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Chloroplast biogenesis relies on the import of thousands of nuclear encoded proteins into the organelle and proper sorting to their sub-organellar compartment. The majority of nucleus-encoded chloroplast proteins are synthesized in the cytoplasm and imported into the organelle via the Toc-Tic translocation systems of the chloroplast envelope. In many cases, these proteins are further targeted to subcompartments of the organelle (e.g. the thylakoid membrane and lumen or inner envelope membrane) by additional targeting systems that function downstream of the import apparatus. The inner envelope membrane (IEM) plays key roles in controlling metabolite transport between the organelle and cytoplasm, and is the major site of lipid and membrane biogenesis within the organelle. In contrast to the protein import and thylakoid targeting systems, our knowledge of the pathways and molecular mechanisms of protein targeting and integration at the IEM are very limited. Previous reports have led to the conclusion that IEM proteins are transferred to the IEM during protein import via a stop-transfer mechanism. Recent studies have shown that at least two components of the Tic machinery (AtTic40 and AtTic110) are completely imported into the stroma and then re-inserted into the IEM in a post-import mechanism. This led me to investigate the mechanisms and pathways involved in the integration of chloroplast IEM proteins in more detail. I selected candidates (AtTic40 for post-import and IEP37 for stop-transfer) that are predicted to have only one membranespanning helix and adopt the same IEM topology to facilitate my analysis. My studies confirm the existence of both stop-transfer and post-import mechanisms of IEM protein targeting. Furthermore, I conclude that the IEP37 transmembrane domain (TMD) is a stop-transfer signal and is able of diverting AtTic40 to this pathway in the absence of AtTic40 IEM targeting information. Moreover, the IEP37 TMD also functions as a topology determinant. I also show that the AtTic40 targeting signals are context dependent, with evidence that in the absence of specific information in the appropriate context, the AtTic40 TMD behaves as a stop-transfer signal. This is an indication that the stop-transfer pathway is the default mechanism of protein insertion in the IEM.

Le chloroplaste est un composant majeur de la cellule végétale. Cet organite est le fruit d’une endosymbiose, survenue entre une cellule eucaryote et une cyanobactérie. Ainsi, 95% des gènes codant pour les protéines plastidiales ont été transférés vers le génome nucléaire au cours de l’évolution. En conséquence, la plupart des protéines chloroplastiques sont aujourd’hui codées par le noyau, synthétisées dans le cytosol sous forme de précurseurs dotés d’une une extension N-terminale clivable (le "peptide de transit") et ensuite importées sans les chloroplastes via le système TOC/TIC (Translocons localisés au niveau des membranes externe et interne de l'enveloppe des chloroplastes). Jusqu'à récemment, toutes les protéines destinées aux compartiments chloroplastiques internes étaient censées posséder une séquence d’adressage N-terminale clivable et engager la machinerie d’import général TOC/TIC. Cependant, des études récentes reposant sur des approches protéomiques ont révélé l’existence de plusieurs protéines chloroplastiques dépourvues de la séquence additionnelle clivable. La première évidence de telles protéines dites non canoniques a été fournie par notre équipe, étudiant le protéome de l’enveloppe du chloroplaste d’Arabidopsis, qui a conduit à l’identification d’une protéine quinone oxidoréductase homologue nommée « ceQORH ». Bien que dépourvues de peptide de transit clivable, il s’est avéré que ces protéines sont capables de rejoindre les compartiments chloroplastiques internes. D’autre part, il a été également montré que l’import de ces protéines dans le chloroplaste n’est pas médiée par la machinerie de translocation générale TOC/TIC. De plus, il s’est avéré que ces protéines ont la particularité d’être multilocalisées dans les cellules de différents tissus de la feuille. Cependant, les mécanismes moléculaires qui contrôlent la localisation sub-cellulaire de telles protéines chloroplastiques non canoniques demeurent encore inconnus. Pour mieux caractériser fonctionnellement les composantes des systèmes d’import alternatifs de protéines chloroplastiques non canoniques, nous avons adopté une approche directe qui reposait sur des techniques biochimiques combinant le crosslink chimique, la purification par affinité et la spectrométrie de masse. Cette stratégie nous a permis d’identifier un partenaire, impliqué dans le contrôle de l’adressage de la protéine ceQORH dans le chloroplaste. Alternativement, nous avons réalisé une bio-analyse du protéome de l’enveloppe du chloroplaste et qui nous a permis de revisiter la composition du protéome de l’enveloppe du chloroplaste. Afin d’expliquer la localisation sub-cellulaire variable de la protéine ceQORH, les membres de l’équipe ont émis l’hypothèse d’une interaction probable de cette protéine avec un partenaire cytosolique. Dans la dernière partie de cette étude, nous avons validé l’interaction, in planta, entre ceQORH et son partenaire par une approche génétique qui portait sur l’analyse de l’impact de l’absence de ce dernier sur la régulation de la localisation sub-cellulaire de la protéine ceQORH. / Chloroplasts are a major component of plant cells. Their origin traces back to a cyanobacterial ancestor that was engulfed by an ancient eukaryotic cell and eventually integrated as an organelle during evolution. As a result, more than 95% of the ancestral cyanobacterial genes were transferred to the host cell nucleus. Proteins encoded by these relocated genes need to return to internal chloroplast compartments. This import is mainly achieved by the general TOC/TIC machinery located at the chloroplast surface. Until recently, all proteins destined to chloroplast were believed to possess an N-terminal and cleavable chloroplast targeting peptide, and to engage the TOC/TIC machinery. However, recent studies have revealed the existence of several non-canonical preproteins, lacking cleavable transit peptides. The first evidence for such ‘non-canonical’ chloroplast proteins was provided by our team studying the Arabidopsis chloroplast envelope proteome, leading to the identification of a quinone oxidoreductase homologue termed « ceQORH ». Furthermore, a few such proteins were demonstrated to use alternative targeting pathways, independent of the TOC/TIC machinery. To better characterize components of such alternative targeting machineries, a targeted study combining affinity purification and mass spectrometry aiming to identify alternative receptors at the chloroplast surface has been performed. This study allowed us to identify new “partner” involved in the control of chloroplast targeting of ceQORH protein. Alternatively, we also revisited the chloroplast envelope proteome composition and initiated a gene candidate approach. In addition, some non-canonical proteins are shared by plastids and other cell compartments. However, molecular mechanisms controlling subcellular localization of these non-canonical plastid proteins remain unknown. In order to explain the variable subcellular localization of ceQORH protein, our team hypothesized a probable interaction of ceQORH with a cytosolic partner. In the last part of this study, we validated the interaction between ceQORH and its partner in planta by a genetic approach analyzing the impact of the absence of the cytosolic partner on the regulation of the sub-cellular localization of ceQORH protein.

