Diffusion of phycobilisomes on the thylakoid membranes of the cyanobacterium Synechococcus 7942. Effects of phycobilisome size, temperature, and membrane lipid composition.

A variant of fluorescence recovery after photobleaching allows us to observe the diffusion of photosynthetic complexes in cyanobacterial thylakoid membranes in vivo. The unicellular cyanobacterium Synechococcus sp. PCC7942 is a wonderful model organism for fluorescence recovery after photobleaching, because it has a favorable membrane geometry and is well characterized and transformable. In Synechococcus 7942 (as in other cyanobacteria) we find that photosystem II is immobile, but phycobilisomes diffuse rapidly on the membrane surface. The diffusion coefficient is 3 x 10(-10) cm(2) s(-1) at 30 degrees C. This shows that the association of phycobilisomes with reaction centers is dynamic; there are no stable phycobilisome-reaction center complexes in vivo. We report the effects of mutations that change the phycobilisome size and membrane lipid composition. 1) In a mutant with no phycobilisome rods, the phycobilisomes remain mobile with a slightly faster diffusion coefficient. This confirms that the diffusion we observe is of intact phycobilisomes rather than detached rod elements. The faster diffusion coefficient in the mutant indicates that the rate of diffusion is partly determined by the phycobilisome size. 2) The temperature dependence of the phycobilisome diffusion coefficient indicates that the phycobilisomes have no integral membrane domain. It is likely that association with the membrane is mediated by multiple weak interactions with lipid head groups. 3) Changing the lipid composition of the thylakoid membrane has a dramatic effect on phycobilisome mobility. The results cannot be explained in terms of changes in the fluidity of the membrane; they suggest that lipids play a role in controlling phycobilisome-reaction center interaction.