A novel mode of photoprotection mediated by a cysteine residue in the chlorophyll protein IsiA
Oxygenic photosynthetic organisms have evolved multitude mechanisms for protection against high light stress. IsiA, a chlorophyll a-binding cyanobacterial protein serves as an accessory antenna complex for photosystem I. Intriguingly, IsiA can also function as an independent pigment protein complex in the thylakoid membrane and facilitate dissipation of excess energy, providing photoprotection. The molecular basis of IsiA-mediated excitation quenching mechanism remains poorly understood. In this study, we demonstrate that IsiA uses a novel cysteine-mediated process to quench excitation energy. The single cysteine in IsiA in the cyanobacterium Synechocystis 6803 was converted to a valine. Ultrafast fluorescence spectroscopic analysis showed that this single change abolishes the excitation energy quenching ability of IsiA, thus providing direct evidence of the crucial role of this cysteine residue in the energy dissipation from excited chlorophylls. Under stress condition, the mutant cells exhibited enhanced light sensitivity, indicating that the cysteine-mediated quenching process is critically important for photoprotection.