Evolution of mitochondrial TAT translocases illustrates the loss of bacterial protein transport machines in mitochondria

Abstract Background Bacteria and mitochondria contain translocases that function to transport proteins across or insert proteins into their inner and outer membranes. Extant mitochondria retain some bacterial-derived translocases but have lost others. While BamA and YidC were integrated into general mitochondrial protein transport pathways (as Sam50 and Oxa1), the inner membrane TAT translocase, which uniquely transports folded proteins across the membrane, was retained sporadically across the eukaryote tree. Results We have identified mitochondrial TAT machinery in diverse eukaryotic lineages and define three different types of eukaryote-encoded TatABC-derived machineries (TatAC, TatBC and TatC-only). Here, we investigate TatAC and TatC-only machineries, which have not been studied previously. We show that mitochondria-encoded TatAC of the jakobid Andalucia godoyi represent the minimal functional pathway capable of substituting for the Escherichia coli TatABC complex and can transport at least one substrate. However, selected TatC-only machineries, from multiple eukaryotic lineages, were not capable of supporting the translocation of this substrate across the bacterial membrane. Despite the multiple losses of the TatC gene from the mitochondrial genome, the gene was never transferred to the cell nucleus. Although the major constraint preventing nuclear transfer of mitochondrial TatC is likely its high hydrophobicity, we show that in chloroplasts, such transfer of TatC was made possible due to modifications of the first transmembrane domain. Conclusions At its origin, mitochondria inherited three inner membrane translocases Sec, TAT and Oxa1 (YidC) from its bacterial ancestor. Our work shows for the first time that mitochondrial TAT has likely retained its unique function of transporting folded proteins at least in those few eukaryotes with TatA and TatC subunits encoded in the mitochondrial genome. However, mitochondria, in contrast to chloroplasts, abandoned the machinery multiple times in evolution. The overall lower hydrophobicity of the Oxa1 protein was likely the main reason why this translocase was nearly universally retained in mitochondrial biogenesis pathways

A signal sequence suppressor mutant that stabilizes an assembled state of the twin arginine translocase

The twin-arginine protein translocation (Tat) system mediates transport of folded proteins across the cytoplasmic membrane of bacteria and the thylakoid membrane of chloroplasts. The Tat system of Escherichia coli is made up of TatA, TatB and TatC components. TatBC comprise the substrate receptor complex, and active Tat translocases are formed by the substrate-induced association of TatA oligomers with this receptor. Proteins are targeted to TatBC by signal peptides containing an essential pair of arginine residues. We isolated substitutions, locating to the transmembrane helix of TatB that restored transport activity to Tat signal peptides with inactivating twin arginine substitutions. A subset of these variants also suppressed inactivating substitutions in the signal peptide binding site on TatC. The suppressors did not function by restoring detectable signal peptide binding to the TatBC complex. Instead, site specific crosslinking experiments indicate that the suppressor substitutions induce conformational change in the complex and movement of the TatB subunit. The TatB F13Y substitution was associated with the strongest suppressing activity, even allowing transport of a Tat substrate lacking a signal peptide. In vivo analysis using a TatA-YFP fusion showed that the TatB F13Y substitution resulted in signal peptide independent assembly of the Tat translocase. We conclude that Tat signal peptides play roles in substrate targeting and in triggering assembly of the active translocase

The TatC component of the twin-arginine protein translocase functions as an obligate oligomer

The Tat protein export system translocates folded proteins across the bacterial cytoplasmic membrane and the plant thylakoid membrane. The Tat system in Escherichia coli is composed of TatA, TatB and TatC proteins. TatB and TatC form an oligomeric, multivalent receptor complex that binds Tat substrates, while multiple protomers of TatA assemble at substrate-bound TatBC receptors to facilitate substrate transport. We have addressed whether oligomerisation of TatC is an absolute requirement for operation of the Tat pathway by screening for dominant negative alleles of tatC that inactivate Tat function in the presence of wild-type tatC. Single substitutions that confer dominant negative TatC activity were localised to the periplasmic cap region. The variant TatC proteins retained the ability to interact with TatB and with a Tat substrate but were unable to support the in vivo assembly of TatA complexes. Blue-native PAGE analysis showed that the variant TatC proteins produced smaller TatBC complexes than the wild-type TatC protein. The substitutions did not alter disulphide crosslinking to neighbouring TatC molecules from positions in the periplasmic cap but abolished a substrate-induced disulphide crosslink in transmembrane helix 5 of TatC. Our findings show that TatC functions as an obligate oligomer.</p

