Thylakoids are complex sub-organellar membrane systems whose role in photosynthesis makes them critical to life. Thylakoids require the coordinated expression of both nuclear- and plastid-encoded proteins to allow rapid response to changing environmental conditions. Transport of cytoplasmically synthesized proteins to thylakoids or the thylakoid lumen is complex; the process involves transport across up to three membrane systems with routing through three aqueous compartments. Protein transport in thylakoids is accomplished by conserved ancestral prokaryotic plasma membrane translocases containing novel adaptations for the sub-organellar location. This review focuses on the evolutionarily conserved chloroplast twin arginine transport (cpTat) pathway. An overview is provided of known aspects of the cpTat components, energy requirements, and mechanisms with a focus on recent discoveries. Some of the most exciting new studies have been in determining the structural architecture of the membrane complex involved in forming the point of passage for the precursor and binding features of the translocase components. The cpTat system is of particular interest because it transports folded protein domains using only the proton motive force for energy. The implications for mechanism of translocation by recent studies focusing on interactions between membrane Tat components and with the translocating precursor will be discussed.

The chloroplast twin arginine transport (cpTat) system distinguishes itself as a protein transport pathway by translocating fully folded proteins, using the proton‐motive force (PMF) as the sole source of energy. The cpTat pathway is evolutionarily conserved with the Tat pathway found in the plasma membrane of many prokaryotes. The cpTat (Escherichia coli) system uses three proteins, Tha4 (TatA), Hcf106 (TatB), and cpTatC (TatC), to form a transient translocase allowing the passage of precursor proteins. Briefly, cpTatC and Hcf106, with Tha4, form the initial receptor complex responsible for precursor protein recognition and binding in an energy‐independent manner, while a separate pool of Tha4 assembles with the precursor‐bound receptor complex in the presence the PMF. Analysis by blue‐native polyacrylamide gel electrophoresis (BN‐PAGE) shows that the receptor complex, in the absence of precursor, migrates near 700 kDa and contains cpTatC and Hcf106 with little Tha4 remaining after detergent solubilization. To investigate the role that Hcf106 may play in receptor complex oligomerization and/or stability, systematic cysteine substitutions were made in positions from the N‐terminal transmembrane domain to the end of the predicted amphipathic helix of the protein. BN‐PAGE analysis allowed us to identify the locations of amino acids in Hcf106 that were critical for interacting with cpTatC. Oxidative cross‐linking allowed us to map interactions of the transmembrane domain and amphipathic helix region of Hcf106. In addition, we showed that in vitro expressed, integrated Hcf106 can interact with the precursor signal peptide domain and imported cpTatC, strongly suggesting that a subpopulation of the integrated Hcf106 is participating in competent cpTat complexes.

1 2 Insights into substrate-mediated assembly of the chloroplast TAT 3 receptor complex 4 5

17The chloroplast Twin arginine transport (cpTat) system distinguishes itself as a protein 18 transport pathway by translocating fully-folded proteins, using the proton-motive force 19(PMF) as the sole source of energy. The cpTat pathway is evolutionarily conserved with 20the Tat pathway found in the plasma membrane of many prokaryotes. The cpTat (E. 21coli) system uses three proteins, Tha4 (TatA), Hcf106 (TatB), and cpTatC (TatC), to 22 form a transient translocase allowing the passage of precursor proteins. Briefly, cpTatC 23and Hcf106, with Tha4, form the initial receptor complex responsible for precursor 24 protein recognition and binding in an energy-independent manner, while a separate pool 25of Tha4 assembles with the precursor-bound receptor complex in the presence the 26 PMF. Analysis by blue-native polyacrylamide gel electrophoresis (BN-PAGE) shows 27that the receptor complex, in the absence of precursor, migrates near 700 kDa and 28 contains cpTatC and Hcf106 with little Tha4 remaining after detergent solubilization. To 29investigate the role that Hcf106 may play in receptor complex oligomerization and/or 30 stability, systematic cysteine substitutions were made in positions from the N-terminal 31 transmembrane domain to the end of the predicted amphipathic helix of the protein. PAGE analysis allowed us to identify the locations of amino acids in Hcf106 that were 33 critical for interacting with cpTatC. Oxidative cross-linking allowed us to map 34interactions of the transmembrane domain and amphipathic helix region of Hcf106. In 35 addition, we showed that in vitro expressed, integrated Hcf106 can interact with the 36 precursor signal peptide domain and imported cpTatC, strongly suggesting that a sub-37 population of the integrated Hcf106 is participating in competent cpTat complexes. 38 39