Intracellular Compartments

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Intracellular Compartments

• Exocytosis and Endocytosis
• Reading Becker, ch. 12, pp. 342-352

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Exocytosis

• Release of secreted molecules into extracellular
  space
• Constitutive or regulated
• Secretory vesicles or secretory granules fuse
  with plasma membrane and dump contents into
  extracellular space

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Exocytosis
4
Regulated Secretion
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Polarized Secretion

• Secretion occurs only from specific sites on the
  cell
• Examples
• Release of neurotransmitters only from axon
  terminals
• Release of digestive enzymes only from lumenal
  side of cells in the small intestine

6
Endocytosis

• Internalization of molecules from extracellular
  environment
• Nutrition
• Defense
• Clearance of damaged cells
• Recycling of membrane components

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Endocytosis
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Phagocytosis

• Internalization of large particles (gt0.5 mm)
• Food acquisition for single-celled eukaryotes
• Defense or removal of damaged cells in
  multi-cellular organisms
• Phagocytes
• neutrophils
• macrophages

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Phagocytosis
antibody

• Pseudopodia formation involves in actin
  polymerization within the phagocyte
• Formation of a phagosome or phagocytic vesicle

receptor
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Phagocytosis
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Phagocytosis
12
Clathrin-Independent Endocytosis(Fluid-Phase
Endocytosis)

• Continual process
• Randomly engulfs fluid from extracellular
  environment
• Recycling of membrane
• Compensation for amount of membrane added during
  secretion
• Maintenance of cells surface area

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Clathrin-Dependent Endocytosis(Receptor-Mediated
Endocytosis)

• Concentration and ingestion of extracellular
  molecules
• Uses specific receptors on the extracellular
  surface of the cell

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Nomenclature

• Receptor
• protein
• usually present within a membrane
• specifically binds to another molecule (usually
  a protein)
• Ligand
• molecule that specifically binds to the
  receptor
• Lock and key fit

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Clathrin-Dependent Endocytosis
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Clathrin-Dependent Endocytosis
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Clathrin-Dependent Endocytosis
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Clathrin-Dependent Endocytosis
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Variations on a Theme

• Receptor only binds adaptin and becomes
  concentrated in coated pit if ligand is bound.
  Internalization only occurs if receptor is
  ligand-bound.
• Unliganded receptors bind adaptin and become
  concentrated in coated pits. Binding of ligand
  triggers the process of internalization.
• Receptors are concentrated and internalized
  regardless of whether ligand is bound.
  Constitutive internalization.

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Clathrin Structure
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Clathrin Structure Triskelions

• Triskelions are multimeric proteins
• 3 heavy chains (mw 192,000)
• 3 light chains (mw 30,000-36,000)
• Tips of each of 3 heavy chains are
• attached to form a central vertex
• Each heavy chain bends in the middle
• (the knee)
• Each heavy chain has a terminal globular domain
  (the foot)
• Each heavy chain is bound to 1 of the 3
• light chains (at the thigh)

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Clathrin Structure Triskelions
Each side of each hexagon is comprised of 2
thighs and a calf (from the heavy chains) and 2
light chains. Each vertex of each Hexagon is
made up of 2 knees and 1 triskelion vertex.
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Assembly Protein Complex(Adaptor Protein Complex)

• Multimeric proteins comprised of 4 subunits
• 2 adaptins (adaptin 1 or 2)
• medium chain
• small chain
• Components of the AP complexes are what
  determines the specificity for the materials that
  are internalized in the clathrin-coated vesicle.

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Dynamin

• A cytosolic GTPase
• What is a GTPase? A molecule that binds the
  nucleotide GTP and hydrolyzes it (gets rid of a
  phosphate group). Breaking the high energy
  phosphoanhydride bond releases energy that can be
  used to drive chemical reactions.
• GTP-bound dynamin forms a helical noose around
  the neck of a budding vesicle. GTP hydrolysis
  tightens the noose, sealing off the vesicle and
  separating it from the parent membrane.

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Coated Vesicles in Cellular Transport Processes

• Endocytic vesicles are not the only vesicles
  which have protein coats
• Other intracellular vesicles have coats
• Identity of proteins present in coat reflects the
  origin and determines the destination of the
  vesicle.

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Coated Vesicles in Cellular Transport Processes
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The SNARE Hypothesis

• How do vesicles know which target membrane to
  fuse to?
• v-SNAREs (vesicular SNAP REceptors)
• t-SNAREs (target SNAP REceptors)

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The SNARE Hypothesis