Post on 28-Dec-2015
Summary
1. Rough ER and smooth ER;2. Signal hypothesis, translocation into ER;3. Single-span and multi-span membrane proteins;4. Glycosylation;5. Protein folding;6. Lipid synthesis
Lecture 8 Vesicular trafficking
from ER to Golgi
Endocytic and biosynthetic-secretory pathways
Transport vesicles
(Ten or more chemically distinct, membrane-enclosed compartments)
The biosynthetic-secretory and endocytic pathways
Various types of coated vesicles
Golgi apparatusPlasma
ER and Golgi Cisternae
Assembly of a clathrin coat
triskelionCoated pitsand vesicleson the cytosolic surface of membranes
Freeze-etch
36 triskelions12 pentagons6 hexagons
Inner layer binds adaptins
Adaptin binds to cargo receptor and clathrin triskelion
Dynamin pinches off the bud
Four types of adaptinsAuxillin-activatedATPase is requiredTo remove the clathrin coat
Vesicles can have different shapes
Dynamin pinches of the vesicles
GTPase
Shibire mutanthas coated pitsbut no budding offof synaptic vesicles
ARF proteins: COPI&clathrinSar1 protein: COPII
GTP causes Sar1 toBind to membrane
Assembly and disassembly of coat by GTPases
Coat-recruitment GTPases GTPase works like a timerAnd cause disassembly shortlyAfter the budding is completed
Guidance of vesicular transport
SNAREs: specificity and fusion
Rab GTPases: initial docking and tethering of vesiclesto target membranes and matching of v- and t- SNAREs
SNARE proteins guide vesicular transport20 SNAREs, v-SNAREs, t-SNAREs
SNAREs specify compartment identity and control specificity
4 helicesin trans-SNAREcomplexes
Rab proteins ensure the specificity of vesicle docking
>30 RabsOn cytosolic surface
C-terminal regions are variable:Bind to other proteins, including GEFs
SNAREs may mediate membrane fusion
SNARE complex
After docking
The entry of enveloped viruses into cells
HIV
Similar to SNAREs
Proteins leave the ER in COPII-coated transport vesicles
ER exit sites(no ribosomes)
Selectiveprocess
Only properly folded and assembled proteins can leave the ER
Chaperones cover up exit signals
Homotypic membrane fusion
to form vesicular tubular clusters
Vesicular tubular clusters
Lacks many of the ER proteins
COPI-coated
Retrograde transport:
Short-lived
carry back the ER resident proteinsthat “leaked” out
ER retrieval signals: KKXX in ER membrane proteins,
KDEL sequence in soluble ER resident proteins
Membrane proteins in Golgi and ER have shorter TM domains (15 aa)Cholesterol
pH controls affinity of KDEL receptors
Ordered series of Golgi compartments
Cisternae, tubular connections
Plant cell
MT is required to localize near the cell nucleus close to the centrosome (in animal cells)
Plant cells
Two main classes of N-linked glycosilation
core
complexoligosaccharides
high-mannoseoligosaccharides
Oligosaccharide processing in the ER and the Golgi
Why glycosylation?
FoldingTransportStability RecognitionRegulatory roles(Notch)
Histochemical stains: biochemicalCompartmentalization of the Golgi
Functional compartmentalization
Transport through the Golgi may occur by vesiculartransport or cisternal maturation (not mutually exclusive)
Collagen rodsScales in algae
Summary
1. Vesicular transport, biosynthetic-secretory andendocytic pathways;
2. Coated vesicles;3. Coat assembly and disassembly, budding, dynamin,
coat-recruitment GTPases;4. Targeting and fusion by Rab GTPases, SNAREs;5. ER to Golgi: COPII, folding, fusion (cluster), retrograde;6. Golgi apparatus structure and polarity;7. Continuation of glycosylation;8. Compartmentalization of Golgi cisternae;9. By now we have introduced gated transport, transmembrane
transport and vesicular transport.