Crystallization of Intact and Subunit L-Deficient Monomers from Synechocystis PCC 6803 Photosystem I

Photosystem I monomers from wildtype cells of Synechocystis PCC 6803 and from a mu­tant deficient in the psaL gene were crystallized. PsaL encodes for the hydrophobic subunit L, which has been proposed to constitute the trimerization domain in the PS I trimer. The absence of subunit L facilitated crystallization of the PS I monomer. The unit cell dimensions and the space group for the crystals from this preparation could be determined to be a = b = 132 Å , c -525 Å, α = β = 90°, y = 120°, the space group is P61 or P65. The results show the potential of using specifically designed deletion mutants of an integral membrane protein for the systematic improvement of crystal structure data

Biogenesis and assembly of photosystem I

351-359<span style="font-size:14.0pt;font-family:HiddenHorzOCR; mso-hansi-font-family:" times="" new="" roman";mso-bidi-font-family:hiddenhorzocr;="" mso-ansi-language:en-us;mso-fareast-language:en-us;mso-bidi-language:ar-sa"="">Photosystem I (PS I) is a multi subunit membrane protein complex consisting of  11 to 14 different subunits. In addition, several cofactors, such as chlorophylls, phylloquinones, carotenoids and iron-sulfur clusters are bound by this complex. We now have a detailed understanding of the structural basics, yet we know very little about the molecular details of the assembly process that finally yields functional PS I<span style="font-size:14.0pt; font-family:Arial;mso-fareast-font-family:HiddenHorzOCR;mso-ansi-language:EN-US; mso-fareast-language:EN-US;mso-bidi-language:AR-SA">. Moreover, not much is known about the molecular dynamics of PS I in the thylakoid membrane or its regulated degradation. These areas have become the focus of recent work and first results have emerged. In this minireview we describe the latest findings in this fascinating and rapidly evolving field.</span

Isolation and Structural Characterization of Monomeric and Trimeric Photosystem I Complexes (P700·FA/FB and P700·Fx) from the Cyanobacterium Synechocystis PCC 6803

An isolation procedure was developed for the cyanobacterium Synechocystis 6803 (and 6714) which yields both monomeric and trimeric photosystem I complexes (P700·FA/FB complexes) depleted of the stroma-exposed subunits PsaC, -D, and -E (P700·FX complexes). Analysis by high resolution gel electrophoresis in combination with immunoblotting and N-terminal sequencing reveals the selective and quantitative removal of PsaC, -D, and -E from the P700·FA/FB complex, containing PsaA, -B, -C, -D, -E, -F, -K, -L and at least two subunits ≤4 kDa. Monomeric and trimeric P700·Fx complexes show an identical subunit composition and an identical charge recombination half-time of 750 ± 250 µs as determined by flash-induced absorption change measurements, reflecting the quantitative loss of iron-sulfur clusters FA/FB and the presence of cluster FX. The existence of a stable trimeric P700·FX complex enables a detailed structural analysis by electron microscopy with high resolution. Comparison of averaged top and side view projections of P700·FX and P700·FA/FB complexes show that the height of the complex is reduced by about 2.5-3.3 nm upon removal of the three stroma-exposed subunits and indicate the position of these three subunits on the PS I surface. While the outer contours of the stroma exposed mass of PS I agree very well with the three-dimensional crystal analysis recently published for trimeric PS I of Synechococcus elongatus, only the structural analysis presented here is able to assign the stroma-exposed mass exclusively to the subunits PsaC, -D, and -E and to exclude a contribution of other subunits.