Download MBBS Trafficking of Protein V Lecture PPT

Receptor Mediated Endocytosis

? The major mechanism of vesicular transport between ER

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and Golgi.

? Takes place in the regions of the membranes known as

coated pits

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? The coated pits has high concentration of protein

clarthrin and this mechanism of receptor mediated
endocytosis is the clarthin coated vesicle method

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? However there is another method in which the receptor

mediated endocytosis takes place without the clarthin
coated vesicles

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? The SNARE proteins helps in the later type of the

receptor
Some Types of Vesicles and Their Functions

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Vesicle

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Function

COPI

Involved in intra-GA transport and

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retrograde transport from the GA to the

ER

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COPII

Involved in export from the ER to either

ERGIC or the GA

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Clathrin

Involved in transport in post-GA locations

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including the PM, TGN and endosomes

Secretory vesicles

Involved in regulated secretion from

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organs such as the pancreas (eg, secretion

of insulin)

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Vesicles from the TGN

They carry proteins to the PM and are also

to the PM

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involved in constitutive secretion


Steps in a round of anterograde

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transport involving COPII vesicles
Steps in a round of anterograde transport

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involving COPII vesicles

? Step 1: Sar1 is activated when GDP exchanged for

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GTP and it becomes embedded in the ER membrane
to form a focal point for bud formation.

? Step 2: Coat proteins bind to Sar1?GTP and cargo

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proteins become enclosed inside the vesicles.


? Step 3: The bud pinches off, formatting a complete

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coated vesicle. Vesicles move through cells along
microtubules or actin filaments.
? Step 4: The vesicle is

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uncoated when bound GTP is

hydrolyzed to GDP by Sar1.

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? Step 5: Rab molecules are attached to vesicles after

switching of Rab.GDP to Rab.GTP, a specific GEF
.Rab effector proteins on target membranes bind to
Rab?GTP, tethering the vesicles to the target

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membrane.

? Step 6: v-SNAREs pair with cognate t-SNAREs in

the target membrane to form a four helix bundle

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which docks the vesicles and initiates fusion.


? Step 7: When the v- and t-SNARES are closely

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aligned, the vesicle fuses with the membrane and the
contents are released.

GTP is then hydrolyzed to GDP, and the Rab?GDP

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molecules are released into the cytosol.

An ATPase (NSF) and -SNAP dissociate the four-

helix bundle between the v- and t-SNARES so that

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they can be reused.

? Step 8: Rab and SNARE proteins are recycled for

further rounds of vesicle fusion

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Selective transport of proteins to lysosomes

? The

best-characterized

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?These phosphorylated mannose

pathway of protein sorting in

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residues

are

specifically

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the Golgi is the selective

recognized by a mannose-6-

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transport of proteins to

phosphate receptor in the trans

lysosomes.

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Goligi network

? Protein

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destined

for

incorporation into lysosomes

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are modified by mannose
phosphorylation.

? This occurs while the protein

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is still in the cis Golgi
network.
The M-6-P pathway

? In the trans-Golgi network, the phosphorylated

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enzymes bind to M6-P receptors .

? Which direct the enzymes into vesicles coated with

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the fibrous protein clathrin.

? The clathrin lattices is rapidly depolymerized to its

subunits, and the uncoated transport vesicles fuse

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with late endosomes.

? Within this low pH compartment, the phosphorylated

enzymes dissociate from the M6P receptors and then

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are de-phosphorylated.

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Targeting to lysosomes
I-cell disease

? Mucolipidosis II

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? UDP-N -acetyl glucosamine phosphotransferase
? Cultured fibroblasts-deficient in numerous lysosomal

enzymes

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? Inclusions in lysosome
? These enzymes were found to be present in excess in

tissue culture media and in extracellular fluids

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? Psychomotor and skeletal defects



Botulinum Toxin

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? Most lethal toxin known
? Most serious cause of food poisoning
? One component of the toxin is a protease specific

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only to the synaptobrevin

? Thus by inhibiting the v-SNARE the release of

acetylcholine into the NMJ is halted

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Brefeldin ?A

? An anti viral produced by fungus Penicillium

brefeldianum

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? Prevents GTP from binding to ARF in the step

1 of the anterograde pathway that Is the step of
Coat assembly

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? So in the presence of this fungal metabolite

the golgi apparatus appears to disintegrate and
fragments are lost

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Disorders Related to Intracellular Transport

Familial Hypercholesteremia

? Familial hypercholesterolemia, FH (type II

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hyperlipoproteinemia) is an autosomal dominant
disorder

? Results from mutations affecting the structure and

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function of the cell-surface receptor that binds plasma
LDLs (low density lipoproteins) removing them from
the circulation

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? The defects in LDL-receptor (LDLR) interaction

result in lifelong elevation of LDL-cholesterol in the
blood

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1. Receptor null mutation ( lack of receptor synthesis

in the ER

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2. Defective intracellular transport to golgi apparatus
3. Defective extracellular ligand binding
4. Defective endocytosis
5. Failure to release LDL molecules inside the

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endosome