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endomembrane system
chapter 15
internal membranes• endomembrane system
• functions as a coordinated unit• divide cytoplasm into distinct
compartments• controls exocytosis and
endocytosis• movement of molecules which
cannot pass through membrane
transport of proteins• protein synthesis
• begins on a ribosome in cytoplasm
• sorting signals• protein transport
• nuclear transport• protein translocators• vesicular transport
pathways• endocytic
• endosomes & lysosomes• exocytic
• biosynthetic pathway• secretory pathway
• constitutive secretion• regulated secretion
• secretory granules
exocytic endocytic
golgi complex
rough ER
secretory granule
regulated secretory pathway
constitutive secretory pathway
endocytic pathway
endoplasmic reticulum• rough ER
• cisternae -• many bound ribosomes • polarity
endoplasmic reticulum• smooth ER
• functions• synthesis of steroid hormones• liver processing of toxins• sequestration of Ca+
• sarcoplasmic reticulum
biosynthesis in RER• synthesis of proteins
• free ribosomes• cytosolic proteins• peripheral proteins• nuclear proteins
• bound ribosomes• secretory proteins• integral proteins
Completed polypeptide
Incoming ribosomal subunits
Start of mRNA (5′ end)
End of mRNA (3′ end)
Polyribosome
Ribosomes
mRNA
0.1 μm
Growing polypeptides
biosynthesis in RER• bound ribosomes
• bind to ER as needed• SRP (signal recognition particle)• binds to SRP receptor
• translocon
Ribosome
mRNA
Signal peptide
SRP
1
SRP receptor protein
Translocation complex
ER LUMEN
2
3
45
6
Signal peptide removed
CYTOSOL
Protein
ER membrane
biosynthesis in RER• translocon
• SRP interacts with a SRP receptor• ribosome interacts with the translocon• SRP is released
• facilitated by G-proteinsRibosome
mRNA
Signal peptide
SRP
1
SRP receptor protein
Translocation complex
ER LUMEN
2
3
45
6
Signal peptide removed
CYTOSOL
Protein
ER membrane
biosynthesis in RER• Processing
• signal sequence cleaved by signal peptidase• Carbohydrates added
• oligosaccharyltransferase• RER lumen
• chaperones assist in folding• protein disulfide isomerase adds disulfide bonds to
cysteines.
• integral membrane proteins• hydrophobic region generated
• translocon opens laterally• expels transmembrane segment• synthesis continues
biosynthesis in RER
start transfer sequence cleaved
1
2
3
4
biosynthesis in RER• glycosylation
• begins in the RER• membrane bound glycosyltransferases (GTs)
• lipid carrier (dolichol phosphate)• oligosaccharyltransferase -> transfers to polypeptide
• removal of 2 of 3 terminal glucose residues• quality control
• glycoprotein binds with calnexin (chaperone)• terminal glucose removed (by glycosidase II)
• GTs recognize malfolded proteins• tag with glucose --> refolded or destroyed
• transferred to golgi
golgi bodies• golgi apparatus
• functionally distinct cisternae - • cis• trans
• differences in composition between cis and trans face
trans-Golgi network (TGN)
trans cisternae
medial cisternae
cis cisternae
cis- Golgi network
movement of materials• movement across Golgi
• vesicular transport model• vesicles shuttle cargo from
CGN to TGN • cisternal maturation model
• cisternae mature as they move through the apparatus
• combination?• retrograde transport via
vesicles cisternal vesicular
glycosylation• carbohydrate assembly
• glycolipids and glycoproteins• assembled across golgi by glycosyltranferases
vesicle shuttles• vesicle-mediated transport
• donor compartment• acceptor compartment• direction
• anterograde transport• toward plasma membrane
• retrograde transport• away from plasma membrane
• protein coats• two main functions
• cause membrane to curve• select components to be carried
vesicle shuttles• steps in vesicle transport
• (1) cargo selection• sorting mechanisms• coat proteins
