The chloroplast Tat pathway transports substrates in the dark

Routing of thylakoid lumen proteins by the chloroplast twin arginine transport pathway

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 Role of Light-Dark Regulation of the Chloroplast ATP Synthase

The chloroplast ATP synthase catalyzes the light-driven synthesis of ATP and is activated in the light and inactivated in the dark by redox-modulation through the thioredoxin system. It has been proposed that this down-regulation is important for preventing wasteful hydrolysis of ATP in the dark. To test this proposal, we compared the effects of extended dark exposure in Arabidopsis lines expressing the wild-type and mutant forms of ATP synthase that are redox regulated or constitutively active. In contrast to the predictions of the model, we observed that plants with wild-type redox regulation lost photosynthetic capacity rapidly in darkness, whereas those expressing redox-insensitive form were far more stable. To explain these results, we propose that in wild-type plants, down-regulation of ATP synthase inhibits ATP hydrolysis, leading to dissipation of thylakoid proton motive force (pmf) and subsequent inhibition of protein transport across the thylakoid through the twin arginine transporter (Tat)-dependent and Sec-dependent import pathways, resulting in the selective loss of specific protein complexes. By contrast, in mutants with a redox-insensitive ATP synthase, pmf is maintained by ATP hydrolysis, thus allowing protein transport to maintain photosynthetic activities for extended periods in the dark. Hence, a basal level of Tat-dependent, as well as, Sec-dependent import activity, in the dark helps replenishes certain components of the photosynthetic complexes and thereby aids in maintaining overall complex activity. However, the influence of a dark pmf on thylakoid protein import, by itself, could not explain all the effects we observed in this study. For example, we also observed in wild type plants a large transient buildup of thylakoid pmf and nonphotochemical exciton quenching upon sudden illumination of dark adapted plants. Therefore, we conclude that down-regulation of the ATP synthase is probably not related to preventing loss of ATP per se. Instead, ATP synthase redox regulation may be impacting a number of cellular processes such as (1) the accumulation of chloroplast proteins and/or ions or (2) the responses of photosynthesis to rapid changes in light intensity. A model highlighting the complex interplay between ATP synthase regulation and pmf in maintaining various chloroplast functions in the dark is presented. Significance Statement: We uncover an unexpected role for thioredoxin modulation of the chloroplast ATP synthase in regulating the dark-stability of the photosynthetic apparatus, most likely by controlling thylakoid membrane transport of proteins and ions.

Substrate-gated docking of pore subunit Tha4 in the TatC cavity initiates Tat translocase assembly

The twin-arginine translocase (Tat) transports folded proteins across tightly sealed membranes. cpTatC is the core component of the thylakoid translocase and coordinates transport through interactions with the substrate signal peptide and other Tat components, notably the Tha4 pore-forming component. Here, Cys-Cys matching mapped Tha4 contact sites on cpTatC and assessed the role of signal peptide binding on Tha4 assembly with the cpTatC-Hcf106 receptor complex. Tha4 made contact with a peripheral cpTatC site in nonstimulated membranes. In the translocase, Tha4 made an additional contact within the cup-shaped cavity of cpTatC that likely seeds Tha4 polymerization to form the pore. Substrate binding triggers assembly of Tha4 onto the interior site. We provide evidence that the substrate signal peptide inserts between cpTatC subunits arranged in a manner that conceivably forms an enclosed chamber. The location of the inserted signal peptide and the Tha4-cpTatC contact data suggest a model for signal peptide-gated Tha4 entry into the chamber to form the translocase.

