Prerequisites for terminal processing of thylakoidal Tat substrates

The ten amino acids of the oxygen-evolving enhancer of tobacco is sufficient as the peptide residues for protein transport to the chloroplast thylakoid

KEY MESSAGE

The thylakoid transit peptide of tobacco oxygen-evolving enhancer protein contains a minimal ten amino acid sequences for thylakoid lumen transports. This ten amino acids do not contain twin-arginine, which is required for typical chloroplast lumen translocation. Chloroplasts are intracellular organelles responsible for photosynthesis to produce organic carbon for all organisms. Numerous proteins must be transported from the cytosol to chloroplasts to support photosynthesis. This transport is facilitated by chloroplast transit peptides (TPs). Four chloroplast thylakoid lumen TPs were isolated from Nicotiana tabacum and were functionally analyzed as thylakoid lumen TPs. Typical chloroplast stroma-transit peptides and thylakoid lumen transit peptides (tTPs) are found in N. tabacum transit peptides (NtTPs) and the functions of these peptides are confirmed with TP-GFP fusion proteins under fluorescence microscopy and chloroplast fractionation, followed by Western blot analysis. During the functional analysis of tTPs, we uncovered the minimum 10 amino acid sequence is sufficient for thylakoid lumen transport. These ten amino acids can efficiently translocate GFP protein, even if they do not contain the twin-arginine residues required for the twin-arginine translocation (Tat) pathway, which is a typical thylakoid lumen transport. Further, thylakoid lumen transporting processes through the Tat pathway was examined by analyzing tTP sequence functions and we demonstrate that the importance of hydrophobic core for the tTP cleavage and target protein translocation.

Structure and dynamics of thylakoids in land plants

Thylakoids of land plants have a bipartite structure, consisting of cylindrical grana stacks, made of membranous discs piled one on top of the other, and stroma lamellae which are helically wound around the cylinders. Protein complexes predominantly located in the stroma lamellae and grana end membranes are either bulky [photosystem I (PSI) and the chloroplast ATP synthase (cpATPase)] or are involved in cyclic electron flow [the NAD(P)H dehydrogenase (NDH) and PGRL1-PGR5 heterodimers], whereas photosystem II (PSII) and its light-harvesting complex (LHCII) are found in the appressed membranes of the granum. Stacking of grana is thought to be due to adhesion between Lhcb proteins (LHCII or CP26) located in opposed thylakoid membranes. The grana margins contain oligomers of CURT1 proteins, which appear to control the size and number of grana discs in a dosage- and phosphorylation-dependent manner. Depending on light conditions, thylakoid membranes undergo dynamic structural changes that involve alterations in granum diameter and height, vertical unstacking of grana, and swelling of the thylakoid lumen. This plasticity is realized predominantly by reorganization of the supramolecular structure of protein complexes within grana stacks and by changes in multiprotein complex composition between appressed and non-appressed membrane domains. Reversible phosphorylation of LHC proteins (LHCPs) and PSII components appears to initiate most of the underlying regulatory mechanisms. An update on the roles of lipids, proteins, and protein complexes, as well as possible trafficking mechanisms, during thylakoid biogenesis and the de-etiolation process complements this review.

The TatBC complex of the Tat protein translocase in Escherichia coli and its transition to the substrate-bound TatABC complex

The twin-arginine translocation (Tat) system serves to transport folded proteins across membranes of prokaryotes and plant plastids. In Escherichia coli, a complex consisting of multiple copies of TatB and TatC initiates the transport by binding the signal peptides of the Tat substrates. Using blue-native polyacrylamide gel electrophoresis, bands of TatBC-containing complexes can be detected at molecular masses of 440 and 580 kDa. We systematically analyzed the formation of Tat complexes with TatB or TatC variants that carried point mutations at selected positions. Several mutations resulted in specific disassembly patterns and alterations in the 440 kDa:580 kDa complex ratios. The 440 kDa complex contains only TatBC, whereas the 580 kDa complex consists of TatABC. Substrate binding results in a TatBC-Tat substrate complex at ~500 kDa and a TatABC-Tat substrate complex at ~600 kDa. Only the ~600 kDa complex was detected with nonrecombinant substrate levels and thus could be the physiologically most relevant species. The results suggest that some TatA is usually associated with TatBC, regardless of substrate binding.

