 huomenta
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Message du 14-03-2009 @ 14:51
Torasme a écrit
Pas mieux 
On va tous penser à toi pour ce jour... et au jour suivant où tu seras complètement torché 
J'ai une bouteille de chartreuse et d'absinthe qui attendent d'être dégustée  Et pour plus d'effet ce sera peut être dans un sauna  Il devrait surement y avoir une cousine russe aussi 
Merci Mario, si seulement c'était vrai 
Et désolé pour la banquise 
sebb974 a écrit ué, je veux bien lire ta petite intro pour voir ce qu'il en est
merci
Bon courage
Fatty acids (FAs) are essential molecules composed of oxygen, carbon and hydrogen, and are used in many different ways in cells. They can function in energy storage, and as structural elements or building blocks for the synthesis of more complex molecules. They also have regulatory roles in biological processes (Kohlwein et al. 2001, Schneiter et al. 1996, Schneiter et al. 2004, Leonard et al. 2002). FA function and category are determined mostly by the length of the carbon chain: short FAs range from 2 to 6 carbons, medium FAs from 8 to 12 carbons, long chain FAs from 14 to 18, and very long chain fatty acids (VLCFA) from 20 carbons.
In yeast, VLCFA also serve as building blocks for much more complex molecules but are not taken up as nutrients, which makes their endogenous synthesis through the elongation pathway an essential process for the cell. The protein complex catalyzing fatty acid chain elongation is located in the endoplasmic reticulum (ER) membrane (Nugteren 1965) and uses long chain FAs (C16-C18) generated by the cytosolic fatty acids synthase as substrates.
In this work, the yeast S. cerevisiae was used as a model organism to characterize cotton fiber genes involved in the VLCFA elongation pathway. The disturbed VLCFA profile of yeast deletion mutants lacking the elongase genes was restored to wild type when the corresponding cotton homologues were expressed in these mutant cells.
Prior to the initiation of this study, three of the four enzymes of the elongation cycle in yeast were known. The dehydratase, however, had not been identified (Han et al. 2002).
N-linked glycosylation is the main protein modification in eukaryotes and it takes place on nascent proteins in the ER lumen on asparagine side chains in the sequence Asn-X-Ser/Thr, unless X is a proline (Welply et al. 1983). The added glycan increases the mass and solubility of the nascent peptide, prevents the aggregation of hydrophobic patches, and makes proteins more resistant to denaturation and proteolysis (Helenius & Aebi 2004). Glycan synthesis is initiated on the cytosolic side of the ER membrane and proceeds until a structure composed of a dolichol pyrophosphate, two N-acetylglucosamine and five mannose residues (DolPP-GlcNac2-Man5) is formed (Bugg & Brandish 1994). A key step in the glycan synthesis is the translocation of the lipid linked oligosaccharide (LLO) across the ER membrane in an Rft1p-dependent process (Helenius et al. 2002). The glycan facing the ER lumen will be further elongated, by the addition of four mannose and three glucose moieties, up to DolPP-GlcNac2-Man9-Glc3 and thereafter transferred to the nascent peptide.
In this study, a chimeric protein engineered to introduce redundancy in the dehydratase step of the VLCFA elongation pathway suppressed the reported yeast rft1Δ lethal phenotype (Helenius et al. 2002). The data demonstrated that the suppression was due to unspecific LLO flipping. Our observation that inactivation of the ER associated protein degradation pathway (ERAD) rescues the growth of the S. cerevisiae rft1Δ highlights the importance of the malfolded protein response for the specificity of the LLO flipping and membrane integrity in general. The data obtained through functional complementation of the yeast rft1Δ mutant demonstrated that the human homologue was capable of substituting for the yeast Rft1p in flipping (Haeuptle et al. 2008). Furthermore, the results also showed that a human RFT1 variant isolated from a patient diagnosed with a glycosylation defect, was unable to complement the yeast rft1Δ N-glycosylation phenotype. This is the first case of a glycosylation deficiency caused by a loss-of-function mutation mapped to hRft1p.
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