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Endoplasmic means
within the cytoplasm
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The nucleus is in physical contact with
RER & SER
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Functions of ER:
- Biosynthesis of proteins destined for incorporation into plasma membrane
- Synthesis of proteins destined for export from cell
- Biosynthesis of lipids a) cholesterol; b) plasma membrane
- Incorporation into organelles of endomembrane system
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Vesicles move from ER to Golgi, and from Golgi to
cell membrane
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Hormones travel in the bloodstream until they find the target organ
Pituitary gland in the brain controls hormone production
- RER = protein synthesis, hence
- ribosomes
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SER= detoxification and
Post-translational modifications
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Golgi
packaging & shipping. Proteins will gain 3dimensional structure
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Smooth Endoplasmic Reticulum characteristics
- No ribosomes
- Found often in ovary and testes
- SER associated with metabolism of lipids
- Ovaries and testes produce lipids
- Cholesterol is the matrix for making estrogen/testosterone
- Continuous with rough endoplasmic reticulum
- Tubular
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RER is continuous with SER because of
the post-translational modifications (epigenetic modifications)
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SER: Drug Detoxification involves
hydroxylation
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Addition of OH groups are more soluble and
- easier to excrete
- If not soluble, hydrophobic toxins may stay in membrane of cells
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Reduced form of Cytochrome P-450 hydroxylates is an
organic hydroxyl acceptor
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Class of enzymes called mixed-function
- Example:
- Elimination of barbitruate drugs by hydroxylation
- Phenobarbital induces increase of barbiturate detoxifying enzymes in liver
- Extensive formation of smooth ER
- Also hydroxylates useful drugs such as antibiotics
- Hydroxylation increases the solubility of hydrophobic drugs in water
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Carbohydrate Metabolism
SER
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Class of enzymes called mixed-function
- Example:
- Member of Cytochrome P-450 protein family
- Aryl hydrocarbon hydroxylase involved in metabolizing polycyclic hydrocarbons
- Changes carcinogens to chemically active form which results in tumors
- Cigarette smoke is an inducer of aryl hydrocarbon hydroxylase
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Carbohydrate Metabolism: Membrane of smooth ER of hepatocytes contain glucose-6-phosphatase which does what?
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(breaks down glucose-6-phosphate into glucose)
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Glycogen is stored in the?
liver
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Carbohydrate Metabolism
- Allows glucose to leave cell and travel into blood system
- The way in which the cell controls the level of sugar
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Calcium Storage: Sarcoplasmic reticulum in muscle cells is an example of smooth ER that specializes in storage of
calcium
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SER Lumen has ATP dependent calcium ATPases which aids in
muscle contraction
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SER is involved in the synthesis of what steroids?
Cholesterol (liver cells), cortisol (adrenal glands), testosterone (Leydig cells), estrogen (follicular cells)
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Cholesterol (liver cells), cortisol (adrenal glands), testosterone (Leydig cells), estrogen (follicular cells) all share what, and differe by what?
All share a 4-ring structure but differ by hydroxyl groups & carbons side chains
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SER & steroid biosynthesis: P-450 monooxygenases are important in the synthesis of cholesterol but also in its conversion to steroid hormones by
hydroxylation
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Proteins in the RER are anchored by?
Proteins are anchored by hydrophobic interactions or covalent attachments
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RER: Transitional Elements (TE)
plays role in formation of vesicles
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RER: Transition Vesicles
shuttle lipids and/or proteins
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RER: is made of
Flattened sacs
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How do the proteins enter the Lumen of the RER?
Proteins enter lumen cotranslationally
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In the RER Soluble proteins are released into
the lumen
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ER is a source of membrane lipids,
including phospholipids (PHL) and cholesterol.
