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bsci330 > Lecture 16: Vesicular Transport > Flashcards

Lecture 16: Vesicular Transport Flashcards

1
Q

Vesicular transport

A
  • Protein transport between ER, Golgi complex, plasma membrane and vesicles is achieved through vesicular transport.
  • The vesicles bud off of the donor compartment and then travel between compartments in the cell along defined, regulated pathways and fuse specifically with their targets
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2
Q

Vesicular transport #2

A
  • Represents movement between compartments that are topologically similar
  • Budding and fusion are not inverses of each other
  • > In budding, the lumen face touches first
  • > In fusion, the cytosolic face touches first
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3
Q

What does vesicular sorting depend on?

A
  • Depends on the assembly of a special protein coat formed at specific locations along a given donor compartment
  • 3 code proteins that are responsible for this targeting
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4
Q

COPII

A
  • Responsible for vesicle traffic between the ER and the Golgi
  • Coats ER-to-Golgi vesicles
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5
Q

COPI

A
  • Responsible for vesicle traffic within the Golgi

- Coats vesicles moving from Golgi to ER, Golgi to plasma membrane (secretory vesicles), and within the Golgi

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6
Q

Clathrin

A
  • Responsible for vesicle traffic between the plasma membrane and the endosomes; in some cases the Golgi
  • Mainly involved in transport to/from and within the endosomal compartments
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7
Q

Coat Proteins Represent the Initial Step in Vesicle Formation

A
  • Transport vesicles bud off as coated vesicles that have a distinctive cage of proteins covering their cytosolic surface.
  • > The cage of proteins is there to help the vesicle form and to target it
  • Before the vesicle fuses with a target membrane, the coat is discarded, to allow the two cytosolic membrane surfaces to interact directly and fuse.
  • Different coat proteins are involved in transport between different organelles
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8
Q

Phospholipids containing inositol head groups mark organelles and membrane domains

A
  • Inositols can get phosphorylated at various locations by different lipid kinases (often located in distinct organelles, providing specificity)
  • > The inositols can be phosphorylated at different positions and they can be differentiated as well
  • PI’s can recruit various proteins that possess lipid binding domains
  • These lipid binding domains usually only recognize a specific type of PI
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9
Q

Regulation of coat assembly and vesicle stability

A
  • Adapter proteins bind to membrane proteins and recruit coat proteins
  • > Often bind to cargo receptors – transmembrane proteins that bind soluble cargo proteins for transport
  • Coat recruitment GTPases control coat assembly
  • > Monomeric GTPases regulate many steps in vesicular traffic
  • > Sar-1 regulates COPII assembly
  • > Arf proteins regulate COPI and clathrin assembly
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10
Q

Regulation of coat assembly and vesicle stability #2

A
  • Sar1-GDP = cytosol = inactive
  • > Contains an amphipathic helix hidden in this form
  • Sar1-GTP = ER membrane-bound = active
  • Sar1-GEF causes Sar1-GDP to exchange GDP for GTP, which causes a conformation change causing the amphipathic helix to show
  • Active Sar1-GTP then promotes the assembly of coat complexes
  • GTP hydrolysis causes coat disassembly after budding
  • > The GTPase regulates both coat assembly and disassembly
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11
Q

Vesicle docking and targeting

A
  • Membrane traffic needs to proceed in an orderly way
  • > Transport vesicles must be highly selective in recognizing the correct target membrane with which to fuse.
  • Specificity in targeting: surface markers that identify vesicles according to their origin and type of cargo.
  • Target membranes display complementary receptors that recognize the appropriate markers.
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12
Q

SNAREs

A
  • One of the two classes of protein that control recognition of donor vesicles by acceptor membranes
  • Provide specificity & catalyze vesicular the fusion of with the target membrane.
  • v-SNAREs: vesicle
  • t-SNAREs: target membrane
  • The interaction between v-SNAREs and t-SNAREs drives fusion between the vesicle and the target membrane
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13
Q

Rabs (GTPases)

A
  • One of the two classes of protein that control recognition of donor vesicles by acceptor membranes
  • Work together with other proteins to regulate the initial docking and tethering of the vesicle to the target membrane; on the vesicle
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14
Q

Cargo Recruitment

A
  • Entry into vesicles leaving the ER is usually a selective process
  • Membrane proteins: have exit signals in their cytosolic ‘tails’ that are recognized by coat proteins
  • Soluble Proteins: bind to cargo receptors that have exit signals in their cytosolic ‘tails’
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15
Q

Vesicular tubular clusters

A
  • Transport vesicles leaving the ER fuse together to form intermediate compartments called these
  • Travel towards the cis Golgi via motor proteins on microtubule tracks
  • Generate coated vesicles going back to the ER (COPI coat) – retrograde transport
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16
Q

ER retrieval signals

A
  • Membrane ER resident proteins: retrieval signals in their cytosolic tails & recognized by COPI coat proteins
  • Soluble ER resident proteins: Retrieval signals within their structure & bind to receptors
  • > EX: KDEL sequences
  • > Bind to KDEL receptors
17
Q

CISTERNAE = STATIC (Vesicular Transport Model)

