When the structure of the RC was solved in 1985 it was the first X-ray structure of a membrane protein. X-ray structures of the other photosynthetic complexes—the light harvesting LH1 and LH2 complexes—followed. But, says Hunter, “it was a bit like a jigsaw—nobody had seen how the pieces were put together.”
Light energy is initially absorbed by LH2 complexes, each of which is a ring of ∼27 chlorophylls and associated proteins. The energy whizzes around the LH2 ring, hops to neighboring LH2 rings, and transfers to the lower energy states available in any nearby LH1 ring. The energy then circulates around the LH1 complex until the RC, which plugs the donut hole of the LH1 complex, is free to accept it.
The Dutch and English groups used tapping mode AFM to detect the protrusions of membrane proteins and thus map the arrangement of the complexes. (Similar results have also been reported by Scheuring et al. .) Large patches of the smaller LH2 complexes “funnel down to form these narrow channels separating linear arrays of RC-LH1 complexes,” says Hunter. The RC-LH1 complexes are mostly present as linear arrangements of dimers surrounded by the more randomly arranged LH2 complexes.
The interconnection of photosynthetic complexes had been computed previously, based on the frequency at which two excitations met each other. But, says Hunter, “knowing something is there is one thing; seeing it is something else.” The new images suggest that an LH1 faced with a busy RC might resort to sending its energy to a neighboring LH1. The team now hopes to test such ideas by visualizing both structural and spectroscopic information at the same time.
“The astonishing thing about AFM is that you are seeing one molecule,” says Hunter. X-ray and EM imaging methods use averaging so “you've grown accustomed to thinking that all molecules look like each other,” he says. “At the molecular level life is a little more untidy.” ▪