ER, Golgi, Lysosome and Endosome Functions

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ER, Golgi, Lysosome and Endosome Functions
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Functions of the Endoplasmic Reticulum

• Smooth ER (no ribosomes) - prominent in cells
  specialized for
• Metabolism of lipids (for secretion)
• Adrenal gland synthesizes steroid hormones from
  cholesterol
• Liver (hepatocyte) production of lipoproteins
  (HDL, LDL, etc.)
• Detoxification of lipid-soluble drugs and harmful
  products of metabolism (hepatocyte)
• Sequestration of Ca2 ions (sarcoplasmic
  reticulum in muscle)
• Rough ER (studded with ribosomes)
• Synthesizes membrane phospholipids for all
  organelles and assembles membrane bilayers for
  endomembrane system and nucleus
• Synthesizes proteins that function in
  endomembrane system, the majority of plasma
  membrane proteins, and all secreted proteins
  (thus especially prominent in cells specialized
  for secretion)

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Fig. 17-13
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Fig. 17-14
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Two Major Transport Pathways in Endomembrane
System

• Outward Synthetic/Secretory Pathway
• RER --gt Golgi --gt plasma membrane or
  extracellular fluid
• --gt lysosomes (via late endosomes)
• Inward Endocytic Pathway
• Plasma membrane (ECF) --gt endosomes
• (responsible for ingestion and intracellular
  digestion of
• extracellular materials)

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Vesicular Transport

• mediates transport of proteins (plus lipids and
  additional membrane components) between
• ER and Golgi (cis or CGN)
• Golgi compartments (CGN, cis-, medial-, trans-
  cisternae, TGN) - retrograde transport only
• Golgi and lysosomes (via endosomes?)
• Golgi and PM (constitutive vs. regulated
  secretion)
• PM and endosomes and lysosomes (endocytosis)
• involves complex, dynamic budding and fusion of
  membranes
• preserves membrane topology (once outside,
  always outside)

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Protein modifications during intracellular
trafficking

• In addition to cleavage by signal peptidase, most
  proteins will be further modified during
  transport from the ER to the Golgi (and beyond).
• These occur within the lumen, and can affect
  either soluble proteins or the luminal domains of
  membrane proteins.
• In the Endoplasmic Reticulum
• Disulfide bond formation
• In some proteins, disulfide bonds form
  sequentially during translation and translocation
  of the polypeptide chain.
• Other proteins require assistance of PDI (protein
  disulfide isomerase), which is an "ER-resident
  protein that catalyzes S-S bond rearrangements to
  link cysteines that are not sequential.
• Protein folding/Assembly of multimeric complexes
• Promoted by luminal Hsc70, calnexin,
  calreticulin, etc.
• Improperly-folded proteins are not allowed to
  leave the ER (in most cases, they become
  permanently bound to chaperones) may accumulate
  in RER.
• Misfolded polypeptides or unassembled subunits
  are often transported back through the translocon
  into the cytosol to be degraded by proteosomes.

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Fig. 17-27
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Protein modifications during intracellular
trafficking

• In the Endoplasmic Reticulum, continued
• N-linked glycosylation
• Preformed oligosaccharide complex is transferred
  (cotranslationally) from dolichol-phosphate to
  specific Asn residues (at Asn-X-Ser/Thr
  sequence).
• Once attached, some sugars residues (e.g., 3
  terminal glucoses and one mannose) are trimmed
  by enzymes (glycosidases) in the ER.

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Fig. 17-35
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Fig. 17-36
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Protein modifications during intracellular
trafficking

• In the Golgi apparatus
• Additional processing of N-linked
  oligosaccharides
• Further trimming of terminal sugars
• Specific sugars may be added on by
  glycosyl-transferases (transfer sugar from a
  preformed activated carrier linkage to UDP)
• Depending on protein, end result is either
• a high mannose oligosaccharide (trimming only)
• a complex oligosaccharide (remnants of core
  plus variable amounts of additional sugars
  including NAG, galactose, sialic acid, and/or
  fucose)

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Fig. 17-38
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Protein modifications during intracellular
trafficking

• In Golgi, continued
• Golgi cisternae with cargo physically move from
  cis to medial to trans positions within the Golgi
  stack cisternal progression.
• Membrane and luminal components are continually
  retrieved from later cisternae to be returned to
  earlier cisternae by small transport vesicles
  that move in a retrograde direction.
• Thus, different Golgi compartments (cis, medial,
  trans) have distinct "resident" components, which
  results in the spatial compartmentalization of
  biochemical reactions within the Golgi.
• In some cases, O-linked glycosylation (on Ser or
  Thr residues) also occurs (only a few sugars,
  which are added one at a time).
• Additional modifications that may be important
  for the maturation of specific proteins include
• hydroxylation (ex. occurs extensively in the ER
  on lysine residues in collagen)
• proteolytic cleavage (may be organelle-specific
  or extracellular)

