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Abstract

High-resolution top-down mass spectrometry was used to characterize eleven integral and five peripheral subunits of the 750 kDa Photosystem II (PSII) complex from the eukaryotic red alga, Galdieria sulphuraria. The primary separation used liquid chromatography mass spectrometry with concomitant fraction collection (LC-MS+) yielding around 40 intact mass tags (IMTs) at 100 ppm mass accuracy on a low-resolution electrospray-ionization mass spectrometer, whose retention and mass were used to guide subsequent high-resolution top-down nano-electrospray Fourier-transform ion-cyclotron resonance mass spectrometry experiments (FT-MS). Both collisionally activated and electron capture dissociation (CAD, ECD) were used to confirm the presence of eleven small subunits to mass accuracy within 5 ppm; PsbE, PsbF, PsbH, PsbI, PsbJ, PsbK, PsbL, PsbM, PsbT, PsbX and PsbZ. All subunits showed covalent modifications that fall into three classes including retention of initiating formylmethionine, removal of methionine at the N-terminus with or without acetylation, and removal of a longer N-terminal peptide. Peripheral subunits identified by top-down analysis included oxygen evolving complex (OEC) subunits PsbO, PsbU, PsbV, as well as Psb28 (PsbW) and Psb27 (‘PsbZ-like’). Top-down high-resolution mass spectrometry provides the necessary precision, typically less than 5 ppm, for identification and characterization of polypeptide composition of these important membrane protein complexes.