Lifetimes of photosystem I and II proteins in the cyanobacterium Synechocystis sp. PCC 6803

► Use of 15N-labeling and mass spectrometry to determine half-life times of proteins. ► Half-life times of photosystem I and II proteins. ► Fast oligo/monomeric interchange in photosystems, or only one form exists in vivo. ► Small Cab-like proteins do not affect the half-life times of photosynthetic proteins.