Investigation of highly stable Photosystem I chlorophyll-protein complexes from the thermophilic cyanobacterium Phormidium laminosum

The use of native gels for the concomitant determination of protein sequences and modifications by mass spectrometry with subsequent conformational and functional analysis of native proteins following electro-elution

The protocol consists of running a native gel with in-gel digestion by proteases, subsequent mass spectrometrical determination of protein sequence and modifications, followed by electro-elution and conformational analysis using melting point and circular dichroism. Finally, the eluted protein is tested for preserved function. Herein, C1 esterase inhibitor is applied on a native gel; in-gel digestion by proteases is carried out and peptides are identified by nano-LC-ESI-CID/ETD-MS/MS using an ion trap for generation of peptide sequences and protein modifications. Protein from replicate bands from the same gel is electro-eluted and used for determination of the melting point and used for circular dichroism analysis. Additional bands from the native gel are either in-gel digested with asparaginase to generate deamidation or PNGase F for deglycosylation, followed by mass spectrometry, conformational and functional studies. Preserved conformation and function of the C1 esterase inhibitor was shown. This protocol can be completed in 1 week.

Preparative isolation of protein complexes and other bioparticles by elution from polyacrylamide gels

Due to its unmatched resolution, gel electrophoresis is an indispensable tool for the analysis of diverse biomolecules. By adaptation of the electrophoretic conditions, even fragile protein complexes as parts of intracellular networks migrate through the gel matrix under sustainment of their integrity. If the thickness of such native gels is significantly increased compared to the analytical version, also high sample loads can be processed. However, the cage-like network obstructs an in-depth analysis for deciphering structure and function of protein complexes and other species. Consequently, the biomolecules have to be removed from the gel matrix into solution. Several approaches summarized in this review tackle this problem. While passive elution relies on diffusion processes, electroelution employs an electric field to force biomolecules out of the gel. An alternative procedure requires a special electrophoresis setup, the continuous elution device. In this apparatus, molecules migrate in the electric field until they leave the gel and were collected in a buffer stream. Successful isolation of diverse protein complexes like photosystems, ATP-dependent enzymes or active respiratory supercomplexes and some other bioparticles demonstrates the versatility of preparative electrophoresis. After liberating particles out of the gel cage, numerous applications are feasible. They include elucidation of the individual components up to high resolution structures of protein complexes. Therefore, preparative electrophoresis can complement standard purification methods and is in some cases superior to them.

NMR Analysis of the Transient Complex between Membrane Photosystem I and Soluble Cytochrome c6

A structural analysis of the surface areas of cytochrome c(6), responsible for the transient interaction with photosystem I, was performed by NMR transverse relaxation-optimized spectroscopy. The hemeprotein was titrated by adding increasing amounts of the chlorophyllic photosystem, and the NMR spectra of the free and bound protein were analyzed in a comparative way. The NMR signals of cytochrome c(6) residues located at the hydrophobic and electrostatic patches, which both surround the heme cleft, were specifically modified by binding. The backbones of internal residues close to the hydrophobic patch of cytochrome c(6) were also affected, a fact that is ascribed to the conformational changes taking place inside the hemeprotein when interacting with photosystem I. To the best of our knowledge, this is the first structural analysis by NMR spectroscopy of a transient complex between soluble and membrane proteins.

A comparative fluorescence kinetics study of Photosystem I monomers and trimers from Synechocystis PCC 6803

Picosecond time-resolved fluorescence measurements have been performed as a function of emission wavelengths in order to investigate the possible functional differences between monomeric and trimeric Photosystem I (PS I) particles from a cyanobacterium Synechocystis. Applying global analysis, four kinetic components were found necessary to describe the fluorescecne decay for both monomers and trimers of PS I. The lifetimes and spectra of the respective components are quite similar, indicating that they can be attributed to identical processes in both the monomers and trimers. It is concluded that both forms of PS I are capable of efficient energy transfer and charge separation, in agreement with a physiological role of both forms. Small differences in the fluorescence decays are discussed in terms of a slightly higher ratio of red emitting pigments per reaction centre in trimers of PS I. A comparison to Synechococcus PS I particles reveals the higher red chlorophyll content of the latter.

Purification and crystallization of Photosystem I complex from a phycobilisome-less mutant of the cyanobacterium Synechococcus PCC 7002

An active photosystem (PSI) complex was isolated from a phycobilisome-less mutant of the mesophilic cyanobacterium Synechococcus PCC 7002 by a mild procedure. Purification of PS I was achieved using a sucrose density gradient and an isoelectric focussing subsequent to the extraction of PSI from thylakoids with dodecyl-β-maltoside. Electron microscopy and gel filtration HPLC suggested that the isolated complex represents a trimeric form of PSI. The trimeric form was resistant to pH or detergent exchange. A 'molecular weight' of 690 kDa to 760 kDa has been determined for the complex by gel filtration HPLC in several detergents or mixtures of detergents.The PSI complex contains the polypeptides of the psaA, psaB, psaC, psaD, psaE, psaL gene products and two small polypeptides as determined by SDS-PAGE and N-terminal sequencing; its antenna size is 77±2 Chl a/P700. The full set of Fe-S clusters (FA, FB and FX) was observed by EPR-spectroscopy. A preliminary characterization of crystals obtained from this preparation was carried out using SDS-PAGE, optical and EPR spectroscopy.

Does the trimeric form of the Photosystem 1 reaction center of cyanobacteria in vivo exist?

When isolating Photosystem 1 from the thylakoid membranes of cyanobacteria, a multiplicity of photosystem complexes can be encountered, which has not yet been observed in any other phototrophic organisms: After solubilisation of thylakoid membranes with detergents, trimeric, dimeric and monomeric forms of Photosystem 1 can be separated. The question must now be answered, which of the stable Photosystem 1 forms is the functional form in vivo-monomeric or trimeric? The two possibilities are discussed, though we mainly present arguments for the existence of the trimeric form of Photosystem 1 in cyanobacterial thylakoid membranes.