Tat transport of a Sec passenger leads to both completely translocated as well as membrane-arrested passenger proteins

We have studied the membrane transport of the chimeric precursor protein 16/33, which is composed of the Tat(1)-specific transport signal of OEC16 and the Sec passenger protein OEC33, both subunits of the oxygen-evolving system associated with photosystem II. Protein transport experiments performed with isolated pea thylakoids show that the 16/33 chimera is transported in a strictly Tat-dependent manner into the thylakoid vesicles yielding mature OEC33 (mOEC33) in two different topologies. One fraction accumulates in the thylakoid lumen and is thus resistant to externally added protease. A second fraction is arrested during transport in an N-in/C-out topology within the membrane. Chase experiments demonstrate that this membrane-arrested mOEC33 moiety does not represent a translocation intermediate but instead an alternative end product of the transport process. Transport arrest of mOEC33, which is embedded in the membrane with a mildly hydrophobic protein segment, requires more than 26 additional and predominantly hydrophilic residues C-terminal of the membrane-embedded segment. Furthermore, it is stimulated by mutations which potentially affect the conformation of mOEC33 suggesting that at least partial folding of the passenger protein is required for complete membrane translocation.

Multiple fates of non-mature lumenal proteins in thylakoids

Most proteins found in the thylakoid lumen are synthesized in the cytosol with an N-terminal extension consisting of transient signals for chloroplast import and thylakoid transfer in tandem. The thylakoid-transfer signal is required for protein sorting from the stroma to thylakoids, mainly via the cpSEC or cpTAT pathway, and is removed by the thylakoidal processing peptidase in the lumen. An Arabidopsis mutant lacking one of the thylakoidal processing peptidase homologs, Plsp1, contains plastids with anomalous thylakoids and is seedling-lethal. Furthermore, the mutant plastids accumulate two cpSEC substrates (PsbO and PetE) and one cpTAT substrate (PsbP) as intermediate forms. These properties of plsp1-null plastids suggest that complete maturation of lumenal proteins is a critical step for proper thylakoid assembly. Here we tested the effects of inhibition of thylakoid-transfer signal removal on protein targeting and accumulation by examining the localization of non-mature lumenal proteins in the Arabidopsis plsp1-null mutant and performing a protein import assay using pea chloroplasts. In plsp1-null plastids, the two cpSEC substrates were shown to be tightly associated with the membrane, while non-mature PsbP was found in the stroma. The import assay revealed that inhibition of thylakoid-transfer signal removal did not disrupt cpSEC- and cpTAT-dependent translocation, but prevented release of proteins from the membrane. Interestingly, non-mature PetE2 was quickly degraded under light, and unprocessed PsbO1 and PsbP1 were found in a 440-kDa complex and as a monomer, respectively. These results indicate that the cpTAT pathway may be disrupted in the plsp1-null mutant, and that there are multiple mechanisms to control unprocessed lumenal proteins in thylakoids.

Exclusively membrane-inserted state of an uncleavable Tat precursor protein suggests lateral transfer into the bilayer from the translocon

In bacteria, the export of proteins by the twin-arginine translocase (Tat) pathway is directed by cleavable N-terminal signal peptides. We studied the relationship between transport and maturation using a substrate, YedY, that contains an Ala > Leu substitution at the -1 position of the signal peptide. This blocks maturation and leads to the accumulation of a membrane-bound precursor form with the mature domain exposed to the periplasm. Its accumulation does not block transport of other Tat substrates, indicating that exit from the translocation channel has taken place, and the precursor protein is fir mLy integrated into the membrane bilayer. The membrane-integrated nature of the precursor, and complete absence of precursor protein in the periplasm, strongly suggest that the precursor has undergone lateral transfer into the bilayer during translocation. We propose that subsequent proteolytic processing releases the mature protein into the periplasm. A delay in processing results in an inhibition of cell growth, emphasizing a requirement for efficient maturation of Tat substrates.