A
  • Vesicles travel between them

- The identity of each cisternae is fixed (cis stays cis and trans stays trans

18
Q

CISTERNAE = DYNAMIC (Cisternal Maturation Model)

A
  • Move upward, changing their properties slightly as they migrate
  • The membrane of the Golgi will mature into the membrane that is next to them (like a treadmill)
  • These models are not mutually exclusive; some proteins can move by the static model, while others can travel via the dynamic model
19
Q

Transport from the Trans Golgi Network to the Cell Exterior

A
  • Many proteins leaving the Golgi need to get to the plasma membrane.
  • Vesicles carrying such proteins fuse with the plasma membrane via exocytosis.
  • Vesicle cargo:
  • > Membrane proteins and the lipids: become part of the plasma membrane
  • > Soluble proteins: are secreted into the extracellular space
20
Q

Constitutive secretory pathway

A
  • One of 2 pathways for transport from the Trans Golgi Network to the Cell Exterior
  • The vesicles bud off of the Golgi and fuse with the plasma membrane and the contents get secreted into the extracellular space
  • This doesn’t require any external signals and it happens all the time
21
Q

Regulated secretory pathway

A
  • One of 2 pathways for transport from the Trans Golgi Network to the Cell Exterior
  • Special vesicles that store secretory proteins are synthesized and move to the plasma membrane
  • They then wait for an extracellular signal to then fuse with the plasma membrane and then released their contents
22
Q

Regulated Secretion

A
  • Cells that are specialized for secreting some of their products rapidly and “on demand” concentrate and store these products in secretory vesicles.
  • These secretory vesicles only release their contents to the cell exterior in response to extracellular signals.
23
Q

Clathrin-Coated Vesicles

A
  • Used for transport between plasma membrane, endosomal system, and Golgi
  • Clathrin was the first coat protein to be identified:
  • > Composed of 3 copies each of heavy chain and light chain
  • > Arranged in a “triskelion” (three-armed pinwheel shape)
24
Q

Clathrin coats

A
  • Clathrin
  • Adaptor proteins
  • Transmembrane cargo receptors
  • Together, they capture and package cargo molecules within the donor compartment into a budding vesicle
  • Clathrin coat is rapidly lost shortly after forming
25
Q

Dynamin

A
  • The pinching-off of clathrin-coated vesicles is controlled in part by this cytoplasmic protein
  • Lipid binding (specific phosphoinositide) & GTPase
  • Wraps (oligomerizes) around the stem of the budding vesicle
  • Brings inner leaflet membranes of vesicles together
  • Fusion of these membranes severs the vesicle from the donor compartment.
  • GTP hydrolysis regulates rate of vesicle pinching-off
26
Q

Lysosomal Enzyme Cargo

A
  • Soluble
  • Carry a unique marker: mannose 6-phosphate (M6P) groups
  • Added to N-linked oligosaccharides in cis-Golgi
  • Recognized by M6P-receptor in TGN and packaged into clathrin-coated vesicles for delivery to lysosomes
27
Q

Targeting of Lysosomal Enzyme Cargo to Lysosomes

A
  • M6P groups are recognized by cargo receptors and recruited into clathrin-coated vesicles
  • > Acidic compartment of early/late endosomes dissociates M6P receptors from cargo
  • > Receptors recycled back to Golgi.
28
Q

Endosomes

A
  • Intermediate organelles in vesicular transport pathways
  • Vary in size/shape
    Receive cargo from Golgi and PM
  • 3 main classes:
    -> Early
    -> Late
    -> Recycling
  • Early endosomes ‘mature’ into Late endosomes, which ‘mature’ into lysosomes in part by becoming increasingly acidic
29
Q

Endosomes #2

A
  • Also characterized by the presences of internal vesicles
  • Such late endosomes are called ‘multivesicular bodies’
  • > One of the ways that vesicle membranes can be recycled for other purposes
  • The budding and internalization of vesicles from the plasma membrane is called endocytosis
30
Q

Vesicle cargo

A
  • Membrane proteins and the lipids:
  • > Are removed from the plasma membrane
  • > Some will be Recycled back to the surface
  • > Some will be Degraded
  • Soluble proteins
  • > from extracellular space
  • > carried in the lumen
  • Common membrane protein cargo: receptors +/- their ligands
31
Q

Phagocytosis (“cellular eating”)

A
  • The ingestion of large particles, such as microorganisms or dead cells via
  • Vesicles called phagosomes (generally >250 nm in diameter)
32
Q

Pinocytosis (“cellular drinking”)

A
  • The ingestion of fluid and solutes
  • Vesicles called pinocytic vesicles (about 100 nm in diameter).
  • Includes receptor-mediated endocytosis
  • Most eukaryotic cells are continually ingesting fluid and solutes by pinocytosis
  • Cholesterol gets into cells via receptor-mediated endocytosis
33
Q

Epidermal growth factor receptor (EGFR)

A
  • Gets degraded, along with its ligand, but transferrin receptor (TfnR) gets recycled to plasma membrane
  • The degradation of EGFR ends the signal
34
Q

Transcytosis

A

Molecules internalized at one end of a ‘polarized’ cell are transported to a different end (and vice versa)

bsci330 (12 decks)

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