Abstract
We have used the nonionic detergent octyl-beta-d-glucopyranoside in combination with sodium dodecyl sulfate to isolate two novel Photosystem I (PSI) complexes from spinach (Spinacea oleracea L.) thylakoid membranes. These complexes have been characterized as to their spectral properties, content of PSI reaction center chlorophyll P(700), and protein composition. PSI-B, purified from solubilized membranes by sucrose density gradient centrifugation, is a putative native PSI complex. PSI-B contains four polypeptides between 21 and 25 kilodaltons in addition to the components of the PSI antenna complex (LHCI); three of these polypeptides have not previously been associated with PSI. A second complex, CPI(*), is purified from octyl glucoside/sodium dodecyl sulfate solubilized thylakoids by two cycles of preparative gel electrophoresis under mildly denaturing conditions. Electrophoresis under these conditions releases a discrete set of polypeptides from PSI producing a complex composed only of the PSI reaction center and the LHCI antenna.In addition, the PSI reaction center complex CPI isolated from preparative gels and PSI-B were reconstituted into lecithin liposomes for structural analysis using freeze-fracture electron microscopy. The results suggest that the native PSI complex produces 12- to 13-nanometer particles, while the PSI reaction center, depleted of LHCI and peripheral proteins, produces particles with an average diameter of 10 nanometers.