Conserved motifs reveal details of ancestry and structure in the small tim chaperones of the mitochondrial intermembrane space

The mitochondrial inner and outer membranes are composed of a variety of integral membrane proteins, assembled into the membranes posttranslationally. The small translocase of the inner mitochondrial membranes (TIMs) are a group of ∼10 kDa proteins that function as chaperones to ferry the imported proteins across the mitochondrial intermembrane space to the outer and inner membranes. In yeast, there are 5 small TIM proteins: Tim8, Tim9, Tim10, Tim12, and Tim13, with equivalent proteins reported in humans. Using hidden Markov models, we find that many eukaryotes have proteins equivalent to the Tim8 and Tim13 and the Tim9 and Tim10 subunits. Some eukaryotes provide "snapshots" of evolution, with a single protein showing the features of both Tim8 and Tim13, suggesting that a single progenitor gene has given rise to each of the small TIMs through duplication and modification. We show that no "Tim12" family of proteins exist, but rather that variant forms of the cognate small TIMs have been recently duplicated and modified to provide new functions: the yeast Tim12 is a modified form of Tim10, whereas in humans and some protists variant forms of Tim9, Tim8, and Tim13 are found instead. Sequence motif analysis reveals acidic residues conserved in the Tim10 substrate-binding tentacles, whereas more hydrophobic residues are found in the equivalent substrate-binding region of Tim13. The substrate-binding region of Tim10 and Tim13 represent structurally independent domains: when the acidic domain from Tim10 is attached to Tim13, the Tim8–Tim13¹⁰ complex becomes essential and the Tim9–Tim10 complex becomes dispensable. The conserved features in the Tim10 and Tim13 subunits provide distinct binding surfaces to accommodate the broad range of substrate proteins delivered to the mitochondrial inner and outer membranes

