Photosystem I from Synechococcus elongatus: preparation and crystallization of monomers with varying subunit compositions

Monomers of Photosystem I (PS I) from the thermophilic cyanobacterium Synechococcus elongatus were prepared and characterized. Using both the zwitterionic sulfobetain 12 and the nonionic β-dodecyl maltoside as detergents, PS I was extracted in trimeric form from the thylakoid membrane. The trimers dissociate into monomers during gel-filtration, apparently due to a change in the osmotic pressure. The PS I monomer prepared in β-dodecyl maltoside exhibits the same subunit composition as the trimeric PS I. The preparation in sulfobetain 12, however, leads to the removal of the two hydrophobic subunits K and L from the monomer. Further treatment of this sample with sulfobetain 12 detaches the subunits F, J and M, associated with the disappearance of a 706 nun band in the absorption spectrum. Both the PS I monomer with complete subunit composition and the monomer lacking subunits K and L were crystallized using batch and vapor diffusion techniques with poly(ethylene glycol) as the precipitating agent.