Reversible dissociation of peripheral subunits from the photosynthetic reaction center of the green sulfur bacterium Chlorobaculum tepidum

The photosynthetic reaction center (RC) of green sulfur bacteria is divided into two parts: the membrane-embedded RC core (RCC) complex (comprising PscA and PscC) and the water-soluble peripheral subunits (comprising the trimeric Fenna-Matthews-Olson [FMO] protein, PscB, and PscD). The peripheral subunits are bound onto the cytoplasmic surface of the RCC complex, and no method has been identified so far to detach the peripheral subunits to maintain the functionality of the RCC complex. The present study proposes biochemical treatments enabling the reversible dissociation of the peripheral subunits. A pH increased to 11.0 effectively removed almost all the peripheral subunits from the RCC complex. NaCl of more than 1 M destabilized their binding; however, the degrees of impact differed between each subunit. Approximately half of FMO and PscB remained on the RCC complex, whereas PscD demonstrated near-complete dissociation. Continuous washing of the detached subunits led to the near-complete dissociation of all subunits, even when treated with 1 M NaCl. The peripheral subunits under the detaching conditions were in an equilibrated state between the bound and unbound states. Neutralization and desalting after detaching treatments reassembled all the dissociated subunits and recovered the charge-separation activity comparable to that before treatment. Insights from atomic resolution structures indicate that electrostatic interaction with PscA stabilized the binding of the peripheral subunits on the RCC complex.