A Colorful Pallet of B-Phycoerythrin Proteoforms Exposed by a Multimodal Mass Spectrometry Approach

Cyanobacteria and red algae represent some of the oldest lifeforms on the planet. During billions of years of evolution, they have fine-tuned the structural details of their light-harvesting antenna, called phycobilisomes, which represents one of the most efficient systems for light harvesting and energy transfer. Yet, the exact details of phycobilisome assembly and energy transfer are still under investigation. Here, we employed a multi-modal mass spectrometric approach to unravel the molecular heterogeneity within B-phycoerythrin, the major phycobiliprotein in the red algae P. cruentum. B-phycoerythrin consists of 12 subunits (alpha beta)(12) arranged in a ring with the central cavity housing a linker (gamma) subunit, which is crucial for stabilizing B-phycoerythrin within the phycobilisome. Using top-down MS, we unravel the heterogeneity in the gamma proteoforms, characterizing the distinct gamma chains and multiple isobaric chromophores they harbor. Our data highlight the key role gamma plays in phycobilisome organization that enables optimal light transmission.