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RER: Biosynthesis and phospholipids incorporation is restricted to
the monolayer facing the cytosol. But the membranes are bilayer. Phospholipids from one layer must be transferred to the other
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RER:Phospholipid translocators (flippases) catalyze
the translocation of phospholipids through ER membranes (fig 7-10)
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RER: Post-translational changes occur to give the proteins
specific functions
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RER: Responsible for
- addition of carbohydrates to proteins
- Folding of proteins
- Recognition and removal of misfolded proteins
- Assembly of multimeric proteins
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RER: Contains enzymes for ________ & ________ modification such as disulfide bonds
posttranslational and cotranslational
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RER: Protein Glycosylation
(the addition of carbohydrate side chains, forming glycoproteins)
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RER: Quality control
- ER associated degradation proteins ERAD (cytoplasmic proteases)
- Post-translational = protein folding
- Co-translational = some types of glycolation
- Rhodopsin
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RER: N-linked glycosylation
oligosaccaride unit to attach to nitrogen on asparagine
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RER: O-linked glycosylation
oligosaccaride unit to attach to oxygen on hydroxyl group of serine or threonine
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RER: Each step of glycosylation is dependent on previous modifications. An error in one step will block the process and lead to a disease on the organism
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Membrane/lumen of RER
- This is the base structure of glycosilation
- Once the protein is glycosilated, it can no longer escape the RER and will continue the process of folding
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Post translational changes are required for signaling and identification. Protein folds to signal to specific receptors. If it does not fold, it will
be non-functional
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Formation of Core Oligosaccharide occurs in the
Cytoplasm
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Dolichol Phosphate inserted into ER membrane then what is added ?
- 2N-acetylgluosime (GlcNAc) are added to dolichol
- Addition of 5 mannose sugars
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N-Glycosylation: Formation of Core Oligosaccharide has what components in the end?
- 2 GlcNAc units
- 9 Mannose
- 3 Glucose
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Section of Golgi that faces rough ER
CGN = Cis Golgi Network
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Formation of Core Oligosaccharide occurs in the lumen of the ER
- Translocation by flippase into lumen
- Addition of 4 mannose sugars (total of 9)
- Addition of 3 glucose units
- Transfer of core to nitrogen of asparagine (oligosaccharyl transferase)
- Modification of core oligosaccaride by removing 3 glucose and 1 mannose sugar
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Section of Golgi that faces plasma membrane =
TGN = Trans Golgi Network
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Stimuli indicate that an enzyme is needed.
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Oligosaccharide is usually added
- cotranslationally
- Promotes proper folding
- Proteins such as calnexin (CNX) and calreticulin (CRT) binds to glycoprotein to form a complex for disulfide bridge
- Thiol oxidoreductase (ERp57) promotes disulfide bonds
- If a protein is missing a sugar, UGGT (UDP-glucose: glycoprotein glucotransferase) will add a sugar so CNX/CRT will promote formation of disulfide bond
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Golgi Apparatus
- Posttranslational Modification
- Terminal glycosylation � removal of a few carbohydrates of core oligosaccharide
- OR/BOTH
- Addition of N-acetylglucosamine and other monosacharides such as galactose, sialic acid and fucose
- Galactosyl transferase is found exclusively in Golgi which adds on galactose
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Antidepressants are taken by people who cannot handle pressure because the brain makes too much of a certain protein.
- Drugs will block the production of the protein in the Golgi
- Post-translational changes are interrupted
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Golgi has 2 major classes of enzymes
- Glucan synthetases � catalyzes formation of oligosaccharide from monosaccharide
- Glycosyl transferases � attach carbohydrates to proteins
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Glucan synthetases
catalyzes formation of oligosaccharide from monosaccharide
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Glycosyl transferases
attach carbohydrates to proteins
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Question: How are the lumenal side of ER and exterior surface of plasma membrane similar?
