PURIFICATION AND PROPERTIES OF AN ANTENNA-REACTION CENTER COMPLEX FROM HELIOBACTERIA

Antenna-reaction center complexes were prepared by incubation of membranes of Heliobacterium chlorum and Heliobacillus mobilis with the detergent sulfobetaine-12, followed by sucrose gradient centrifugation in the presence of sulfobetaine-12 and sodium cholate. Similar results were obtained with n-octyl β-d-glucopyranoside. Further purification was obtained by size-exclusion HPLC. From the elution volume, the molecular masses for the purified complexes solubilized with sulfobetaine-12 were estimated to be 335 kDa for both species, not corrected for the weight of the associated detergent molecules. SDS-polyacrylamide gel electrophoresis showed the presence of a dominant peptide located at 94 kDa, together with minor and variable bands near 50 kDa and above 100 kDa. The spectral properties of the isolated complexes were very similar for the two species and also almost identical to those of the membranes used as starting material. Fluoresence emission and excitation spectra showed efficient energy transfer within the complexes. The reaction center activity, measured by the yield and extent of photo-oxidation of the primary electron donor P-798, was completely conserved. These observations indicate that the structure of the antenna and of the reaction center was fully retained during solubilization and isolation. It is concluded that the complexes probably contain the complete pigment complement of a single photosynthetic unit, together with the associated reaction center. Although the estimated molecular weights of the antenna-reaction center complexes are similar to that of the Photosystem I core, the peptide composition appears to suggest a basically different structure. © 1990.