(294g) Predicting and Evaluating Protein Binding Motifs in the Chlorosomes of Green Sulfur Bacteria

We present the first model of the chlorosome baseplate in the green sulfur bacteria Chlorobaculum tepidum binding to the Fenna-Matthews-Olson protein (FMO).  Our model of the baseplate includes the photosynthetic pigment Bacteriochlorophyll a (BChl a) bound to chlorosome protein A (CsmA).  Until now, atomistic structural models have been difficult to obtain for this system due to limitations in experimentally characterizing the large and complicated photosynthetic antenna complex.  Our multiscale predictive modeling approach uses a combination of protein-ligand docking and semi-empirical quantum calculations to predict and evaluate potential BChl a binding sites on CsmA and protein-protein docking and molecular dynamics free energy methods to predict and evaluate the potential CsmA binding sites on FMO.  Our computational approaches refine existing models and agree with available experimental data, including newly-identified CsmA binding sites on FMO.  Our model consists of a Bchl a molecule, with the magnesium center of the bacteriochlorin ring located within 3 Å of the imidazole nitrogen of CsmA's HIS25 residue, and the phytyl tail situated on the hydrophobic side of the CsmA α-helix so that it is near the non-polar residues (TRP26, VAL29, LEU32, and PHE33).  Our predicted structure reproduces the experimentally observed absorption redshift when comparing the absorption of the bare BChl a pigment in an organic solvent environment to our computed model of the pigment in its native protein environment.  The rigorous understanding of the interaction of a single BChl a pigment with a single CsmA protein is necessary for the construction of CsmA multimers.  In turn, these CsmA multimers bind to FMO, forming the baseplate of the photosynthetic antenna complex present in green sulfur bacteria.  The stability of this baseplate-FMO complex in its native environment, along with possible mechanisms of assembly, will be discussed.  This structure prediction is a necessary first step in characterizing and understanding more complicated photosynthetic systems composed of multiple pigment-protein aggregates.