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Fig. 17-30(a)
Fig. 17-32
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Functions of Glycosylation

• May stiffen and extend polypeptide chain during
  folding.
• Protein stability (protease resistance)
  oligosaccharides can shield protein surfaces
• On plasma membrane
• protective, lubricated coat (glycocalyx) (Fig.
  8-18 FOB)
• specific recognition by lectins proteins that
  bind carbohydrates) (Fig. 8-19 FOB)
• binding of infectious microbes to host cells
  (Fig. 8-20 FOB)
• Targeting of proteins to lysosomes

Fig. 8-17 in FOB
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Protein Sorting in the Golgi

• Many proteins follow the constitutive secretion
  pathway, in that they move (by default) from ER
  to Golgi to cell surface.
• Only requirement for protein transport from the
  RER to Golgi is that the protein is properly
  folded and assembled quality control by the
  RER!!
• Proteins that are sorted by Golgi include
• ER-resident proteins transported back to ER from
  cis-Golgi (-KDEL sorting signal)
• Retrograde transport between Golgi cisternae
• Proteins that are sequestered in secretory
  vesicles (and later released at cell surface by
  stimulus-driven exocytosis).
• Plasma membrane proteins in polarized cells
• Lysosomal proteins (mannose-6-phosphate sorting
  signal)

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Fig. 17-42(a)
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Fig. 17-42(b)
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Fig. 17-41
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Fig. 17-13
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Fig. 17-43
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Fig. 17-29
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Fig. 15-10
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Fig. 3-18
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Fig. 17-13
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Fig. 5-44
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Lysosomal Targeting

• Lysosomes are membrane-enclosed organelles that
  carry out the intracellular digestion of
  macromolecules.
• Key components of lysosome are lumenal acid
  hydrolases (plus various membrane proteins,
  e.g., the H pump, Cl- channels and transporters
  that export building blocks).
• In the Golgi, mannose-6-phosphate is attached to
  newly synthesized lysosomal proteins M-6-P is
  ticket to lysosome.
• M-6-P is recognized by receptor in Golgi membrane
  (trans Golgi network).
• M-6-P receptor (and bound protein) is packaged
  into clathrin coated vesicles.
• Complex between protein and receptor is unstable
  at low pH and dissociates when vesicle fuses with
  acidic compartment (late endosome/lysosome).
• Lysosomal protein remains in lysosome, while
  M-6-P receptor recycles (returned to Golgi by
  vesicular transport).

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Steps in the Synthesis of the Mannose-6-P Tag
of Lysosomal Enzymes 1. GlcNAc
phosphotransferase has binding sites for signal
patch of lysosomal enzyme (recognition site) and
UDP-GlcNAc (catalytic site).\ 2. GlcNAc
phosphotransferase catalyzes the covalent
attachment of phosphoGlcNAc to a mannose residue
on the lysosomal enzyme, with the release of
UMP. 3. GlcNAc phosphodiesterase then hydrolyzes
phosphodiester bond to release GlcNAc, leaving
the phosphate group attached to the 6C hydroxyl
of mannose.
Fig. 17-39
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Fig. 17-40
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Fig. 17-45
Fig. 17-47
Fig. 17-46
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The Transferrin Cycle.
Fig. 17-48
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Transcytosis of maternal IgG across intestinal
epithelial cells of newborn mice.
Fig. 17-49
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The endosome acts as a sorting station for
inward-moving receptor/cargo complexes

• Some receptors unload cargo in endosomes cargo
  goes on to endolysosomes for degradation and
  receptor returns to the plasma membrane (ex. LDL
  receptor).
• Some receptors stay bound to cargo and both cargo
  and receptor are delivered to endolysosomes for
  degradation example EGF receptor - mechanism
  acts to dampen cell signaling response to EGF
  (epidermal growth factor).
• In above two examples, lysosomes represent a
  fusion of the biosynthetic and the inward
  endocytic streams of protein trafficking.
• In transferrin pathway, Fe dissociates in
  endosomes, and both transferrin (protein "cargo")
  and its receptor return to plasma membrane to be
  re-used.
• In transcytosis, both receptor and cargo pass
  through endosome compartment as complex, to be
  delivered to opposite surface of cell, where
  cargo is released.