Orientation of chlorophyll transition moments in the higher-plant light-harvesting complex CP29

The Q(y) transition dipole moment vectors of all eight chlorophylls in the higher-plant antenna protein CP29 were calculated by an original method on the basis of linear dichroism and absorption spectroscopy. The contribution of individual chromophores was determined from difference spectra between wild type and mutant proteins in which a single chlorophyll has been remove d by mutating pigment-binding residues. Recombinant proteins were construct ed by overexpressing the apoprotein in bacteria and refolding of the pigmen t-protein complex in vitro [Bassi, R., Croce, R., Cugini, D., and Sandona, D. (1999) Proc. Natl. Acad Sci. U.S.A. (in press)]. The spectroscopic data are interpreted on the basis of a protein structural model obtained via the homology with the major antenna complex LHCII [Kuhlbrandt, W., Wang, D. N. , and Fujiyoshi, Y. (1994) Nature 367, 614-621]. The results allow us to de termine the orientation of six chromophores within the protein structure. T he orientations of the two remaining chromophores are inferred by consideri ng the symmetry properties of CP29 and fitting steady state absorption and linear dichroism spectra by independent chlorophyll spectral forms. As a co nsequence, four "mixed" sites with different chlorophyll a and b binding af finities are identified in CP29. Geometrical data and the Forster mechanism for energy transfer suggest that excitation energy equilibrates rapidly am ong chlorophyll "pure" sites while energy preferentially flows outward from chlorophyll "mixed" sites. The orientation of the dipole moments of two ch lorophyll molecules symmetrically located at the center of the protein and parallel to the carotenoid transition vectors suggests a role in energy tra nsfer from xanthophyll to chlorophyll.