Membrane orientation of the FMO antenna protein from Chlorobaculum tepidum as determined by mass spectrometry-based footprinting

Abstract

The high excitation energy-transfer efficiency demanded in photosynthetic organisms relies on the optimal pigment-protein binding orientation in the individual protein complexes and also on the overall architecture of the photosystem. In green sulfur bacteria, the membrane-attached Fenna-Matthews-Olson (FMO) antenna protein functions as a ''wire'' to connect the large peripheral chlorosome antenna complex with the reaction center (RC), which is embedded in the cytoplasmic membrane (CM). Energy collected by the chlorosome is funneled through the FMO to the RC. Although there has been considerable effort to understand the relationships between structure and function of the individual isolated complexes, the specific architecture for in vivo interactions of the FMO protein, the CM, and the chlorosome, ensuring highly efficient energy transfer, is still not established experimentally. Here, we describe a mass spectrometrybased method that probes solvent-exposed surfaces of the FMO by labeling solvent-exposed aspartic and glutamic acid residues. The locations and extents of labeling of FMO on the native membrane in comparison with it alone and on a chlorosome-depleted membrane reveal the orientation. The large differences in the modification of certain peptides show that the Bchl a #3 side of the FMO trimer interacts with the CM, which is consistent with recent theoretical predictions. Moreover, the results also provide direct experimental evidence to confirm the overall architecture of the photosystem from Chlorobaculum tepidum (C. tepidum) and give information on the packing of the FMO protein in its native environment.chemical labeling ͉ energy transfer ͉ FMO protein ͉ mass spectrometry ͉ protein footprinting P hotosynthesis is a fundamental biological process that harvests solar energy to power life on Earth (1). A diverse family of pigment-protein complexes and elegant architectures accomplish the necessary light-harvesting and energy-storage processes (2-5). In photosynthetic green sulfur bacteria, light absorbed by a large antenna complex known as a chlorosome (6-8) is transferred through a protein called Fenna-Matthews-Olson (FMO) (9) to the reaction center, which is embedded in the cytoplasmic membrane (CM). Together, they form a funnel-like architecture to facilitate energy transfer. The specific orientation of the critical linker, the FMO protein, however is unknown (Fig. 1A).The structure of the FMO protein was the first (bacterio)chlorophyll binding protein to be determined by X-ray crystallography. Structures of this protein from 2 species, Prosthecochloris aestuarii 2K (10, 11) and Chlorobaculum tepidum (12) are now available, and they show strong structural and spectral similarities. The FMO protein consists of 3 identical subunits of mass 40 kDa related by a 3-fold axis of symmetry. The 3 monomers form a disc with a C3 symmetry axis perpendicular to the disc plane (Fig. 1B). There are 7 BChl a molecules in a monomer, although an eighth pigment has been resolved in newly solved structures (13,14). Eac...

Isolation and Characterization of Homodimeric Type-I Reaction Center Complex from Candidatus Chloracidobacterium thermophilum, an Aerobic Chlorophototroph

“…The FMO protein is firmly attached to the cytoplasmfacing side of the RC in most GSB, and its orientation has recently been deduced (24). Although the pscB and fmoA genes occur in the same operon as pscA in the Ca.…”

“…The suprastructures of the BChl d and c molecules in chlorosomes of C. tepidum were recently described (23). The FMO protein, which forms a layer between the chlorosomes and RCs (24), functions both as a light-harvesting complex and as a conduit for excitation energy transfer between the chlorosome baseplate and the RC (20 -22). Although its genome predicts that Ca.…”

Microscopic quantum coherence in a photosynthetic-light-harvesting antenna

“…The FMO complex has been assumed to be oriented with BChla 1 and 6 towards the light-harvesting antenna, whereas BChla 3 and 4 define the target region in contact with the reaction centre complex [65,66]. Accordingly, we adopt BChla 1 or 6 as the initial excited pigment for numerical calculations.…”

Ultrastructural Analysis and Identification of Envelope Proteins of "Candidatus Chloracidobacterium thermophilum" Chlorosomes

“…Excitation energy absorbed by BChls moves quickly through these suprastructures due to strong excitonic coupling (48), and excitation energy is eventually transferred to BChl a molecules associated with the baseplate protein, CsmA (42,(46)(47)(48)(49)56). In the photosynthetic apparatus of GSB, an additional BChl a-binding protein, the Fenna-Matthews-Olson (FMO) protein, transfers excitation energy from CsmA to the core antenna molecules associated with the type 1 reaction centers found in the cytoplasmic membrane (4,33,44,46,62,63,(69)(70)(71). High-resolution X-ray crystallographic studies of FMO have established that this protein binds eight BChl a molecules, rather than the seven that were previously suggested (33,62,70).…”