Structure and properties in solution of PsaD, an extrinsic polypeptide of photosystem I

Abstract

PsaD is a small, extrinsic polypeptide located on the stromal side (cytoplasmic side in cyanobacteria) of the photosystem I reaction centre complex. The gene from the cyanobacterium Nostoc sp. PCC 8009 was expressed in Escherichia coli and the structure of the recovered protein in solution investigated. Size-exclusion chromatography, dynamic light scattering and measurement of 15 N transverse relaxation times showed that the protein is a stable dimer in solution, whereas in the reaction centre complex it is a monomer. NMR experiments showed that the dimer is symmetrical and that there are at least two domains, one structured and the remainder unstructured. The structured domain contains a small amount of β-sheet. Three-dimensional heteronuclear NMR spectra of [ 13 C, 15 N]PsaD showed that the structured domain is associated with the central part of the sequence while the N-and C-terminal regions are mobile. Evidence was obtained for a shift in equilibrium between two slightly different conformational states at about pH 6, and the protein was shown to bind to PsaE preferentially at neutral pH. Addition of trifluoroethanol was shown to induce the formation of a small amount of A-helix, and the form present in 30% trifluoroethanol appears to be more closely related to the in situ structure, which has been reported to contain one short helix in crystals [Schubert, W.-D., Klukas, O., Krauss, N., Saenger, W., Fromme, P. & Witt, H. T. (1997) J. Mol. Biol. 272, 741Ϫ769]. The significance of these findings for the assembly of the complex is discussed.Keywords : cyanobacteria ; NMR; photosystem I; PsaD; solution structure.The PSI (photosystem I) reaction centre in cyanobacteria, plastocyanin for oxidation by P700 [4], but its function in cyanobacterial systems is less well established [5, 6]. On the stromal eukaryotic algae and higher plants is a membrane-bound, multiprotein complex which acts as a light driven oxidoreductase [1, side the three extrinsic polypeptides, PsaC, PsaD and PsaE form a ridge that can be observed by electron microscopy [7, 8]. PsaC 2]. It utilises the energy of a single red photon (1.8 eV) to drive the energetically unfavourable reduction of oxidized ferredoxin binds the terminal electron acceptors, F a and Fb, which are [4Fe-4S] clusters. PsaD is known to be necessary for the native struc-(E m ϷϪ420 mV) by reduced plastocyanin or cytochrome c 6 (E m Ϸ370 mV), with a remarkable quantum yield for stable photo-ture of these clusters [9] and has also been implicated in the electrostatic binding of the soluble FeS protein, ferredoxin, for chemical charge separation across the thylakoid membrane (nearly 100 %) and an efficient conversion of the photon energy reduction by F A or F B [10, 11]. PsaE is also thought to play a role in ferredoxin reduction [12Ϫ14], as well as in cyclic to chemical free energy (around 40%). In cyanobacteria, it is known to contain two large, intrinsic membrane subunits (PsaA, electron flow around PSI [15]. Neither PsaD nor PsaE contain a redox cofactor. PsaB of molecular mas...

Breaking biological symmetry in membrane proteins: The asymmetrical orientation of PsaC on the pseudo-C2 symmetric Photosystem I core

“…In contrast to PsaC, the majority of unbound PsaD does not show any extended secondary structure and a significant portion of the protein is unfolded in solution [40]. The NMR solution structure of PsaE agrees closely with that obtained in fully assembled PS I, except for some minor differences in two loop regions of the protein [41].…”

Arbuscular mycorrhizal symbiosis elicits shoot proteome changes that are modified during cadmium stress alleviation in Medicago truncatula

“…In favour of this hypothesis was the down-accumulation of PsaD that was observed in the shoots of Cd-treated M. truncatula plants (Figure 6, column Cd/C). PsaD is a small extrinsic polypeptide of the PSI reaction centre complex that is required for native assembly of PSI reaction clusters [61]. A reduced electron flux due to PSI disassembly can generate direct photoreduction of oxygen, which leads to inactivation of ROS-scavenging enzymes and CO 2 fixation [54,61,62].…”

“…PsaD is a small extrinsic polypeptide of the PSI reaction centre complex that is required for native assembly of PSI reaction clusters [61]. A reduced electron flux due to PSI disassembly can generate direct photoreduction of oxygen, which leads to inactivation of ROS-scavenging enzymes and CO 2 fixation [54,61,62]. PSI disassembly could further drive the destruction of PSII unit and thus a reduction in the density of active RC as presented in Figure 4.…”

Suppressors of the Chloroplast Protein Import Mutant tic40 Reveal a Genetic Link between Protein Import and Thylakoid Biogenesis

“…Clear fluorescence signals were also observed for both STIC2-ALB3 combinations (STIC2-nYFP with ALB3-cYFP; STIC2-cYFP with ALB3-nYFP; Figure 7), while no signals were observed in the negative control, STIC2-nYFP with PSI-D-cYFP (Figure 7). The PSI-D-cYFP fusion is a valid negative control here, as it was shown to be expressed and able to interact with PSI-D-nYFP in chloroplasts (Figure 7; Supplemental Figure 13) (Xia et al, 1998;Kudla and Bock, 2016).…”