Twin-arginine-dependent translocation of folded proteins

Twin-arginine translocation (Tat) denotes a protein transport pathway in bacteria, archaea and plant chloroplasts, which is specific for precursor proteins harbouring a characteristic twin-arginine pair in their signal sequences. Many Tat substrates receive cofactors and fold prior to translocation. For a subset of them, proofreading chaperones coordinate maturation and membrane-targeting. Tat translocases comprise two kinds of membrane proteins, a hexahelical TatC-type protein and one or two members of the single-spanning TatA protein family, called TatA and TatB. TatC- and TatA-type proteins form homo- and hetero-oligomeric complexes. The subunits of TatABC translocases are predominantly recovered from two separate complexes, a TatBC complex that might contain some TatA, and a homomeric TatA complex. TatB and TatC coordinately recognize twin-arginine signal peptides and accommodate them in membrane-embedded binding pockets. Advanced binding of the signal sequence to the Tat translocase requires the proton-motive force (PMF) across the membranes and might involve a first recruitment of TatA. When targeted in this manner, folded twin-arginine precursors induce homo-oligomerization of TatB and TatA. Ultimately, this leads to the formation of a transmembrane protein conduit that possibly consists of a pore-like TatA structure. The translocation step again is dependent on the PMF.

Kinetics of precursor interactions with the bacterial Tat translocase detected by real-time FRET

The Escherichia coli twin-arginine translocation (Tat) system transports fully folded and assembled proteins across the inner membrane into the periplasmic space. Traditionally, in vitro protein translocation studies have been performed using gel-based transport assays. This technique suffers from low time resolution, and often, an inability to distinguish between different steps in a continuously occurring translocation process. To address these limitations, we have developed an in vitro FRET-based assay that reports on an early step in the Tat translocation process in real-time. The natural Tat substrate pre-SufI was labeled with Alexa532 (donor), and the fluorescent protein mCherry (acceptor) was fused to the C terminus of TatB or TatC. The colored Tat proteins were easily visible during purification, enabling identification of a highly active inverted membrane vesicle (IMV) fraction yielding transport rates with NADH almost an order of magnitude faster than previously reported. When pre-SufI was bound to the translocon, FRET was observed for both Tat proteins. FRET was diminished upon addition of nonfluorescent pre-SufI, indicating that the initial binding step is reversible. When the membranes were energized with NADH, the FRET signal was lost after a short delay. These data suggest a model in which a Tat cargo initially associates with the TatBC complex, and an electric field gradient is required for the cargo to proceed to the next stage of transport. This cargo migration away from the TatBC complex requires a significant fraction of the total transport time.

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.

Twin arginine translocation (Tat)-dependent protein transport: the passenger protein participates in the initial membrane binding step

The initial step in twin arginine translocation (Tat)-dependent thylakoid transport of the 16/23 chimera is the interaction of the protein with the lipid bilayer. It results in the formation of the early translocation intermediate Ti-1, which is represented by a protease-protected fragment of 14 kDa. Cys-scanning mutagenesis in combination with in thylakoido and liposome insertion assays was used to precisely map this membrane-interacting and protease-protected fragment within the 16/23 chimera. The fragment comprises 124 residues, which are provided both by the transit peptide (31 residues) and the mature protein (93 residues), demonstrating that the passenger protein directly participates in membrane binding. The implications of this finding on the mechanism of Tat-dependent protein transport are discussed.

The significance of protein maturation by plastidic type I signal peptidase 1 for thylakoid development in Arabidopsis chloroplasts

Thylakoids are the chloroplast internal membrane systems that house light-harvesting and electron transport reactions. Despite the important functions and well-studied constituents of thylakoids, the molecular mechanism of their development remains largely elusive. A recent genetic study has demonstrated that plastidic type I signal peptidase 1 (Plsp1) is vital for proper thylakoid development in Arabidopsis (Arabidopsis thaliana) chloroplasts. Plsp1 was also shown to be necessary for processing of an envelope protein, Toc75, and a thylakoid lumenal protein, OE33; however, the relevance of the protein maturation in both of the two distinct subcompartments for proper chloroplast development remained unknown. Here, we conducted an extensive analysis of the plsp1-null mutant to address the significance of lumenal protein maturation in thylakoid development. Plastids that lack Plsp1 were found to accumulate vesicles of variable sizes in the stroma. Analyses of the mutant plastids revealed that the lack of Plsp1 causes a reduction in accumulation of thylakoid proteins and that Plsp1 is involved in maturation of two additional lumenal proteins, OE23 and plastocyanin. Further immunoblotting and electron microscopy immunolocalization studies showed that OE33 associates with the stromal vesicles of the mutant plastids. Finally, we used a genetic complementation system to demonstrate that accumulation of improperly processed forms of Toc75 in the plastid envelope does not disrupt normal plant development. These results suggest that proper maturation of lumenal proteins may be a key process for correct assembly of thylakoids.

