Most chloroplast proteins are encoded by nuclear genes and synthesized in the cytosol as higher molecular weight precursors with N-terminal transit peptides. These peptides direct proteins to cross membranes via specialized translocation systems to reach their final destinations. One such system is the chloroplast twin-arginine transport (cpTAT) complex, composed of three transmembrane proteins: Tha4 (cpTatA), Hcf106 (cpTatB), and cpTatC. The cpTAT pathway is unique in that it translocates fully folded proteins across ion-tight membranes. Among its components, Hcf106, a single transmembrane protein, is critical for function, yet its structural and dynamic properties remain difficult to resolve using conventional biophysical approaches. Electron paramagnetic resonance (EPR) spectroscopy offers powerful tools to probe protein structure and conformational dynamics. In this study, eleven single-cysteine Hcf106 variants were created by site-directed mutagenesis and subsequently labeled with the nitroxide spin label methyl methanesulfonothioate (MTSL). The labeled proteins were reconstituted into three environments: DPC detergent micelles, phosphatidylcholine (PC) liposomes, and thylakoid-mimetic (ThML) liposomes. Continuous-wave EPR spectra revealed that Hcf106 exhibits more restricted motion in lipid bilayers compared to detergent micelles, reflecting differences in structural constraints. To further examine membrane topology, CW-EPR power saturation experiments were performed to calculate depth parameters for spin-labeled residues in PC liposomes. Results indicated that four of the residues tested are embedded within the transmembrane domain, six localize to the amphipathic helix region, and one is exposed to the stromal side. These findings allowed us to delineate the boundaries of Hcf106 in the lipid bilayer and gain an understanding of its membrane conformation. Overall, our results demonstrate that EPR spectroscopy can provide valuable structural insights into Hcf106 within membranes. Future work will employ double electron-electron resonance (DEER) to measure distances within Hcf106 in liposomes, enabling deeper insight into spatial organization.
Arachchige et al. (Sun,) studied this question.