Crop Updates 2002 - Oilseeds

This session covers twenty seven papers from different authors: 1. Forward and acknowledgements, Dave Eksteen, ACTING MANAGER OILSEEDS PRODUCTIVITY AND INDUSTRY DEVELOPMENT Department of Agriculture PLENARY SESSION 2. GMO canola - Track record in Canada, K. Neil Harker and George W. Clayton,Agriculture and Agri-Food Canada, Lacombe Research Centre, Lacombe, Alberta, R. Keith Downey, Agriculture and Agri-Food Canada, Saskatoon Research Centre, Saskatoon, Saskatchewan 3. GMO canola – Prospects in Western Australia farming systems, Keith Alcock, Crop Improvement Institute, Department of Agriculture 4. Diamondback moth (DBM) in canola, Kevin Walden, Department of Agriculture CANOLA AGRONOMY 5. Getting the best out of canola in the low rainfall central wheatbelt, Bevan Addison and Peter Carlton, Elders Ltd 6. Canola variety performance in Western Australia, Kevin Morthorpe, Stephen Addenbrooke and Alex Ford, Pioneer Hi-Bred Australia P/L 7. Relative performance of new canola varieties in Department of Agriculture variety trials in 2000 and 2001, S. Hasan Zaheer, GSARI, Department of Agriculture, G. Walton, Crop Improvement Institute, Department of Agriculture 8. Which canola cultivar should I sow? Imma Farré, CSIRO Plant Industry, Floreat, Bill Bowden,Western Australia Department of Agriculture 9. The effect of seed generation and seed source on yield and quality of canola, Paul Carmody, Department of Agriculture 10. The accumulation of oil in Brassica species, J.A. Fortescue and D.W. Turner, Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, B. Tan, PO Box 1249, South Perth 11. Potential and performance of alternative oilseeds in WA, Margaret C. Campbell, Centre for Legumes in Mediterranean Agriculture 12. Comparison of oilseed crops in WA, Ian Pritchard and Paul Carmody, Department of Agriculture, Centre for Cropping Systems, Margaret Campbell, Centre for Legumes in Mediterranean Agriculture 13. Identifying constraints to canola production, Dave Eksteen, Canola Development Officer, Department of Agriculture 14. Boron – should we be worried about it? Richard W. BellA, K. FrostA, Mike WongB, and Ross BrennanC , ASchool of Environmental Science, Murdoch University, BCSIRO Land and Water, CDepartment of Agriculture PEST AND DISEASE 15. Yield losses caused when Beet Western Yellows Virus infects canola, Roger Jones and Jenny Hawkes, Department of Agriculture, and Centre for Legumes in Mediterranean Agriculture 16. Influence of climate on aphid outbreaks and virus epidemics in canola, Debbie Thackray, Jenny Hawkes and Roger Jones, Centre for Legumes in Mediterranean Agriculture and Department of Agriculture 17. The annual shower of blackleg ascospores in canola: Can we predict and avoid it? Moin U. Salam, Ravjit K. Khangura, Art J. Diggle and Martin J. Barbetti, Department of Agriculture 18. Environmental influences on production and release of ascospores of blackleg and their implications in blackleg management in canola, Ravjit K. Khangura, Martin J. Barbetti , Moin U. Salam and Art J. Diggle, Department of Agriculture 19. WA blackleg resistance ratings on canola varieties form 2002, Ravjit Khangura, Martin J. Barbetti and Graham Walton, Department of Agriculture 20. Bronzed field beetle management in canola, Phil Michael, Department of Agriculture 21. DBM control in canola: Aerial versus boom application, Paul Carmody, Department of Agriculture 22. Effect of single or multiple spray trearments on the control of Diamondback moth (Plutella xylostella) and yield of canola at Wongan Hills, Françoise Berlandier, Paul Carmody and Christiaan Valentine, Department of Agriculture ESTABLISHMENT 23. GrainGuardÔ - A biosecurity plan for the canola industry, Greg Shea, Department of Agriculture 24. Large canola seed is best, particularly for deep sowing, Glen Riethmuller, Rafiul Alam, Greg Hamilton and Jo Hawksley, Department of Agriculture 25. Canola establishment with seed size, tines and discs, with and without stubble, Glen Riethmuller, Rafiul Alam, Greg Hamilton and Jo Hawksley, Department of Agriculture WEEDS 26. Role of Roundup ReadyÒ canola in the farming system, Art Diggle1, Patrick Smith2, Paul Neve3, Felicity Flugge4, Amir Abadi5, Stephen Powles3 1Department of Agriculture, 2CSIRO, Sustainable Ecosystems, 3Western Australian Herbicide Resistance Initiative, University of Western Australia, 4Centre for Legumes in Mediterranean Agriculture, University of Western Australia, 5Touchstone Consulting, Mt Hawthorn FEED 27. Getting value from canola meals in the animal feed industries: Aquaculture, Brett Glencross and John Curnow, Department of Fisheries - Government of Western Australia and Wayne Hawkins, Department of Agricultur

New insights into the Tat protein transport cycle from characterising the assembled Tat translocon

The twin-arginine protein translocation (Tat) system transports folded proteins across the bacterial cytoplasmic membrane and the thylakoid membrane of chloroplasts. The Tat translocation site is transiently assembled by the recruitment of multiple TatA proteins to a substrate-activated TatBC receptor complex in a process requiring the protonmotive force. The ephemeral nature of the Tat translocation site has so far precluded its isolation. We now report that detergent solubilization of membranes during active transport allows the recovery of receptor complexes that are associated with elevated levels of TatA. We apply this biochemical analysis in combination with live cell fluorescence imaging to Tat systems trapped in the assembled state. We resolve sub-steps in the Tat translocation cycle and infer that TatA assembly precedes the functional interaction of TatA with a polar cluster site on TatC. We observe that dissipation of the protonmotive force releases TatA oligomers from the assembled translocation site demonstrating that the stability of the TatA oligomer does not depend on binding to the receptor complex and implying that the TatA oligomer is assembled at the periphery of the receptor complex. This work provides new insight into the Tat transport cycle and advances efforts to isolate the active Tat translocon

Minor modifications and major adaptations: The evolution of molecular machines driving mitochondrial protein import

AbstractBacterial endosymbionts gave rise to mitochondria in a process that depended on the acquisition of protein import pathways. Modification and in some cases major re-tooling of the endosymbiont's cellular machinery produced these pathways, establishing mitochondria as organelles common to all eukaryotic cells. The legacy of this evolutionary tinkering can be seen in the homologies and structural similarities between mitochondrial protein import machinery and modern day bacterial proteins. Comparative analysis of these systems is revealing both possible routes for the evolution of the mitochondrial membrane translocases and a greater understanding of the mechanisms behind mitochondrial protein import. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes