Vesicular Transport Flashcards
Steps in endocytosis
Step 1: The cell membrane invaginates, enclosing extracellular fluid.
Step 1: Bacteria or cell debris bind to cell receptors.
3 types of endocytosis
- Phagocytosis - plasma membrane forms pseudopodium and engulfs solid particles to form phagosome (food vacuole)
- Pinocytosis - plasma membrane invaginates and engulfs extracellular fluid to form a vesicle.
- Receptor-mediated endocytosis - cells engulf molecules by binding them to cell surface receptors, which then cluster and bud inward, to form coated vesicles (have coat proteins) containing the bound molecules
________ allows Proteins and Vesicles to Reach their Destinations
Budding -> Fusion
Vesicular Trafficking
different coat proteins involved in intracellular vesicular transport
Clathrin
COPI
COPII
Retromer
Mediates anterograde transport (forward transport) from the ER to the Golgi.
COPII
Functions in retrograde transport (backward transport) from the Golgi to the ER and within Golgi compartments.
COPI
Involved in vesicle formation at the trans-Golgi network (TGN) and plasma membrane.
Functions in endocytosis (vesicle budding from the plasma membrane) and Golgi-to-endosome transport.
Clathrin
Involved in endosome-to-Golgi retrieval and recycling of specific cargoes from endosomes.
Retromer
flattened, membrane-bound sacs (or pouches) that form the stacked structure of the Golgi apparatus, a cellular organelle responsible for processing and packaging proteins and lipids
Golgi cisternae
Made up of clathrin
* three large and three small polypeptide chains that together form a three-legged structure (__________)
Clathrin-coated vesicles; triskelion
Light chain or heavy chain?
The primary structural component, forming the legs of the triskelion.
Clathrin heavy chains (CHC)
Light chain or heavy chain?
Regulate clathrin assembly, vesicle formation, and endocytosis of specific receptors.
Clathrin Light Chains (CLC)
Required to bind the clathrin coat to the membrane and to trap various transmembrane proteins
Adaptor protein (AP)
Receptor-mediated endocytosis using clathrin:
- Coat assembly and cargo selection (Cargo receptors outside the membrane and adaptor proteins inside the membrane)
- Bud formation
- Vesicle formation
- Uncoating (coated vesicle membrane becomes naked transport vesicle)
is a phosphatidylinositol phosphate (PIP) present in the plasma membrane, which acts as a signal for AP2 recruitment.
Phosphoinositide PI(4,5)P₂
AP2 exists in a locked state in the cytosol and undergoes a conformational change upon binding to PI(4,5)P₂
Which Phosphoinositide is this:
Found in early endosomes, involved in endosomal sorting and phagocytosis.
PI(3)P
Which Phosphoinositide is this:
Enriched in the Golgi, regulates vesicular trafficking and exocytosis.
PI(4)P
Which Phosphoinositide is this:
Located in the plasma membrane, crucial for endocytosis and actin cytoskeleton regulation
PI(4,5)P₂
Which Phosphoinositide is this:
Found in late endosomes/lysosomes, important for vesicle maturation.
PI(3,5)P₂
Which Phosphoinositide is this:
Regulates signaling pathways at the plasma membrane.
PI(3,4,5)P₃
Exocytosis: Secretion of molecules either in response to signals (______) or continuously (_______)
regulated; constitutive
AP2 interacts with which PIP to enable clathrin-mediated endocytosis
PI(4,5)P₂
soluble cytoplasmic protein
assemble as a ring around
the neck of each bud and
pinch off the membrane to
release the clathrin-coated
vesicles
Dynamin
Sculpt phosphoinositide-rich membranes to generate membrane protrusions or invaginations
Bin-Amphiphysin-Rvs (BAR) domains
________ inhibit the lateral diffusion of PI(4,5)P₂
F-BAR
Adaptor proteins (_______) bind clathrin and receptors, acting as a bridge
adaptin
________ and ______ uncoat the vesicle.
Hsc70 chaperone; auxillin
Transport vesicles are highly selective
*Specificity in targeting is ensured because all transport vesicles display ____________ that identify them according to their origin and type of cargo, while target membranes display _________ that recognize the appropriate markers
surface markers; complementary receptors
have a central role both in providing specificity and in catalyzing the fusion of vesicles with the target membrane.
SNARE proteins
Type of snare protein that is incorporated in the membranes of transport vesicles during budding
v-SNARES (vesicles)
Type of snare protein that is located in the membrane of target compartments
t-SNARES (target)
When a v-SNARE interacts with a t-SNARE, the helical domains of one wrap around the helical domains of the other to form stable _______________, which lock the two membranes together
t-SNARE (syntaxin)
t-SNARE (Snap25)
v-SNARE (synaptobrevin)
trans-SNARE complexes
The formation of the SNARE complex may work like a _______, using the energy that is freed when the interacting helices wrap around each other to pull the membrane faces together, while simultaneously squeezing out water molecules from the interface.
winch
Form stalk
Hemifusion
Fusion
Direct the vesicle to a specific spot on the target membrane
Rab proteins
Tethering: Rab proteins on the vesicle are recognized by tethering proteins (Rab effector) on the target membrane
Docking: recognition of v-SNARE and t-SNARE
Fusion: v-SNARE and t-SNARE wrap around each other tightly
Tethering
Docking
Fusion
is an ATPase that dissociates SNARE pairs
NSF (N-ethylmaleimide-sensitive factor)
together with accessory proteins and ATP
After they dissociate, ADP + P
HIV enters through _______
membrane fusion
Transport from the ER through the Golgi Apparatus
A secretory protein is synthesized inside the rough endoplasmic reticulum, migrates through it, and exits inside a vesicle.