The twin-arginine subunit C in Oscarella: origin, evolution, and potential functional significance

The twin-arginine translocation (Tat) pathway is a protein transport system that moves completely folded proteins across lipid membranes. Genes encoding components of the pathway have been found in the genomes of many Bacteria, Archaea, and eukaryotic organelles including chloroplasts, plant mitochondria, and the mitochondria of many protists. However, with a single exception, Tat genes are absent from the mitochondrial genomes of all animals. The only exception comes from the homoscleromorph sponges in the family Oscarellidae, whose mitochondrial genomes encode a gene for tatC, the largest subunit of the complex. Here, we explore the origin and evolution of the mitochondrial tatC gene in Oscarellidae, and use bioinformatic approaches to evaluate its functional significance. We conclude that tatC in Homoscleromorpha sponges was likely inherited from the ancestral proto-mitochondrial genome, implying multiple independent losses of the mitochondrial Tat pathway during the evolution of opisthokonts. In addition, bioinformatic evidence suggests that tatC comprises the entire Tat pathway in Oscarellidae, and that the Rieske Fe/S protein of mitochondrial complex III is its likely substrate.

Role of vesicle-inducing protein in plastids 1 in cpTat transport at the thylakoid

VIPP1 has been shown to be required for the proper formation of thylakoid membranes. However, studies on VIPP1 itself, as well as on PspA, its bacterial homolog, suggests that this protein may be involved in a number of additional functions, including protein translocation. The role of VIPP1 in protein translocation in the chloroplast has not been investigated. To this end, we conducted in vitro thylakoid protein transport assays to look at the effect of VIPP1 on the cpTat pathway, which is one of three translocation pathways found in both the chloroplast and its bacterial progenitor. We found that VIPP1 does indeed enhance protein transport through the cpTat pathway by up to 100%. The VIPP1 effect on cpTat activity occurs without interacting with the substrates or components of the translocon, and does not alter the energy potentials driving this translocation pathway. Instead, VIPP1 greatly enhances the amount of substrate bound productively to the thylakoids. Moreover, the presence of increasing VIPP1 concentrations in the reactions resulted in greater interactions between thylakoid membranes. Taken together, these results demonstrate a stimulatory role for VIPP1 in cpTat transport by enhancement of substrate binding, probably to the membrane lipid regions of the thylakoid. We propose a model in which VIPP1 facilitates reorganization of the thylakoid structure to increase substrate access to productive binding regions of the membrane as an early step in the cpTat pathway.

The Tat-dependent protein translocation pathway

The twin-arginine translocation (Tat) pathway is found in bacteria, archaea, and plant chloroplasts, where it is dedicated to the transmembrane transport of fully folded proteins. These proteins contain N-terminal signal peptides with a specific Tat-system binding motif that is recognized by the transport machinery. In contrast to other protein transport systems, the Tat system consists of multiple copies of only two or three usually small (∼8–30 kDa) membrane proteins that oligomerize to two large complexes that transiently interact during translocation. Only one of these complexes includes a polytopic membrane protein, TatC. The other complex consists of TatA. Tat systems of plants, proteobacteria, and several other phyla contain a third component, TatB. TatB is evolutionarily and structurally related to TatA and usually forms tight complexes with TatC. Minimal two-component Tat systems lacking TatB are found in many bacterial and archaeal phyla. They consist of a ‘bifunctional’ TatA that also covers TatB functionalities, and a TatC. Recent insights into the structure and interactions of the Tat proteins have various important implications.

Protein transport in organelles: Protein transport into and across the thylakoid membrane

The chloroplast thylakoid is the most abundant membrane system in nature, and is responsible for the critical processes of light capture, electron transport and photophosphorylation. Most of the resident proteins are imported from the cytosol and then transported into or across the thylakoid membrane. This minireview describes the multitude of pathways used for these proteins. We discuss the huge differences in the mechanisms involved in the secretory and twin-arginine translocase pathways used for the transport of proteins into the lumen, with an emphasis on the differing substrate conformations and energy requirements. We also discuss the rationale for the use of two different systems for membrane protein insertion: the signal recognition particle pathway and the so-called spontaneous pathway. The recent crystallization of a key chloroplast signal recognition particle component provides new insights into this rather unique form of signal recognition particle.