In green-sulfur bacterial photosynthesis, excitation energy absorbed by a peripheral antenna structure known as the chlorosome is sequentially transferred through a baseplate protein to the Fenna-Matthews-Olson (FMO) antenna protein and into the reaction center, which is embedded in the cytoplasmic membrane. The molecular details of the optimized photosystem architecture required for efficient energy transfer are only partially understood. We address here the question of how the baseplate interacts with the FMO protein by applying hydrogen/deuterium exchange coupled with enzymatic digestion and mass spectrometry analysis to reveal the binding interface of the FMO antenna protein and the CsmA baseplate protein. Several regions on the FMO protein, represented by peptides consisting of 123-129, 140-149, 150-162, 191-208, and 224-232, show significant decreases of deuterium uptake after CsmA binding. The results indicate that the CsmA protein interacts with the Bchl a #1 side of the FMO protein. A global picture including peptide-level details for the architecture of the photosystem from green-sulfur bacteria can now be drawn.