The ER v-SNAREs are required for GPI-anchored protein sorting from other secretory proteins upon exit from the ER

Glycosylphosphatidylinositol (GPI)-anchored proteins exit the ER in distinct vesicles from other secretory proteins, and this sorting event requires the Rab GTPase Ypt1p, tethering factors Uso1p, and the conserved oligomeric Golgi complex. Here we show that proper sorting depended on the vSNAREs, Bos1p, Bet1p, and Sec22p. However, the t-SNARE Sed5p was not required for protein sorting upon ER exit. Moreover, the sorting defect observed in vitro with bos1–1 extracts was also observed in vivo and was visualized by EM. Finally, transport and maturation of the GPI-anchored protein Gas1p was specifically affected in a bos1–1 mutant at semirestrictive temperature. Therefore, we propose that v-SNAREs are part of the cargo protein sorting machinery upon exit from the ER and that a correct sorting process is necessary for proper maturation of GPI-anchored proteins

Trophic state alters the mechanism whereby energetic coupling between photosynthesis and respiration occurs in Euglena gracilis

- The coupling between mitochondrial respiration and photosynthesis plays an important role in the energetic physiology of green plants and some secondary-red photosynthetic eukaryotes(diatoms), allowing an efficient CO2 assimilation and optimal growth. - Using the flagellate Euglena gracilis, we first tested if photosynthesis–respiration coupling occurs in this species harbouring secondary green plastids (i.e. originated from an endosymbiosis between a green alga and a phagotrophic euglenozoan). Second, we tested how the trophic state (mixotrophy and photoautotrophy) of the cell alters the mechanisms involved in the photosynthesis–respiration coupling. - Energetic coupling between photosynthesis and respiration was determined by testing the effect of respiratory inhibitors on photosynthesis, and measuring the simultaneous variation of photosynthesis and respiration rates as a function of temperature (i.e. thermal response curves). The mechanism involved in the photosynthesis–respiration coupling was assessed by combining proteomics, biophysical and cytological analyses. - Our work shows that there is photosynthesis–respiration coupling and membrane contacts between mitochondria and chloroplasts in E. gracilis. However, whereas in mixotrophy adjustment of the chloroplast ATP/NADPH ratio drives the interaction, in photoautotrophy the coupling is conditioned by CO2 limitation and photorespiration. This indicates that maintenance of photosynthesis–respiration coupling, through plastic metabolic responses, is key to E. gracilis functioning under changing environmental conditions

A TSPO-related protein localizes to the early secretory pathway in Arabidopsis, but is targeted to mitochondria when expressed in yeast

AtTSPO is a TspO/MBR domain-protein potentially involved in multiple stress regulation in Arabidopsis. As in most angiosperms, AtTSPO is encoded by a single, intronless gene. Expression of AtTSPO is tightly regulated both at the transcriptional and post-translational levels. It has been shown previously that overexpression of AtTSPO in plant cell can be detrimental, and the protein was detected in the endoplasmic reticulum (ER) and Golgi stacks, contrasting with previous findings and suggesting a mitochondrial subcellular localization for this protein. To ascertain these findings, immunocytochemistry and ABA induction were used to demonstrate that, in plant cells, physiological levels of AtTSPO colocalized with AtArf1, a mainly Golgi-localized protein in plant cells. In addition, fluorescent protein-tagged AtTSPO was targeted to the secretory pathway and did not colocalize with MitoTracker-labelled mitochondria. These results suggest that the polytopic membrane protein AtTSPO is cotranslationally targeted to the ER in plant cells and accumulates in the Trans-Golgi Network. Heterologous expression of AtTSPO in Saccharomyces cerevisiae, yeast devoid of TSPO-related protein, resulted in growth defects. However, subcellular fractionation and immunoprecipitation experiments showed that AtTSPO was targeted to mitochondria where it colocalized and interacted with the outer mitochondrial membrane porin VDAC1p, reminiscent of the subcellular localization and activity of mammalian translocator protein 18 kDa TSPO. The evolutionarily divergent AtTSPO appears therefore to be switching its sorting mode in a species-dependent manner, an uncommon peculiarity for a polytopic membrane protein in eukaryotic cells. These results are discussed in relation to the recognition and organelle targeting mechanisms of polytopic membrane proteins in eukaryotic cells

Targeting of Sna3p to the Endosomal Pathway Depends on Its Interaction with Rsp5p and Multivesicular Body Sorting on Its Ubiquitylation

Rsp5p is an ubiquitin (Ub)-protein ligase of the Nedd4 family that carries WW domains involved in interaction with PPXY-containing proteins. It plays a key role at several stages of intracellular trafficking, such as Ub-mediated internalization of endocytic cargoes and Ub-mediated sorting of membrane proteins to internal vesicles of multivesicular bodies (MVBs), a process that is crucial for their subsequent targeting to the vacuolar lumen. Sna3p is a membrane protein previously described as an Ub-independent MVB cargo, but proteomic studies have since shown it to be an ubiquitylated protein. Sna3p carries a PPXY motif. We observed that this motif mediates its interaction with Rsp5p WW domains. Mutation of either the Sna3p PPXY motif or the Rsp5p WW3 domain or reduction in the amounts of Rsp5 results in the mistargeting of Sna3p to multiple mobile vesicles and prevents its sorting to the endosomal pathway. This sorting defect appears to occur prior to the defect displayed in rsp5 mutants by other MVB cargoes, which are correctly sorted to the endosomal pathway but missorted to the vacuolar membrane instead of the vacuolar lumen. Sna3p is polyubiquitylated on one target lysine, and a mutant Sna3p lacking its target lysine displays defective MVB sorting. Sna3p undergoes Rsp5-dependent polyubiquitylation, with K63-linked Ub chains

Functional study of the plasma membrane H+-ATPase PMA2 isoform from Nicotiana plumbaginifolia expressed in the yeast Saccharomyces cerevisiae

Doctorat en sciences agronomiques et ingénierie biologique - UCL, 199

Mitochondrial Proteomics of a Secondary Green Alga

Euglena gracilis is an alga that derives from a secondary endosymbiosis with a green alga. Our general objective is to study the interactions established between the chloroplast and the mitochondrion during the endosymbiosic event and to determine the phylogenetic origin of the genes encoding the proteins involved in these interactions. As a first step, we performed a high-throughput analysis of the mitochondrial proteome of Euglena gracilis. Our MS/MS experiments mostly recover mitochondrial proteins representing 15 mitochondrial pathways, which indicates that our mitochondrial extracts are relatively pure, but the phylogenetic origins of the corresponding genes are surprisingly diverse

Mitochondrial Proteomics of a Secondary Green Alga.

Euglena gracilis is an alga that derives from a secondary endosymbiosis with a green alga. Our general objective is to study the interactions established between the chloroplast and the mitochondrion during the endosymbiosic event and to determine the phylogenetic origin of the genes encoding the proteins involved in these interactions. As a first step, we performed a high-throughput analysis of the mitochondrial proteome of Euglena gracilis. Our MS/MS experiments mostly recover mitochondrial proteins representing 15 mitochondrial pathways, which indicates that our mitochondrial extracts are relatively pure, but the phylogenetic origins of the corresponding genes are surprisingly diverse