Design and Assembly of Functional Light-Harvesting Complexes

Abstract

Two complementary model systems are described, which are used to study the assembly of functional light-harvesting (LH) complexes. One system is based on rational design of cofactor-binding motifs and their capacity to assembly model LH2 complexes via expression in native-like membranes. The second takes advantage of the highly reversible self-assembly of the LH1 complex in artificial membranes and provides a convenient tool for design of model complexes with modified cofactors. In essence, re-design of the cofactor binding pockets in LH2 enables exploration of the underlying principles that enable particular amino acid combinations to sustain stable and functional assembly of LH-active arrays. Cofactor-binding motifs predicted in silico are tested in the context of the LH2 complex. In this way, H-bonding at the bacteriochlorophyll (BChl)/protein interface and the presence of aromatic residues were identified as critical for assembly of BChl and carotenoid (Crt). Moreover, the volumes of particular residues in the vicinity of BChl were shown to be critical for fine-tuning the spectroscopic properties. The LH1 reconstitution system, on the other hand, provides new information on the cofactor-related determinants of formation and functioning of this LH complex. Using the excitation trap approach, the coupling between BChl and excitation delocalization over the LH1 ring could be evaluated, while, by the replacement of Crts, their contribution to the assembly was assessed and for the first time a Crt-binding intermediate of LH1 assembly was identified. A new challenge is to make the two model approaches more interchangeable, thus allowing us to compare the same factors in different LH complexes, and eventually to identify on a molecular level what renders these apparently similar complexes so different.