MVU11 - INTRACELLULAR TRAFFICKING - 2 Flashcards
(32 cards)
why are the two mechanisms that ensure that vesicles transport their contents to the correct acceptor membrane?
- Rab GTPase proteins which provide specificity of vesicle targeting and attachment to acceptor membrane
- SNARE fusion proteins provide specificity during fusion of vesicles with acceptor membrane
how is the GA organised?
into a stack of membranes
cis (closest to ER)
medial
trans (closer to PM)
what is the old model of the transport through the GA?
vesicles transport cargo between layers
what is the new model of transport through GA?
each layer matures and becomes the next layer
trans makes the vesicles that go out, runs out of membrane, disappears
medial becomes the new trans one
cis is made by the new incoming vesicles
where do CCV carry cargo to?
to PM and endosomes
what maintains the organisation of the stack?
a cytosolic protein matrix
how are N-linked glycans modified in the GA?
removal of mannoses and addition of different sugars, often with negative charges
other complex sugars are attached to Ser and Thr side chains: O-linked glycosylation
many different combinations: specificity, heterogeneity
what is the effect of glycosylation in the GA?
promotes protein folding:
makes folding intermediates more soluble (prevents aggregation)
sequential modifications -> progression of folding or degradation
sugars have limited flexibility ->protect from proteases, stabilise protein structure (protective coat in the cytosol)
signaling hubs (regulations of development)
rich in OH groups, can be phosphorylated
what is the function of proprotein convertases?
some PM and extracellular proteins are made as a longer inactive form in the ER
cut by proprotein convertases into a shorter active form at the GA
proteases recognise specific patterns of AAs (not specific motifs)
cleavage activates protein by removing inhibitory region
what is an example of a protein made active by being cut short?
proinsulin made into an active polypeptide
convertases remove the middle section, the two remaining sections form the active insulin
prevents premature signaling by insulin at the ER
disulfide bonds bring together C and N termini
what is an example of a protein in the signalling pathway that is activated by being cut?
ATF6 is activated by proteolysis in the GA
regulation is by trafficking
BiP covers ER exit signal on ATF6
proteases are only present in the GA
what are Rab proteins?
large sub-family of Ras-related proteins
different organelles and vesicle types in secretory pathway have unique sets of rab proteins
what does Rab-GTP bind to?
binds to a large number of Rab-effector proteins which mediate vesicle targeting
what are the different functions of Rab?
- rabs can act at several steps in vesicle targeting
- assist cargo selection and coat formation during vesicle budding
- connect vesicle to motors on cytoskeleton for transport
- tether vesicles to acceptor membrane –> specificity
- recruit SNARE fusion protein
what are the specific rabs at each vesicle transport stage of the secretory pathway?
Rab1: ER to GA
Rab 2: GA to ER
Rab7,9: GA and early to late endosome
Rab5: endocytosis from PM to enosome
Rab4,11: recycling from endosome to PM
Rab3: exocytosis of secretory vesicles
explain the Rab membrane anchor
Rabs have two prenyl lipid groups attached at their C termini
in the GDP bound state Rab is soluble and not associated with the membrane -> lipid is covered up by other proteins (GDI, GDF)
in the GTP bound state lipid modifications are exposed and anchor Rab to the membrane
the Rab effector proteins become attached to membrane through Rab-GTP
how is Rab activated?
specific GEF on membrane produces anchored, active Rab-GTP
GEF is linked to formation of vesicle coat
how does RabGTP work through vesicle proteins?
attach vesicle to motor proteins
tether vesicle to target membrane
activate PI kinases and GEFs to make more Rab GTP in clusters on acceptor membrane
what is the vesicle Rab cycle?
- Vesicle Rab is activated by
GEF on donor membrane
and packaged onto vesicles
– some interact with cargo,
assist uncoating, or attach
to motors - Vesicle Rab-GTP binds
specifically to tethers on
acceptor membrane - After fusion,GAPs on target
membrane inactivate Rab - Inactive vesicle Rab-GDP is
recycled through cytosol to
donor membrane
what runs through the cytosol and provides structure to the cell?
protein filaments (actin) and microtubules (tubulin)
connected to each other and anchored to organelles and PM (cortical cytoskeleton)
what are characteristics of actin filaments?
shorter, clustered at PM, highly crosslinked into network
what are characteristics of microtubules?
longer, thicker, organised around centrosome near nucleus
how do motor proteins transport vesicles?
along cytoskeleton
myosins on actin, dyneins and kinesins on microtubules
different families and mechanisms but ATP dependent
(dyneins are AAA proteins, other motors are not)
what do motors do?
bring vesicles to general location of acceptor membrane but do not provide targeting specificity
