PROGRESS WORK ON THE CRYSTALLISATION OF B800-820 LIGHT-HARVESTING COMPLEXES. K. McLuskey1, N.W. Isaacs1, R. J. Cogdell2, S.M. Prince1, A. A. Freer1.Dept. of Chemistry1 and Biochemistry2, University of Glasgow, G12 8QQ.
In photosynthetic bacteria the cell membrane contains a reaction centre (RC) stoichiometrically associated with a light-harvesting (LH1) complex. In certain bacteria a second, so called LH2, complex is produced to increase the light-harvesting capacity. Depending on growth conditions, some bacteria are able to produce a second form of LH2 where the Bchl a absorption is observed at 800 and 820 nm, rather than at 800 and 850 nm.
The B800-820 complex is a more efficient light-harvesting complex than its B800-850 counterpart, being more effective in terms of energy transfer between the internal pigment molecules and more effectual in directing energy to LH1 and hence towards the RC. It has been suggested that the change in absorption is modulated by the apoprotein and it has been shown, using mutants, that changing the hydrogen-bonding patterns between protein residues and the Bchl a, causes a blue shift in the spectrum. A structure of a B800-820 complex would help us to understand the mechanisms of energy transfer in more detail and the role that proteins play in determining the characteristics of light harvesting.
LH2 B800-820 complexes from Rps. acidophila strain 7750 and Rps. cryptolactis have been crystallised (Guthrie, N. et al., J. Mol Biol. 224, 527-528, (1992) and Halloreen, E. et al., Photo. Res. 44, 149-155, (1995)). These crystals diffracted to resolutions of no higher than 5Å. The purification protocol employed was based mainly on charge, whereas experience, with the B800-850 complex, indicated that purification based on size exclusion gives better quality crystals. This work involves refining the protocol for both purification and crystallisation in order to attain crystals that diffract to the highest possible resolutions and results will be reported.