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BIS104 Slide5 - Lecture 5 Lipid and Protein trafficking...

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Lecture 5 Lipid and Protein trafficking Part 2
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Figure 12-50 Molecular Biology of the Cell (© Garland Science 2008) Post-translational modifications in ER N-glycosylations Adding a group of oligosaccharides (simple sugars) to the NH group of the side chain of asparagine GlcNAc: monosaccharide derivative of glucose
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Figure 12-52 Molecular Biology of the Cell (© Garland Science 2008) Synthesis of the core oligosaccharide for N-glycosylation Step-by-step reaction on a dolichol foundation via glycosyl transferases -use nucleoside diphosphate sugar precursors
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Figure 12-51 Molecular Biology of the Cell (© Garland Science 2008) Further modified in rough ER -sugar trimming Some to smooth ER and Golgi for more modifications
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What is the purpose of protein glycosylation? 1. N-linked glycosylation prevents protein aggregation before they are properly folded (more soluble) 2. N-linked oligosaccharide establish a “ glyco-code ” that marks the progression of protein folding and mediates the binding to chaperones A. Promote protein folding B. protective 1. Glycosylated proteins are more resistant to proteolytic enzymes (a protective coat on the cell surface) C. Signal transduction, cell-cell recognition/adhesion
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Figure 12-53 Molecular Biology of the Cell (© Garland Science 2008) N-linked glycosylation help to retain incompletely folded proteins in the ER -continuous glucose trimming/addition cycles
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Mis-folded proteins are exported out of ER then targeted for degradation in the cytoplasm Figure 12-54 Molecular Biology of the Cell (© Garland Science 2008)
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Figure 12-56 Molecular Biology of the Cell (© Garland Science 2008) Formation of GPI anchored proteins present on PM This permits these proteins to be specifically shed from PM under certain conditions through phospholipases More protein processing in ER….
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-by linkage to g lycosyl phosp hatidyli nositol (GPI anchor) Figure 10-19 Molecular Biology of the Cell (© Garland Science 2008) Proteins associate with membrane in different ways
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Figure 12-57 Molecular Biology of the Cell (© Garland Science 2008) ER is also the site for the synthesis and assembly of membrane lipids -on the cytosolic side of ER where the necessary substrates are found
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Scramblase -Phospholipid transferase -catalizes the flipping of lipids assembled on the cytosolic side of ER to the lumen side -non-specific Figure 12-58 Molecular Biology of the Cell (© Garland Science 2008)
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Flippase -head group specific (e.g. PS vs PC) -determines the asymmetry of lipid components in the inner and outer faces of the plasma membrane Figure 12-58 Molecular Biology of the Cell (© Garland Science 2008)
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What happen to lipids & proteins after ER? ER, Golgi and PM are in constant inter-communication through shuttling membrane vesicles - Golgi is the central hub and switch station in this system Figure 13-3b Molecular Biology of the Cell (© Garland Science 2008)
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Red : Exocytosis from ER to Golgi to PM ( biosynthetic secretory pathway )
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