Intracellular Vesicle Traffic

  1. -euk only 

    -adapting to env change

    -respond to and sense env stimuli

    -membrane transport 

    -mem maintenance 

    -transport btwn compartments 
           ~ER <--> Golgi
           ~Trans Golgi <--> lysosomes 


    Intracellular Vesicle Traffic
  2. 2 types 

    defined by composition of their membranes 

    maintained by vesicle transport 

    specific combination of molecular markers determine which membranes can fuse (molecular address)
  3. Retrieval pathway: retrieve things by accident

            ~4rm early endosomes and returned to cell surface for reuse

            ~some molecules are retrieved 4rm early and late endosomes and returned to Golgi.

            ~some retrieved 4rm Golgi and returned to ER.
    The biosynthetic secretory/endocytic roadmap
  4. Secretory pathway: making protein, bring them into cell

               ~protein molecules transported 4rm ER to plasma membrane or via endosome to lysosomes.
    The biosynthetic secretory/endocytic roadmap
  5. Edocytic pathway: stuff coming into cell --> endosome --> lysosome 

             ~molecules are ingested in vesicles derived from the plasma membrane and delivered to early endosomes and via late endosomes to lysosomes
    The biosynthetic-secretory/endocytic roadmap
  6. -coated in cages of specific proteins 

    -coat is discarded when vesicles fuse 

    -function of coat:
            ~concentrate membrane proteins 
            ~mold vesicle 

    -3 types 
           ~clathrin-coated: mediate transport 4rm Golgi apparatus and 4rm plasma membrane
           ~COPI-coated: mediate transport 4rm ER and from Golgi cisternae
           ~COPII-coated: mediate transport 4rm ER and 4rm Golgi cisternae 

    -100 nm vesicle
    Coated vesicles
  7. (most buds from) plasma membrane -> endosomes 

    endosomes -> (bud from) golgi
  8. -secretory pathway

    -retrieval pathway

    -buds from Golgi
  9. -secretory pathway

    -buds from ER
  10. Encased by clathrin proteins 

    -heavy + light chain make triscelion (3H3L hexamer)

    -can assemble into cages without membrane

    -pull a 'pit' on inside of cytoplasmic membrane

    -protein determines geometry of cage

    -relatively uniform vesicle size

    Adaptor Protein
    -btwn membrane and clathrins

    -trap trans-membrane proteins 

    -trap cargo receptors 

    -diff kinds <--> distinct kinds of vesicles
    Clathrin vesicles
  11. -vesicles with less define coat 

    -specialized protein assemblies that form dimers stabilizing curvature 

    -carry cargo originates from endosomes and carry cargo back to Golgi
            ~acid hydrolase receptor 
            ~manose 6-phosphate receptor

    only assemble when...
    -can bind to cargo receptors 
    -can directly interact with membrane 
    -can bind to phosphorylated phosphatidylinsitol lipid
  12. -inositol phospholipids <10% of membrane 

    -can undergo phosphorylation on 3',4', and 5' C.
    -high compartmentalized 
         ~PI and PIP kinases
         ~PIP phosphotases

    -transport proteins bind specific PIPs
    -bar code for type of vesicle / membrane surface 
    Phosphoinositides (PIPs)
  13. -forms ring that helps bud vesicle 

    -membrane domain that binds PI(4,5)P2

    -GTPase domain provides energy for pinching 

    -recruits other proteins 

    *once vesicle is released, clathrin coat is released 

    -Hsp70 chaperone protein

    -ATP hydrolysis energy used to peal clathrin 

    -PIP phosphotase cleaves P from PI(4,5)P2

    -GTP hydrolysis of coat recruiting GTP binding proteins
  14. -high concentrations in cytosol

    -assembly of COPI, COPII, and clathrin coat 

    -binding of GTP exposes helix that inserts into membrane 

    -binds to linker / COPI / cargo receptor 

    -GTP hydrolysis leads to coat disassembly
    Coat recruiting GTPases
  15. -highly selective to target membrane 

    -achieved with surface markers that identify origin & cargo

    -achieved with 2 protein classes
    ~Rab proteins 
        -monomeric GTPases 
        -GTP-bound --> to membranes with lipid anchor 
        -bind to Rab effectors on target membrane 