This thesis describes experiments investigating the translational recoding activities and the novel dual signalling properties of eukaryotic ribosome skipping 2A sequences. Over twenty years ago, the 19 amino acid 2A region of a Picornavirus; namely, Foot-and-Mouth Disease Virus (FMDV) polypeptide was shown to possess apparent “self-cleaving” abilities, cutting at its own C-terminus during translation (Ryan et al., 1991). Active FMDV 2A-like sequences were subsequently found in a number of related viruses (Luke et al., 2008), with several now utilised as essential biotechnology multi-gene transfer tools (Luke et al., 2010b). Then, in 2006, eukaryotic 2A-like sequences were identified from trypanosome non-LTR sequences. These were found to be functional in vitro (Heras et al., 2006). I have been able to identify over 400 putative eukaryotic 2A-like sequences through searching the freely available online proteomic and genomic databases. Data is presented to show that these 2As were encoded in frame with non-LTRs, or metabolic, or immune function genes, from a wide range of eukaryotic organisms; but I could not discern any obvious phylogenetic distribution for 2A. I have discovered that the majority of eukaryotic 2A sequences tested can mediate ribosome skipping in vitro. Modelling in silico indicated that active 2A-like sequences possessed the propensity to form a central alpha-helical region, whereas the models suggested that inactive 2A-like sequences would be essentially unstructured. I also report that some of these eukaryotic 2A peptides constitute a novel form of dual protein targeting as they play a dual role as exocytic pathway signal peptides mediating extracellular protein trafficking. I have shown that this protein trafficking ability is evolutionarily conserved, with an echinoderm sequence able to direct protein targeting in both plant and mammalian cells. I therefore propose that these novel eukaryotic 2A sequences could potentially become extremely valuable in biotechnological engineering.

