THE STRUCTURE OF AN INTEGRAL MEMBRANE PROTEIN LIGHT-HARVESTING COMPLEX N.W. Isaacs, R.J. Cogdell^#, A.A. Freer, A.M. Hawthornthwaite-Lawless^*, M.Z. Papiz^*, S.M. Prince, and G. McDermott. Depts of Chemistry and ^#Biochemistry, University of Glasgow, Glasgow, G12 8QQ, UK. and ^*CCLRC Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK.

The crystal structure of the integral membrane light-harvesting complex from Rhodopseudomonas acidophila strain 10050 showed that the active assembly consists of two concentric cylinders of single helical protein subunits which enclose the pigment molecules. Eighteen bacteriochlorophyll a (Bchl a) molecules are sandwiched between these membrane-spanning helices to form a continuous overlapping ring with their bacteriochlorin planes perpendicular to the plane of the membrane surface. A further nine Bchl a are positioned between the outer helices with their bacteriochlorin ring planes parallel to the membrane plane. Carotenoid molecules span the assembly making van der Waals contacts to both types of Bchl a pigments as well as the protein helices. A close analysis of the structure shows a beautiful interlocking of the protein and pigment molecules to allow for the optimisation of energy transfer between pigments in a single complex and between adjacent complexes, connecting to the reaction centre where charge separation takes place.

This work has been supported by the BBSRC.