    ~Snare proteins: catalyze membrane fusion rxns in vesicular transport 
          -provide add'l layer of specificity in transport process by helping to ensure only correct targeted vesicles fuse.
          -35 dif SNAREs in animal cell
          -transmembrane proteins exist as complementary sets ---

    vSNAREs: found on vesicle membranes (single polypeptide chair)
    tSNAREs: found on target molecules (consists of 2-3 protein)
    Targeting vesicles
  16. -require to bring membranes close enough 

    -1.5 nm

    -overcome energetically unfavorable merging 

    -many different kinds; specific to vesicles / membranes 



    -energy comes from snare protein interaction
    Snare proteins mediate membrane fusion
  17. -COPII vesicles 

    -vesicles originate from ER exit sites 

    -cargo proteins have signal surfaces 
         ~glycosylation plays a role 

    -cargo receptors have similar exit signals on cytosolic side 

    -some mis-packaging (leakage) occurs, but mostly packaging is specific

    -incorrectly misfolded proteins bind to chaperones that mask signals 
           ~export back to cytosol --> degradation
    Transport: ER --> Golgi
  18. Homotypic fusion
    -fusion of vesicles / membranes of the same type 

    Heterotypic fusion
    -fusion of vesicles / membranes of diff compartments 

    Vesicular tubular clusters
    -formed by homotypic fusion of ER COPII vesicles 
    -immediately start forming COPI vesicles 
          ~transport back receptors / other proteins to ER
    Mechanisms of Vesicular Transport
  19. -C-terminal AA signal

    -KKXX (Lys-Lys-X-X) -- membrane proteins 

    -KDEL (Lys-Asp-Glu-Leu) -- dissolved proteins 
        ~if removed, proteins are secreted 

    -binds directly to COPI coat associated receptors 
        ~KDEL receptors 

    -KDEL receptor affinity 
        ~high in Golgi
        ~low in ER
    ER retrieval signals 
  20. -diff enzymes in diff portions of the stack (immuno gold)

    -all transmembrane proteins
    Oligosacharide processing in Golgi is an organized sequence
  21. Complex oligosacharides 
    -modified in Golgi

    High manose oligosacharides 
    -little modification in Golgi
    Mammalian Glycoproteins
  22. N-linked glycans
    -N of asparagine or arginine side-chains 
    -added en-block in ER 
    -modified in ER and Golgi 

    O-linked glycans 
    -OH of serene, threonine, tyrosine
    -added in golgi 
    -proteoglycans, mucins 

    **N-linked oligosaccharides also found in archaeal cell membranes
  23. Suggested by:

    -complicated pathways 

    -many enzymes 

    -abundance of glycoproteins 

    Acts via:

    -promoting folding 

    -making folding intermediates more soluble 

    -'glyco-code' marks folding progress 

    -mediating binding to chaperones/lectins

    -helping sorting ER-->Golgi 

    In general Sugars

    -prevent approach of other macromolecules 

    -protect from proteolytic digestion

    -allow protective mucus coat without rigid cell wall 

    -recognition surfaces
    Glycosylation Functions
  24. 1. Vesicular transport model

    2. Cisternal maturation model 
           -dominant bc more simple 

    *matrix proteins 
    -determine location in cell
    -responsible for assembly of golgi mitotic protein kinase disrupts assembly
    2 Models of Transport thru Golgi
  25. Destination of golgi processed proteins.

    -digestion of phagocytosed material

    -breakdown of intra/extra cellular debris 

    -soluble hydrolytic enzymes (40 types)
          ex: digestion of macromolecules 

    -pH 4.5-5
          ~enzyme not active at pH 7.0
          ~maintained by vacuolar V-type ATPase 

    -membrane proteins highly glycosylated for protection

    -transporters move monomers to cytosol 

    -confluences / recycling with late endosomes 

    -whole family of organelles in it
  26. functionally related to lysosomes (enlarged lysosomes)

    -contain hydrolytic enzymes 

    -store waste and nutrients

    -increase cell size 

    -maintain turgor pressure

    -maintain cellular pH
  27. -digestion of particles inside cell 
          ~ex: obsolete organelles 

    -helps restructure cells in development

    -vesicles fuse to form double membrane autophagosome 

    -then fusion with lysosome
  28. N-linked mannose-6-P (M6P) marker
           ~requires signal patch in protein/recognized by phosphotransferase 

    M6P binds to receptor proteins on TGN
          ~binds at pH >6.5
          ~releases at pH <6

    Clathrin coated vesicles deliver cargo 

    Receptor mediated endocytosis rescues false transport by default pathway
    Targeting proteins to lysosomes
  29. Achieved with endocytotic vesicle 

