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endoplasmic reticulum (ER)
synthesis of most lipids; synthesis of proteins for distribution to many organelles and to the plasma membrane
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Golgi apparatus
modification, sorting, and packaging of proteins and lipids for either secretion or deliver to another organelle
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endosomes
sorting of endocytosed material
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Cytosol % of total cell volume and approximate number per cell
54% cell volume, 1 per cell
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Mitochondria % of total cell volume and approx. number per cell
22% cell volume, 1700 per cell
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Endoplasmic reticulum % of total cell volume and approximate number per cell
12% cell volume, 1 per cell
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Nucleus % of total cell volume and approximate number per cell
6% cell volume, 1 per cell
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Golgi Apparatus % of total cell volume and approximate number per cell
3% cell volume, 1 per cell
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3 general types of transport
- 1) nuclear pores such as mRNA
- 2) across intracellular membranes into other organelles, or across plasma membrane from outer environment
- 3) by vesicles to either the Golgi or another membrane bound organelle, or to the plasma membrane for secretion
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Signal sequence of import into ER
Leucine and hydrophobic amino acids
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Signal sequence of retention of lumen in ER
negatively charged amino acids and contains asp
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Signal sequence of import into mitochondria
positively charged and arginine
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signal sequence into the nucleus
positively charged and lysine
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Perinuclear space of nucleus is analogous to
the ER lumen
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transport into the nucleus
- proteins must have a nuclear signal sequence.
- then becomes bound by a nuclear transport receptor
- nuclear transport receptor and nuclear pore interactions pull protein into the nucleas where a GTP then binds to the receptor
- receptor and GTP leave together while the protein stays inside
- GTP is then hydrolyzed to GDP
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transport into mitochondria
- initial contact is between signal sequence and receptor protein on outer mitochondrial membrane (also attached to a protein translocator)
- entire complex diffuses in membrane until it reaches another contact site on inner membrane that has another protein transporter
- chaperones pull protein in and refold it, then signal sequence is cleaved
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transport into ER lumen
- Signal recognition particle (SRP) binds to the ER signal sequence on growing polypeptide
- complex then binds to SRP receptor located in the ER membrane
- entire protein passes through a translocation channel and SRP dissociates and recycles
- As polypeptide chain passes through translocation as a loop at some point the signal sequence is cleaved by peptidase
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peptidase
cleaves off signal sequence of polypeptide chain
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what anchors a transmembrane protein in the membrane
hydrophobic stop-transfer sequence
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Clathrin coated pits
- receptors bind to their ligands, then are bound by adaptins
- adaptins then bind to clathrin and vesicle is pinched off by dynamin
- once inside uncoating occurs to leave only the ligand~receptor complex inside the cell
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Clathrin+adaptin 1 origin and destination
- origin = Golgi appartus
- destination = lysosomes (via endosomes)
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Clathrin+adaptin 2 origin and destination
- origin = plasma membrane
- destination = endosomes
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COP (coat proteins) origin and destination
- origin = ER, Golgi cisterna, and Golgi apparatus
- destination = Golgi apparatus, Golgi cisterna, and ER
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Delivery of an uncoated vesicle to its destination:
- Tethering - involves a family of proteins called Rab, that bind to outer membrane of the vesicles. Rab engages tethering proteins on the target membranes
- Docking - mediated by SNARE proteins (v-SNARE and t-SNARE) which are embedded in the vesicle and target membranes
- Fusion - membrane fusion and ligand delivery
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SNARE proteins
serve to bring vesicle and target membranes into close contact prior to membrane fusion
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N-linked oligosaccharides
- within the ER lumen, a common glycolipid, dolichol, donates its oligosaccharides to new proteins at Asn residues through a transferase
- this modification of this common oligosaccharide chain yields protein with specific sugar motifs attached
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What if there is an excess of proteins in the lumen?
- the ER only expands up to a point and if there is too much the apoptotic program is triggered
- controlled by unfolded protein response (UPR)
- ER doesn't allow immature or misfolded proteins to pass cytosol or to be packaged in vesicles for transport.
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Cis golgi
- adjacent to the ER
- vesicles either move through the stacks of cisterna towards the trans Golgi or are routed back to the ER
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trans golgi
- oriented towards the plasma membrane
- vesicles are destined for either lysosomes or the plasma membrane for export of contained products
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Constitutive secretion
replacement of lipids/proteins for the plasma membrane, basement membrane components, etc.
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regulated secretion
- specialized products
- hormones, mucus, digestive enzymes, etc. Insulin. Secretion in response to a signal
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Macrophages
professional phagocytes that engulf microbial invaders in phagocytic vacuoles, which then fuse with lysosomes
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lysosomes
- acidification due to H+ ATPase which brings pH to 5.0
- also contain other degradative enzymes that destroy endocytosed or phagoytosed organisms
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Phagolysosome
- lysosomes fused with phagosomes for microbial destruction
- these can be beaten by pathogens that inhibit phagolysosomal fusion
- ex. Mycobacteria multiply inside the phagosome
- Listeria break out of phagolysosome into the cytoplasm
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Mycobacteria
inhibit phagolysosomal fusion by inserting waxy lipids into the phagosomal membrane
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Receptor mediated phagocytosis
- low density lipoprotein (LDL) is a protein that binds cholestorl and is implicated in cardiovascular disease
- LDL is bound by cell surface receptor in a clathrin coated vesicle. vesicle uncoats and fuses with endosome
- receptor buds off within transport vesicle for recycling to the cell surface
- LDL~cholesterol is transferred to a lysosome, where hydrolytic enzymes destroy LDL and free cholesterol into the cytosol
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Different receptor pathways from endocytic compartments
- 1) recycling of receptors can occur from early endosomes. Late endosomes found near nucleus or fused with lysosomes
- 2) degradation of receptors due to endosomal-lysosomal fusion
- 3) transcytosis of endosomal contents in transport veislces that bud off early endosomes. this usually takes place in polarized cells like gut epithelial cells
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Pathways to lysosomal function
- 1)phagocytosis - phagosomes fuse with lysosomes to yield phagolysosomes -> destruction of microbial invaders
- 2) endocytosed materials from outside can be taken in as endosome, early endosome -> late endosome -> incorporation into a lysosome for destruction
- 3) autophagy of old mitochondria which is very important since any mitochondrial leaks lead to apoptosis. tip off is finding cytochrome C which shouldn't be there
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