HIV infection process:
- CD4 attachment - HIV’s surface glycoprotein gp120 binds to the CD4 receptor on the host cell membrane.
- Chemokine receptor binding - Once gp120 binds to the chemokine receptor, the viral membrane gets closer to the host cell membrane.
- Membrane insertion - inserting its hydrophobic tail into the host cell membrane
- Fusion
- sorting and dispatching station for the products of the ER
- major site of carbohydrate synthesis, as well as a pectin and hemicellulose, glycosaminoglycans
Golgi apparatus
- a cis face or entry face
- a trans face or exit face
Transport from ER to GA
- Proteins Leave the ER in COPII-coated Transport Vesicles
*cargo proteins display exit (transport) signals on their surface that are recognized by complementary receptor proteins
- proteins must be properly folded and, if they are subunits of multimeric protein complexes, they may need to be completely assembled
*Those that are misfolded or incompletely assembled are retained in the ER, where they are bound to chaperone proteins
- common inherited disease, produces a plasma membrane protein necessary for Cl- transport that is only slightly misfolded
*Misfolded protein is retained in the ER
Cystic fibrosis
Symptoms of cystic fibrosis
- Thickened smooth muscle in the trachea
- Increased mucus secretion
- Narrow lumen
- Inflammation
- fusion of membranes from the same compartment
- Requires matching SNAREs
Homotypic
- one compartment fuses with the membrane of a different compartment
Heterotypic
ER-derived vesicles fused to form
__________
vesicular tubular clusters (forms the cis Golgi network)
pH gradient of ER and Golgi network
ER = neutral
cis Golgi network = weakly acidic
Golgi stack = acidic
trans Golgi network = highly acidic
- Membrane proteins needed to form COPII vesicles can be retrieved from the cis-Golgi by ________.
COPI vesicles
- One of the sorting signals that directs membrane proteins into COPI vesicles is a _______, which binds to subunits of the COPI coat.
KKXX sequence
- Many soluble ER-resident proteins contain a _______. Binding of this retrieval sequence to a specific receptor protein in the cis-Golgi membrane recruits missorted ER proteins into retrograde COPI vesicles
KDEL sorting signal
Oligo processing in Golgi compartments
cis Golgi network = Sorting - phosphorylation of oligosaccharides on lysosomal proteins
cis cisterna = removal of Man
medial cisterna = removal of Man & addition of GlcNAc
trans cisterna = addition of Gal & addition of NANA
trans Golgi network = Sorting - sulfation of tyrosines and carbohydrates
2 models of transport in GA
A. Vesicular transport model
B. Cisternal maturation model
- 40 types of hydrolytic enzymes, including proteases, nucleases, glycosidases, lipases, phospholipases, phosphatases, and sulfatases
- Major site for degradation and recycling of proteins, nucleic acids, lipids and whole organelles
Lysosomes
- The __________ are hydrolytic enzymes that are active under acidic conditions.
acid hydrolases
- The lumen is maintained at an acidic pH by an H+ ATPase in the membrane that pumps H+ into the lysosome
pH in lysosome lumen = 5.0
pH in the cytosol = 7.2
The low pH in the late endosome dissociates the lysosomal hydrolases from these receptors, making the transport of the hydrolases unidirectional
Transport from GA-lysosome
- ___________ recognizes Lysosomal Proteins in the Trans Golgi Network
Mannose 6-Phosphate Receptor
- Coarse facial features, skeletal abnormalities and mental retardation
*almost all of the hydrolytic enzymes are missing from lysosomes, and their undigested substrates accumulate in lysosomes, which consequently form large “inclusions”
Inclusion-cell disease
- Gives the cell structure and shape
- Moves vesicles, and separates chromosomes during cell division
- Facilitates muscle contraction
Cytoskeleton
Cytoplasm is crowded
responsible for intracellular transport and positioning of membrane vesicles and organelles provide long tracks and vesicles are transported by _______ and ___________
Microtubules; kinesin; dynein motors
- walk towards the + and –ends of microtubules
- Anterograde axonal transport, rapid movement of mitochondria, secretory vesicle precursors, synapse components of the axon to the nerve terminals
Kinesin
- Minus end-directed microtubule motors
- Organelle and mRNA transport, positioning the centrosomes and nucleus, construction of microtubule spindle
Dynein
- Kill cells
- Microtubule depolymerization
Colchicine