The chloroplast is the organelle within a plant cell where photosynthesis takes place. This organelle originates from a cyanobacterium that was engulfed by a eukaryotic cell. During the transition from endosymbiont to organelle most of the cyanobacterial genes were transferred to the nuclear genome of the host cell, resulting in a chloroplast with a much reduced genome that requires massive import of gene products (proteins) back to the organelle. The majority of these proteins are translated in the cytosol as pre-proteins containing targeting information that directs them to a translocon complex in the chloroplast envelope, the Toc-Tic system, through which these proteins are transported. We have identified a protein in the model plant Arabidopsis thaliana, CAH1, that is trafficked via the endomembrane system (ER/Golgi apparatus) to the chloroplast instead of using the Toc-Tic machinery. This transport is partly mediated by canonical vesicle trafficking elements involved in ER to Golgi transport, such as Sar1 and RabD GTPases. Analysis of point mutated variants of CAH1 showed that both N-linked glycans and an intra-molecular disulphide bridge are required for correct folding, trafficking and function of the protein. Since chloroplasts lack N-glycosylation machinery, we propose that a route for chloroplast proteins that require endomembrane-specific post-translational modifications for their functionality exists as a complement to the Toc-Tic system. We also show that mutant plants with disrupted CAH1 gene expression have reduced rates of CO2 uptake and accumulate lower amounts of starch compared to wild-type plants, indicating an important function of the CAH1 protein for the photosynthetic capacity of Arabidopsis. Further study of CAH1 will not only be important to reveal its role in photosynthesis, but characterization of this novel protein pathway to the chloroplast can also shed light on how the plant cell evolved and clarify the purpose of keeping several chloroplast import pathways working in parallel. In addition, knowledge about this pathway could increase the opportunities for using plants as bio-factories for production of recombinant glycoproteins, which make up the vast majority of the bio-pharmaceutical molecules. / Kloroplasten är den organell i växtcellen där fotosyntesen sker. Denna organell härstammar från en cyanobakterie som togs upp av en eukaryot cell. Under omvandlingen från endosymbiont till organell har de flesta av den ursprungliga cyanobakteriens gener flyttats över till växtcellens eget kärngenom, vilket resulterat i en kloroplast som endast kan producera ett fåtal av de proteiner den behöver och som istället kräver att en mängd genprodukter (proteiner) transporteras tillbaka till organellen. De flesta av dessa proteiner syntetiseras i cytosolen som polypeptider innehållande en speciell signal för kloroplasten, och tranporteras över kloroplastens dubbelmembran (envelop) med hjälp av ett specifikt importsystem (Toc-Tic). Vi har identifierat ett protein i modellväxten Arabidopsis thaliana (CAH1) som istället för att använda Toc-Tic tranporteras via det endomembrana systemet (ER/Golgi). Transporten sker delvis med hjälp av faktorer involverade i normal vesikeltransport, t.ex. Sar1 och RabD GTPaser (mellan ER och Golgi). Genom att uttycka och analysera punktmuterade varianter av CAH1 har vi kunnat visa att både sockergrupper kopplade till proteinet, samt en intern svavelbrygga, är nödvändiga för korrekt veckning, transport och funktion av proteinet. Då kloroplasten saknar eget maskineri för att koppla sådana sockergrupper till proteiner så föreslår vi att anledningen till att denna rutt existerar, som ett komplement till Toc-Tic, är för att proteiner beroende av denna typ av modifiering ska kunna finnas i kloroplasten. Vi visar också att muterade växter som inte kan uttrycka genen som kodar för CAH1 uppvisar lägre upptag av CO2, samt ackumulerar mindre stärkelse än vildtypplantor, vilket antyder att CAH1 har en viktig funktion för den fotosyntetiska förmågan hos Arabidopsis. För att kunna fastställa den exakta funktionen för CAH1 kommer ytterliga studier att vara nödvändiga. En fördjupad karaktärisering av transportvägen som CAH1 följer till kloroplasten kan dessutom ge kunskap om hur växtcellen uppkom, samt besvara varför flera importvägar arbetar till synes parallellt med varandra. Kunskap om denna transportväg kan även bidra med användbar information i försöken att nyttja växter till att uttrycka rekombinanta N-glykosylerade proteiner, t. ex. antikroppar och vacciner.

Mitochondria carry out several important functions in eukaryotic cells such as energy metabolism, iron-sulfur cluster assembly, apoptosis, signaling pathways, protein quality control etc. Most mitochondrial proteins are synthesized on the cytosolic ribosomes and transported to the organelles by the cytosolic chaperones and mitochondrial protein import machinery based on specific targeting signals. Although, the basic principles of protein import have been explained, many questions remain unanswered, particularly for highly modified mitochondria such as hydrogenosomes. The aim of the study was to investigate protein translocation into hydrogenosomes of a human parasite, Trichomonas vaginalis (Tv) with a focus on the composition, function and structure of protein translocases and the role of targeting signals. The translocase of the outer membrane (TOM) is responsible for the import of most proteins into the organelle. Even though, the presence of a TOM complex in trichomonad hydrogenosomes was predicted, its components were not known. Moreover, the generic structure of the mitochondrial TOM complex was not resolved. This study showed that the TvTOM complex is highly divergent consisting of two modified core subunits - channel- forming TvTom40 isoforms and a Tom22-like protein, and two...

Yes / The ribosome exit site is a focal point for the interaction of protein-biogenesis factors that guide the fate of nascent polypeptides. These factors include chaperones such as NAC, N-terminal-modifying enzymes like Methionine aminopeptidase (MetAP), and the signal recognition particle (SRP), which targets secretory and membrane proteins to the ER. These factors potentially compete with one another in the short time-window when the nascent chain first emerges at the exit site, suggesting a need for regulation. Here, we show that MetAP contacts the ribosome at the universal adaptor site where it is adjacent to the α subunit of NAC. SRP is also known to contact the ribosome at this site. In the absence of NAC, MetAP and SRP antagonize each other, indicating a novel role for NAC in regulating the access of MetAP and SRP to the ribosome. NAC also functions in SRP-dependent targeting and helps to protect substrates from aggregation before translocation. / This work was supported by grants from the BBSRC [H007202/1] and Wellcome Trust [097820/Z/11/A].

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