Endocytosis%20-%20Exocytosis

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Endocytosis - Exocytosis
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Endocytosis

• Phagocytosis solid
• Pinocytosis liquid (general)
• Endocytosis
• Uptake of substances
• Transport of protein or lipid components of
  compartments
• Metabolic or division signaling
• Defense to microorganisms

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Phagocytosis (1)

• Predominant cells
• unicellular cells
• macrophages
• osteoslats
• throphoblasts

• Functions
• uptake of food partickles
• immuneresponses
• elimination of aged cells (RBC)

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Phagocytosis (2)

• Required
• signal
• membrane receptor (Fc receptor for Ab)
• formation of pseudopodium
• cortical actin network
• The formed vesicle phagosome (hetero- auto-)

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Endocytosis

• Clathrin-coated vesicles
• Non-clathrin coated vesicles
• Macropinocytosis
• Potocytosis

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Clathrin coated pits/vesicles
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Function of clathrin coated vesicles

• Receptor mediated endocytosis
• Selective uptake of molecules
• (low environmental conc.)
• Membrane receptors
• Concentration of ligand (1000x)

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Components of coated vesicles
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Receptor-mediated endocytosis of LDL
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Sorting signals of secreted and membrane
proteins to transport vesicles
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Selective incorporation of membrane proteins Into
the coated vesicles
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LRO
dER
TGN
EE
LE
L
RE
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-AP-2
-AP180
-clathrin
-PIP2
-synaptotagmin
-dynamin
-actin
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Endosomal-Lysosomal compartmentStructure

• tubular, vesicular
• acidic pH - vacuolar H ATP-ase - proton pump
• early-endosome (EE) and late-endosomes (LE) and
  lysosomes (L)
• EE pH 6 LE pH5
• in EE no lysosomal membrane proteins or enzymes
  (in contrast LE)

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Endosomal-Lysosomal compartmentFunction

• sorting
• transport
• degradation
• removal of clathrin layer
• formation of EE
• in the EE
• dissociation of receptor-ligand complex -
  receptor-recycling (e.g. LDL, transferrin)
• receptor-ligand complex transported together -
  receptor down regulation (e.g. EGF)

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• Pathway of LDL
• insulin or other
• hormones
• in receptor
• mediated
• endocytosis

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Fate of LDL internalized by receptor-mediated
endocytosis
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The transferrin-cycle
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Partcipating components in budding of coated
vesicles
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Late endosome

• early endosomes, TGN and autophagosomes feed late
  endosomes
• lysosomal enzymes M-6-P signal is changed, the
  phosphate group is cleaved - receptors can not
  bind enzymes
• the enzyme content of vacuoles is in the lumen

lysosomes
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Dissociation of receptor-ligand complex in late
endosomes
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De Duve, Ch. Nobel-prize - 1974
Lysosomes (TEM)
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Lysosomes

• enzymes - acidic hydrolases
• e.g. protease, nuclease, glycosidase,
  phosphatese
• more than 40 types of enzymes
• membrane proteins - highly glycosilated protects
  from the enzymes
• transport molecules of the membranes - transports
  the products of proteolytic cleavage into the
  cytoplasm
• the waste products are released or stored in the
  cytoplasm (inclusion - residual body)

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LAMP lysosome associated membrane proteins

• integrant membrane proteins of
• the lysosome
• LAMP-2 tarnsport of cholesterol
• LAMP-2 defficiency- autophagy

www.helsinki.fi/bioscience/biochemistry/eskelinen
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Autophagy - Autophagosome

• intake of own components
• regulates the number of organells
• toxic effects can also induce it

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Formation of autophagosome
www.helsinki.fi/bioscience/biochemistry/eskelinen
E
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Non-clathrin coated vesicles

• There is no receptor or clathrin in the membrane
• The uptake of substances is less selective
• Primairly liquide-phase endocytosis

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Macropinocytosis

• Ruffling of the surface membrane forms
  inclusions
• These vacuoles have no membrane
• Size 0.2-5 mm - the mass/surface ratio is very
  good
• Significance
• Liquide-phase pinocytosis
• Taking probes from the
• environment
• antigene recognition
• in macrophages

Film produced by F. Vilhardt and M. Grandahl.
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Caveolae

• 50-80 nm, bottle-like infoldings of the surface
  membrane
• endothels, adipocytes
• caveolin
• potocytosis - caveolae close but not
  internalized,
• the materials enter the cytoplasm by a
  special carrier molecule e.g. vitamine B4
• some other caveolae enter the cell !!!

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Caveolae
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Caveolin oligomers and caveolae assembly
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Dynamincs of caveolae-formation
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Functions of dynamin
Clathrin-mediated endocytosis
Membrane retrieval
Endosome- to-Golgi transport
Secretory vesicle formation in TGF
Caveolae
Fluid phase endocytosis
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Dynamin in the cell
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Structure of dynamin
Interaction with membranes
Interaction with cytosceleton
Activation of GTP-ase domain
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Dynamin requires GTP hydrolysis to pinching off
coated vesicles

• The not-hydrolysable GTP-gS is added
• Dots represent binding of anti-dynamin
  antibodies
• The long neck shows that however the coated pit
  was formed,
• in the absence of GTP hydrolysis its pinching
  off is absence

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Carrier mediated proteolysis

• some molecules can enter lysosome directly from
  the cytoplasm
• the signal of entry KFERQ (Lys-Phe-Glu-Arg-Gln)

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Proteasome

• non-lysosomal cleavage of proteins
• cylindric, multienzyme complex
• parts ATP binding-, substrate binding-,
  regulator-domain
• location close to the external part of
  ER-translocon
• ubiquitin - degradation-signal - is required
• the non-properly folded or damaged proteins
• regulator - eliminator - role e.g. cyclins
• cystic fibrosis - Cl- fac. transp. is affected as
  the responsible membrane protein is broken down
  in proteosome

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Proteasomes
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Ubiquitation - proteasome
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Exocytotic processes
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The mannose-6-P pathway and lysosomal enzymes
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Exocytosis in TEM
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Apical and basolateral targeting in epithelial
cell
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Transcytosis

• the ligands walk around the endosomal compartment
• ligands transported from one surface to the other
• e.g. immunoglobulins of the colostrum cross the
  intestinal epithelium by transcytosis

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Release of neurotransmitters