For the first time, data are presented on the time-resolved disassembly reaction of a highly organized membrane protein complex in vitro. The photosynthetic core light-harvesting complex of the bacterial strain Rhodospirillum rubrum G9 consists of 12-16 dimeric subunits that in vivo are associated with the photosynthetic reaction center in a ringlike manner. Isolated in a detergent solution, its appearance either as a ringlike complex (called B873 and absorbing at 873 nm), subunit dimer (called B820 and absorbing at 820 nm), or monomeric form (called B777 and absorbing at 777 nm) is strongly temperature-dependent. In thermodynamic equilibria between B820 and B873, intermediate-sized complexes have also been observed that have absorption maxima around 850 nm. It is unknown whether these structures appear as intermediates in the kinetic B820-B873 (dis)assembly reaction. In this paper disassembly of the light-harvesting complex into its dimeric subunits was followed spectroscopically on a time scale up to 200 ms, upon applying an infrared laser-induced temperature jump. The full dissociation process appears to take place on a time scale of tens to hundreds of milliseconds, the rates becoming faster at higher starting temperatures. Applying the same technique, the dissociation reaction of dimeric subunits into monomers also could be established. This dissociation process occurred on a much faster time scale and took place within the 500 μs response time of our detection system.
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