The outside of the plasma membrane is made in the RER. They have the same proteins and chemical compositions
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The Golgi Complex
- (1898) Camillo Golgi
- Flattened sacs or cisternae
- Cis (CGN) or Trans face of Golgi (TGN)
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Between cis & trans is medial cisternae of Golgi where ________ occurs
processing
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Two Models depect flow of lipids & proteins
- Stationary Cisternae Model � shuttle vesicles bind & fuse
- Cisternal Maturation Model � Golgi cisternae transform from CGN (with aid of ER vesicles) and accumulate specific enzymes. CGN transform to medial cisternae then trans cisternae Golgi network with new enzymes.
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Time elapsed fluorescence microscopy support maturation model
Anterograde & Retrograde Transport
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Anterograde -
vesicles from ER fuse with Golgi then plasma membrane
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Retrograde -
- flow from plasma to Golgi to ER
- Happens when a protein is made by two different polypeptides which have different post-translational modifications
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Golgi Apparatus/ER (rough) Protein Trafficking from ER to Golgi
- Membrane-bound & soluble proteins must be directed to a variety of intracellular locations including ER, Golgi, endosomes, and lysosomes
- Each protein has a tag for a particular vesicle
- Tag may be amino acid sequence, oligosaccharide chain, hydrophobic domain
- Membrane lipids may be tagged by phosphate group by a kinase
- For each tag, we have a specific receptor
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Retrieval tags placed on proteins that should return to ER. Receptors on Golgi will bind to KDEL (Lys-Asp-Glu-Leu) or KKXX (Lys-Lys-any-any) and return proteins to
ER
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Retention Tags
RXR (arginine, any, arginine) tag is placed on protein and stays in ER
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Question: If proteins are returned to ER, why go to Golgi?
This is a huge area of current biomedical research possible cure for many monogenetic diseases
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how can we retag proteins that lack the tag due to genetic mutation
- genetic therapy � changing the DNA - difficult
- so we are trying to retag proteins
- They go to the Golgi to be tagged in order to return to the ER
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The key of life is in another area of research: how do sperm cells know which cell is an egg?
�
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Proteins of Golgi: Some have _________or _________ attached to proteins so they will remain or come back to Golgi
retention or retrieval tags
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Proteins of Golgi: Specific proteins are
integral proteins spanning the membrane (hydrophobic regions)
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Hydrophobic regions is a third way to keep proteins in
Golgi
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Proteins will migrate from CGN to TGN until the thickness of membrane exceeds length of
hydrophobic membrane-spanning domain
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All post-translational changes have a function. More post-translational changes take more time in the
Golgi
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Endosome Pathway: Lysosomal specific proteins have mannose-6-phophate formed from
mannose by 2 Golgi specific enzymes
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In TGN of Golgi are mannose-6 phosphate receptors (pH 6.4) where enzymes bind and packaged into transport vesicle...These vesicles form
Early Endosome which bind to plasma membrane
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Excocytosis � textbook p342
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Endocytosis � textbook p343
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Regulated Secretion: Vesicles fuse with plasma membrane only in response to
extracellular signals
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Secretory Pathways: 1967 James Jamieson & George Palade labeled amino acids and watched movement
- After 3 minutes found in ER
- After 7 minutes found in Golgi
- 37 minutes into vesicles
- 117 minutes vesicles discharge protein into extracellular matrix
- After budding from TGN, secretory vesicles constantly move to cell membrane and
- release proteins by exocytosis
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Experiment removed KDEL retrieval tag from ER proteins and they did not return to ER but were secreted. This is strong evidence for
the existence and function of tags
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Vesicles wait around plasma membrane until signal response tells it to
- fuse and release
- Ex: release of insulin from pancreatic B cells OR release of zymogens (precursors of hydrolytic enzymes)
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Exocytosis
- Release of cellular products destined for secretion (triggered by calcium or hormones)
- Vesicles fuse with plasma membrane
- Discharge contents
- Inner surface of vesicle (lumen of ER and Golgi) become outer surface of plasma membrane
- Addition of lipids and proteins to cell membrane
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