Structural interactions of TLP18.3 and Psb27-H1 to the luminal CP43 and Rubredoxin-ENH1 to the stromal side of Photosystem II in higher plants

TLP18.3 and Psb27 are known proteins on the luminal side of photosystem II.The structural locations of these two proteins are still absent in the currently available higher plant photosystem II cryo-EM structures.We interrogated the structural locations of these proteins using chemical cross-linking followed by liquid chromatography/tandem MS analysis.Structural mass spectrometry results then provided chemical restrains to direct structural modelling to determine the collective binding/stabilization of these two proteins to the luminal PSII CP43 protein.Using this pipeline, we also found the structural location of a Rubredoxin protein on the stromal side of PSII.Discovery of this redox active iron-sulfur protein in the vicinity of PSII subunit D1/D2 proteins, greatly showcases the importance of the redox processes that are potentially involved in PSII assembly or less known steady state functionality or photoprotection.This structural mass spectrometry platform high-lights its powerful applicability in protein complex discovery.