High-Resolution Tandem Mass Spectrometry Indicates Rubisco Activase is Associated with PS I-LHC I-LHC II Membranes

During proteomic investigations examining the mobile LHC II chlorophyll antenna complex associated with Photosystem I (PS I), we identified, with very high confidence, the association of Rubisco Activase with PS I-LHC I-LHC II membranes. Using very rigorous criteria (p-values ≤ 10-5), a total of fifty-three high-quality Rubisco Activase peptides were identified by high resolution tandem mass spectrometry in two biological replicates, each digested with either trypsin or chymotrypsin, independently. Using these criteria, and searching the entire spinach proteome, only proteins previously known to be associated with PS I (the PS I subunits PsaD and PsaL, Lhca1-4, Lhcb1-3), the monomeric Lhcb proteins, Lhcb4-6 (which will be discussed elsewhere), and Rubisco Activase were identified. The presence of Rubisco Activase closely associated with PS I has important implications with respect to the activation of this enzyme and, consequently, the Calvin cycle by PS I, which will be discussed. ### Competing Interest Statement The authors have declared no competing interest.