    -large vesicles (>250nm)
    -only some cells 
    -food/ dead cells/ invasion defense 

    -small vesicles (<100nm)
    -fluid ingestion
    -most cells 
    -cells maintain fluid and membrane balance
  30. -requires receptors
    -occurs continually
           ~shaped by actin polymerization
           ~Pi-kinases-> PI(4,5)P2

    -PI 3 Kinase 
    -PI(4,5)P2 -> PI(4,5)P3

    Self cells have 'don't eat me' signals 
    -recruits phosphotase
  31. Vesicle formation 

    Surprisingly high rate 
          ~up to 30% of membrane / minute (macrophages)

    Balanced with exocytosis (& endocytosis)

    Starts with clathrin coated pits 
    -clathrin shed w/in seconds 
    -fuse with early endosome
  32. Most Cells 

    Used for
            -transport to other cell side (transcytosis)
            -transport to caveosome

    Formed using lipid rafts 

    Cargo not recruited by coat proteins 
         ~lipid rafts 

    involved structural proteins: caveolins

         ~no low pH or hydrolytic enzyme like endosomes/lysosomes

    -0.2 um : small so if it carries a bacteria by accident it will not get infected by badly/quickly
  33. Efficient uptake of macromolecules 

    Receptor complex in clathrin coated vesicles 

    selective concentrating mechanism 

         -transported as cholesteryl esters 
         -lipid-protein particles 
                   ~low-density lipoproteins (LDLs)
         -LDL receptors bind in exterior and release in low pH of endosome 
         -esters are hydrolyzed in lysosomes --> cholesterol for membrane synthesis
    Receptor mediated endocytosis
  34. -4rm early endosome 

    -before pH is low enough to activate degradative enzymes 

    -most receptors retrieved to origin 
         ~some sent to other membrane 
         ~some sent to lysosome for degradation
    Membrane proteins are retrieved to the cell membrane
  35. Ion transport into cell 

    Ligand (transferrin) is recycled 

    ATP (mannose-6-phosphate :: similar to transferrin)
    The Transferrin Cycle
  36. EX: epidermal growth factor (EGF)
    -elicits regulator response 
    -binds to receptor 
    -transported to endosome/degraded in lysosome 
    -receptor is not recycled, making cells less sensitive to EGF stimulation
    Receptor down regulation
  37. -form from early endosome
    -fuse with late endosome 

    -carry endocytosed membrane proteins for degradation 
          ~Ubiquitin tags

    -ESCRT proteins involved in sorting proteins into invaginations
    Multi-vescular bodies
  38. -regulate uptake activity 

    -ex: glucose transporters
                 ~transported to membrane in response to insulin stimulus
    Recycling endosomes can be used as protein storage
  39. Polarized epithelial cells 
       -apical domain
       -basolateral domain

    Allow recycling to proper domain 

    Can be sent in different directions
    More than one early endosome type
  40. New Lipids / proteins for cell membrane 

    soluble proteins for excretion 

    secretion of glycoproteins/proteoglycans for matrix 

    Constitutive secretory pathway 
             -always on 

    Regulated secretory pathway
             -secretion on demand 
                      ~digestive enzymes
  41. Cell membrane 

           -Mannose 6-phosphate tag

           -unknown signal
    Destinations for Golgi proteins
  42. AKA secretory granules/dense-core vesicles 

    -Originate with trans-golgi network (TGN)

    -Signal unknown / selective aggregation

    -Can contain 
           ~small molecule 

    -Too big to be achieved with cargo receptors 

    -Fuse; retrograde retrieval of membrane to TGN
           ~Clathrin coated vesicles 

    -Net concentration of contents
    Secretory Vesicles
  43. Activates certain peptide products 
     -Hydrolytic enzymes 

    Starts in Golgi / mostly in vesicles 

    Often in N-terminal pro-peptide cleaved 

    Sometimes multiple prod from same peptide 

        ~too short to be transported thru ER membrane 
        ~can be harmful
    Proteolytic processing
  44. (bring protein to surface)

    -deliver new membrane 

    -cell division



    -wound repair (triggered by Ca2+ influx)
    Exocytosis to enlarge plasma membrane
  45. 1. Targeted transport 

    2. Indiscriminate transport 
             -Selective exclusion
             -Selective retention
    3 ways for sorting protein in diff membrane domains
Card Set
Intracellular Vesicle Traffic
chapter 13