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Interaction of a Tat Substrate and a Tat Signal Peptide with Thylakoid Lipids at the Air–Water Interface

The Tat machinery enables folded proteins to be translocated across biological membranes. In vitro studies have shown that Tat substrates can interact with membranes prior to translocation. In this study we investigated the initial states of this interaction with thylakoid lipid monolayers at the air–water interface by using monolayer techniques combined with infrared reflection‐absorption spectroscopy (IRRAS). We used enhanced green fluorescent protein (EGFP) as a model substrate and the signal peptide SP16 from the 16 kDa protein of the spinach oxygen‐evolving complex (OEC16). We found that the signal peptide is essential for the interaction of the model substrate with lipid monolayers. IRRA spectroscopy showed an increased amount of α‐helical secondary structure elements for the chimeric model substrate i16/EGFP (SP16 fused to EGFP) compared with EGFP; this can be attributed to the signal peptide.