THE CRYSTAL STRUCTURE OF THE B800-820 LIGHT-HARVESTING COMPLEX FROM RHODOPSEUDOMONAS ACIDOPHILA STRAIN 7050
Karen Mc Luskey1, Stephen M.Prince1, Richard J.Cogdell2 and Neil W.Isaacs1
1 Department of Chemistry
and
2 Institute for Biomedical and Life
Sciences, University of Glasgow, Glasgow, Scotland, UK G128QQ
e-mail: karenm@chem.gla.ac.uk
Keywords: Photosynthesis,
membrane protein, light-harvesting
The crystal structures of the light-harvesting complexes (LH2), from Rps. acidophila strain 10050 [1] and Rhodospirillum molischianum [2], revealed two concentric cylinders of single hydrophobic apoproteins which enclose the pigment molecules, bacteriochlorophyll a(Bchl a) and carotenoid. Within these complexes the Bchl a molecules are divided into two spectrally distinct forms whose absorption maxima occur at ca 800 nm and 850 nm. Consequently, these are also known as B800-850 light-harvesting complexes.
When grown under ''stressed'' conditions, such as lower light intensities, some bacteria are able to produce a second form of LH2 where the Bchl a absorption is observed at ca 800 nm and 820 nm. This B800-820 complex is found to restrict back transfer of energy from the core light-harvesting complex considerably more than its B800-850 counterpart [3], producing a more efficient system. The reason for the spectral shift is unclear, although site-directed mutagenesis has shown that breakage of two hydrogen bonds, between Bchl a molecules and the protein, induces a blue-shift in the absorption spectrum. The primary sequence of the apoproteins from the B800-820 complex from Rps. acidophila strain 7050 is approximately 70% identical to that of the B800-850 complex from Rps. acidophila 10050 [4].
Here we present the three dimensional crystal
structure of this complex, with possible hypotheses on the role
that the protein plays on determining the characteristic of this
light harvesting system. The purified complex crystallised in
space group R32 with unit cell dimensions a=117.20 A, c=295.14 A
(at 100K). A complete data set was collected to 2.8 A resolution
and molecular replacement methods, using the helical subunits
from the Rps.acidophila strain 10050 structure, were used to
obtain a structure solution.
This work has been supported by the BBSRC.