Tat subunit stoichiometry in Arabidopsis thaliana challenges the proposed function of TatA as the translocation pore

The twin arginine translocation (Tat) machinery which is capable of transporting folded proteins across lipid bilayers operates in the thylakoid membrane of plant chloroplasts as well as in the cytoplasmic membrane of bacteria. It is composed of three integral membrane proteins (TatA, TatB, and TatC) which form heteromeric complexes of high molecular weight that accomplish binding and transport of substrates carrying Tat pathway-specific signal peptides. Western analyses using affinity purified antibodies showed in both, juvenile and adult tissue from Arabidopsis thaliana, an approximately equimolar ratio of the TatB and TatC components, whereas TatA was detectable only in minor amounts. Upon Blue Native-PAGE, TatB and TatC were found in four heteromeric TatB/C complexes possessing molecular weights of approximately 310, 370, 560 and 620 kDa, respectively, while TatA was detected only in a molecular weight range below 200 kDa. The implications of these findings on the currently existing models explaining the mechanism of Tat transport are discussed.

A stromal pool of TatA promotes Tat-dependent protein transport across the thylakoid membrane

In chloroplasts and bacteria, the Tat (twin-arginine translocation) system is engaged in transporting folded passenger proteins across the thylakoid and cytoplasmic membranes, respectively. To date, three membrane proteins (TatA, TatB, and TatC) have been identified to be essential for Tat-dependent protein translocation in the plant system, whereas soluble factors seem not to be required. In contrast, in the bacterial system, several cytosolic chaperones were described to be involved in Tat transport processes. Therefore, we have examined whether stromal or peripherally associated membrane proteins also play a role in Tat transport across the thylakoid membrane. Analyzing both authentic precursors as well as the chimeric 16/23 protein, which allows us to study each step of the translocation process individually, we demonstrate that a soluble form of TatA is present in the chloroplast stroma, which significantly improves the efficiency of Tat-dependent protein transport. Furthermore, this soluble TatA is able to reconstitute the Tat transport properties of thylakoid membranes that are transport-incompetent due to extraction with solutions of chaotropic salts.

Translocation of aprotinin, a therapeutic protease inhibitor, into the thylakoid lumen of genetically engineered tobacco chloroplasts

Aprotinin, a bovine protease inhibitor of important therapeutic value, was expressed in tobacco plastid transformants. This disulphide bond-containing protein was targeted to the lumen of thylakoids using signal peptides derived from nuclear genes which encode lumenal proteins. Translocation was attempted via either the general secretion (Sec) or the twin-arginine translocation (Tat) pathway. In both cases, this strategy allowed the production of genuine aprotinin with its N-terminal arginine residue. The recombinant protease inhibitor was efficiently secreted within the lumen of thylakoids, accumulated in older leaves and was bound to trypsin, suggesting that the three disulphide bonds of aprotinin are correctly folded and paired in this chloroplast compartment. Mass spectrometric analysis indicated that translocation via the Sec pathway, unlike the Tat pathway, led predominantly to an oxidized protein. Translocation via the Tat pathway was linked to a slightly decreased growth rate, a pale-green leaf phenotype and supplementary expression products associated with the thylakoids.

Translocation of a phycoerythrin alpha subunit across five biological membranes

Cryptophytes, unicellular algae, evolved by secondary endosymbiosis and contain plastids surrounded by four membranes. In contrast to cyanobacteria and red algae, their phycobiliproteins do not assemble into phycobilisomes and are located within the thylakoid lumen instead of the stroma. We identified two gene families encoding phycoerythrin alpha and light-harvesting complex proteins from an expressed sequence tag library of the cryptophyte Guillardia theta. The proteins bear a bipartite topogenic signal responsible for the transport of nuclear encoded proteins via the ER into the plastid. Analysis of the phycoerythrin alpha sequences revealed that more than half of them carry an additional, third topogenic signal comprising a twin arginine motif, which is indicative of Tat (twin arginine transport)-specific targeting signals. We performed import studies with several derivatives of one member using a diatom transformation system, as well as intact chloroplasts and thylakoid vesicles isolated from pea. We demonstrated the different targeting properties of each individual part of the tripartite leader and show that phycoerythrin alpha is transported across the thylakoid membrane into the thylakoid lumen and protease-protected. Furthermore, we showed that thylakoid transport of phycoerythrin alpha takes place by the Tat pathway even if the 36 amino acid long bipartite topogenic signal precedes the actual twin arginine signal. This is the first experimental evidence of a protein being targeted across five biological membranes.