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68 Cards in this Set
- Front
- Back
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What is the crucial event in secretion?
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Fusion of membrane vesicles that budded from the golgi, with the plasma membrane.
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What type of secretion occurs in pancreatic exocrine cells?
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Regulated secretion
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What is unregulated secretion called?
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Constitutive secretion
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Does membrane fusion only occur during exocytosis?
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No - obviously it happens within the golgi complex etc.
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What do we best understand membrane fusion for?
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Viruses - like the flu virus
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How does the flu virus enter cells?
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By fusing its membrane envelope with the plasma membrane.
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What mediates this membrane fusion?
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Fusion proteins - Hemagglutinin
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What is created by the fusion proteins being inserted in a target cell?
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A protein bridge which brings the virus/cell into close proximity.
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What brings about the actual fusion?
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Conformational change in the protein complex.
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What are the membrane fusion proteins that mammalian cells use for fusion?
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SNARES
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How do SNARES work?
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-Form large protein complexes at membrane docking sites
-Bring adjacent membranes into close proximity for fusion |
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Do membranes fuse randomly?
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no
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What regulates membrane fusion and provides specificity?
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RAB PROTEINS - GTP binding proteins
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Which RAB protein regulates exocytosis at the plasma membrane?
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RAB 5
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what one regulates fusion between golgi and lysosomes?
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Rab 7
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How do GTP proteins function?
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By cycling between GTP and GDP bound states
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What are the 2 big things that happen when a membrane vesicle fuses with the plasma membrane?
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1. Its contents are released
2. Its membrane fuses with the plasma membrane and gets INCORPORATED INTO IT |
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What are the consequences of the membranes fusing?
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It increases the surface area of the cell - though not a lot.
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What compensates for the increased SA?
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Simultaneous endocytosis.
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Why exactly do we need fusion proteins again?
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Because cell membranes are neg charged and repel; need to bring them close together before they will run together like salad dressing.
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When are RAB proteins in their active state and able to bind fusion complexes?
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When bound to GTP
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When are RAB proteins in their inactive state?
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When bound to GDP
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Fundamental feature of lysosomes and endosomes:
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Acidic pH
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How do lyso/endosomes get their pH acidic?
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By proton pumps
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What can be used to identify lyso and endosomes?
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Chloroquine - they accumulate this weak base.
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What are endosomes associated with?
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Endocytosis.
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3 types of endocytosis:
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-Phagocytosis
-Pinocytosis |
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What type of endocytosis is phagocytosis?
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Receptor-mediated
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What receptor is used in phagocytosis by macrophages?
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Fc receptor
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What is involved in Fluid phase endocytosis?
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Uptake of components in the fluid phase that are not bound specifically to the membrane
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What marker is used to study fluid phase endocytosis?
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Horseradish peroxidase
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What did bersharse show in his lab about macrophage and Lcell endocytosis via horserad peroxd?
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Turnover rate of entire membrane
-Macrophages were every 32 min -Lcells once every 111 min |
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How does receptor mediated endocytosis compare to fluid phase?
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It is more specific
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What is the classic example of receptor mediated endocytosis?
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LDL-receptor
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What happens when LDL binds its LDL receptor?
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The complex is internalized to form a COATED (clathrin) vesicle
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Where does the coated vesicle go?
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To fuse with an endosome and be uncoated
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What happens to the LDL receptor after LDL comes off?
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It's recycled to the cell surface.
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What are the protein complexes formed by clathrin called?
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Triskelions
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What links the clathrin coat to the pit that is the membrane domain of the receptor?
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Adaptin proteins
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What are the 2 fundamental pathways of protein degradation?
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1. Lysosomal
2. Ubiquitination |
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What is the purpose of lysosomes?
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To degrade their components with hydrolytic enzymes
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How does stuff get into lysosomes?
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By endocytosis
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What are proteins that enter lysosomes degraded with?
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Lysosomal proteases
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Where does ubiquitination occur?
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In the cytosol
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How does the ATP-dependent proteolytic machinery know to degrade proteins?
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They are tagged with ubiquitin
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Where are lysosomal enzymes made?
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In the rough ER, like normal
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What happens to lysosomal enzymes at the trans golgi network?
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They diverge from the secretory pathway into pleiomorphic organelles called these LYSOSOMES
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What needs to be on the enzymes to make them go to lysosomes?
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Targeting signals
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What cellular organelles have the longest half lifes?
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-Mitochondria - 6.8 days
-Nucleus - 5.1 days |
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What cellular organelle has the shortest half life?
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The plasma membrane - 1.8 days
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Do lysosomes only contain hydrolases and proteolysing enzymes?
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no, they also have phosphatases, lipases, nucleases, sulfatases, glycosidases, and phospholipases
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Where do lysosomes form again?
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At the trans-golgi network - it buds off just like secretory vesicles.
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What ARE lysosomal enzymes?
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Glycoproteins!
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How does the golgi know not to send lysosomal enzymes to the cell surface?
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The core protein put onto the glycoproteins in the RER gets a MANNOSE-6-PHOSPHATE added to it.
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Where does the Mannose6P get added?
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Between the RER and cis-golgi
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What recognizes the Mannose-6P?
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M6P-receptor in the golgi complex
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Where are lysosomal enzymes first targeted?
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To endosomes, then subsequently lysosomes.
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What makes the lysosomal enzymes dissociate from their M6P receptors after budding from the trans golgi?
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The acidic environment
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What is the function of the small amount of M6PR at the cell plasma membrane?
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To scavenge any lysosomal enzymes that do escape the normal sorting process.
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What are the 2 classic lysosomal storage diseases?
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-Icell disease
-Hurler's disease |
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What is the problem in lysosomal storage diseases?
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Lack or deficiency of a lysosomal enzyme
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Why are they called storage diseases?
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Because the cell is unable to degrade macromolecules and so they accumulate (get stored in) lysosomes.
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What enzyme is lacking in Hurler's disease?
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L-iduronidase
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What can't people with Hurler's disease break down?
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Dermatan sulfate
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Symptoms of hurler's disease:
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-Growth problems
-Mental retardation |
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What is lacking in I-cell disease?
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All acid hydrolases in lysosomes
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Why does Icell disease develop?
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The enzyme for phosphorylating mannose to make the mannose-6P marker is missing.
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What would happen to newly synthesized lysosomal enzymes in I-cell disease?
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They would end up on the cell surface and degrade the extracelular contents - bad.
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