubMed


Until now, the functional and structural characterization of monomeric photosystem 1 (PS1) complexes from Thermosynechococcus elongatus has been hampered by the lack of a fully intact PS1 preparation; for this reason, the three-dimensional crystal structure at 2.5 A resolution was determined with the trimeric PS1 complex [Jordan, P., et al. (2001) Nature 411 (6840), 909-917]. Here we show the possibility of isolating from this cyanobacterium the intact monomeric PS1 complex which preserves all subunits and the photochemical activity of the isolated trimeric complex. Moreover, the equilibrium between these complexes in the thylakoid membrane can be shifted by a high-salt treatment in favor of monomeric PS1 which can be quantitatively extracted below the phase transition temperature. Both monomers and trimers exhibit identical posttranslational modifications of their subunits and the same reaction centers but differ in the long-wavelength antenna chlorophylls. Their chlorophyll/P700 ratio (108 for the monomer and 112 for the trimer) is slightly higher than in the crystal structure, confirming mild preparation conditions. Interaction of antenna chlorophylls of the monomers within the trimer leads to a larger amount of long-wavelength chlorophylls, resulting in a higher photochemical activity of the trimers under red or far-red illumination. The dynamic equilibrium between monomers and trimers in the thylakoid membrane may indicate a transient monomer population in the course of biogenesis and could also be the basis for short-term adaptation of the cell to changing environmental conditions.

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    • "clusters, 4 lipids, and ~200 water molecules. More recent work has indicated that the trimer may contain over 330 chlorophyll molecules [47]. In addition to the high-resolution X-ray structure of PSI, there are many TEM single particle structures [48] [49] and even some high-resolution AFM structures of the PSI complex in native membranes [50] [51]. "
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    • "Monomeric PSI complexes have fewer protein subunits (12) but more chlorophylls (96) than PSII [6]. Cyanobacterial PSI complexes have been found in trimeric and monomeric forms in vitro [2,6–8] and a particularly long-wavelength fluorescence emission form of PSI is found in some cyanobacteria, possibly representing a trimer [8] [9] [10], but trimeric PSI has never been observed in the crystal structure or BN protein gels in plants [11] [12]. PSI supercomplexes, which consist of trimeric PSI associated with IsiA proteins [13], have also been isolated from cyanobacteria. "
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