• COPI, COPII, clathrin• bulk transport
• proteins trapped
Nat
ure
Rev
iew
s M
olec
ular
Cel
l Bio
logy
10,
360
-364
(M
ay 2
009)
vesicle shuttles• steps in vesicle transport
• (2) budding• coat proteins interact with
adaptor proteins• COPII - sec23/24,
sec13/31, Sar1• Sar1
• coat recruitment GTPase• GTP bound Sar1 is
membrane bound
COPII-coated vesicle
sec23/24
cargo receptors
donor membrane
sec13/31
outer coatinner coat
assembly of protein coat1. sec12 (a GEF) in membrane bound to cargo receptor
• cargo receptor binds to cargo2. GDP-bound SAR1 binds to sec12, exchanges GDP for GTP
• sec12 is acting as a GEF here3. SAR1-GTP exposes tail which anchors it to ER membrane4. sec23/24 dimer binds to SAR1-GEF complex (assembles
inner coat)• sec13/31 dimer binds to inner coat • curving of membrane may increase affinity for SAR1
vesicle shuttles• steps in vesicle transport
• (3) scission• scission proteins excise bud
• (4) uncoating• coat proteins detach and get
reused
protein cargo
vesicle shuttles• steps in vesicle transport
• (5) transport • microtubule transport via
motor proteins• dynactin
• (6) tethering• Rab proteins bind to tether
proteins on acceptor membrane
Rab
tethering proteins
x
vesicle shuttles• steps in vesicle transport
• (7) docking• soluble NSF attachment
protein receptor• v-SNAREs• t-SNAREs
• (8) fusion • (9) disassembly
• NSF and SNAP proteins
exocytic pathway• TGN
• proteins sorted• secretory (constitutive &
regulated)• lysosomal
• constitutive secretion• plasma membrane proteins
• polarized cells - apical / basal proteins segregated in TGN
• exocytosis• triggered by increase in Ca2+ • vesicle fuses with plasma
membrane
non-lysosomal proteins• regulated secretion
• aggregate as dense granules at TGN• secretory vesicles (granules)• 2 main functions
• timing, amount
lysosome enzyme sorting• sorting proteins at TGN
• lysosomal proteins tagged• phosphorylated mannose• recognized by MPRs
• released in clathrin-coated vesicles
lysosomes• digestive organelles
• contain acid hydrolases• digest a wide variety of biological
molecules• intracellular digestion
• lysosomal storage disorders• absent or defective enzymes
• buildup of undigested materials• Tay-Sachs
lysosomal disorders• Tay-Sachs disease
• autosomal recessive disorder• probably with multiple origins
• deficiency in hexosaminidase A• buildup of ganglioside GM2
• symptoms include• mental retardation, blindness, death
uptake of materials• phagocytosis• endocytosis
• pinocytosis• receptor-mediated
endocytosis
RME• receptor-mediated
endocytosis• clathrin coat
arranged in geometric pattern
lipoproteins on cell surface
clathrin polygons on
inner surface
invagination of clathrin pit
clathrin structure• clathrin
• triskelion• three heavy chains• three light chains
• binds to AP2 adaptor• binds to cargo receptors
clathrin structure• clathrin
• triskelions and adaptors geometrically arranged• dynamin-ring
• promotes pinching off of coated pit into vesicle• uses GTP to tighten around neck of vesicle
• Hsc70 uses ATP and frees clathrin coat
endocytic pathway• endosomes
• formed from fusion of endocytes• sorts materials arriving from PM
• early endosomes (rab5)• derived from PM vesicles
• fuse with early endosome• proteins dissociate - pH affects
conformation• proton pumps lower pH of
endosome• late endosome (rab7, rab9, MPRs)
• as pH goes down, MPRs release hydrolases - recycle to golgi
• lysosome
lysosomal pathways• lysosomes
• autophagy• digestion of organelles
• autophagosome• organelle wrapped in
double membrane• fuses with lysosome
• autophagolysosome
• phagocytosis• engulfing large particles• phagosome
• fuses with lysosome
plant cell vacuoles• vacuoles
• aqueous storage organelles• tonoplast - water vacuole in plants
• contains active transport system for K+ ions • aquaporins