Download MBBS Trafficking of Protein IV Lecture PPT

Scheme of the events in ERAD

? A target protein which is

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misfolded undergoes retrograde

transport

through

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the

ER

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membrane into the cytosol, where

it

is

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subjected

to

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polyubiquitination.

? Following polyubiquitination, it

enters a proteasome, inside which

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it is degraded to small peptides

that exit.

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? Several proteins, including Sec61,

Derlin 1 and the ERAD E3 ligases,

Hrd1 and Doa10, are potential

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ERAD channel candidates.


the nobel Prize in Chemistry 2004

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The Nobel Prize in Chemistry 2004 was

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awarded jointly to Aaron Ciechanover, Avram
Hershko and Irwin Rose "forthediscoveryof
ubiquitin-mediatedproteindegradation."
Ubiquitin-Dependent Degradation

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? Ubiquitin is a small (8.5 kDa, 76 residue) polypeptide that targets

many intracellular proteins for degradation.

? Ubiquitin molecules are attached by non--peptide bonds formed

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between the carboxyl terminal of ubiquitin and the -amino groups

of lysyl residues in the target protein.

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? The residue present at its amino terminus affects whether a protein

is ubiquitinated.

? Amino terminal Met or Ser residues retard, whereas Asp or Arg

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accelerate ubiquitination

? Attachment of a single ubiquitin molecule to transmembrane

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proteins alters their subcellular localization and targets them for

degradation.

? Subsequent degradation of ubiquitin-tagged proteins takes place in

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the proteasome, a macromolecule that also is ubiquitous in

eukaryotic cells.

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Reactions in attachment of ubiquitin to proteins.

E1 - activating enzyme,
E2 - ligase, and

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E3 - transferase
? The terminal COOH of ubiquitin

first forms a thioester.

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? The coupled hydrolysis of PPi by

pyrophosphatase ensures that the

reaction will proceed readily.

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? A thioester exchange reaction now

transfers activated ubiquitin to E2.

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? E3 then catalyzes the transfer of

ubiquitin to the -amino group of a

lysyl residue of the target protein.

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? Additional

rounds

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of

ubiquitination result in subsequent

polyubiquitination

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Proteasome

? The proteasome consists

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of a macromolecular,
cylindrical complex of
proteins, whose stacked
rings form a central pore

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that harbors the active
sites

of

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proteolytic

enzymes

? For degradation, a protein

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thus must first enter the
central pore.
? Entry into the core is

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regulated by the two outer rings

that recognize polyubiquitinated proteins .

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? The regulatory particle recognizes the ubiquitinated

protein which are unfolded by ATPases present in the

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regulatory particles or caps.

? Protease active sites in the core of the proteosome

attack peptide bonds and degrade the protein.

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? Peptides are released into the cytosol for further

degradation by cytosolic peptidases.

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? Both normally and abnormally folded proteins are

substrates for the proteasome.

? Liberated ubiquitin molecules are recycled.

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? The proteasome plays an

important role in presenting

small peptides produced by degradation of various

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viruses and other molecules to MHC class I molecules,

a key step in antigen presentation to T lymphocytes

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? Genetic disorders that result from defects in the genes

that encode ubiquitin, ubiquitin ligases, or

deubiquitinating enzymes include

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Angelman syndrome
Autosomal recessive juvenile Parkinson's disease
Von Hippel-Lindau syndrome, and
Congenital polycythemia

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TRANSPORT VESICLES

? Proteins

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that

are

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synthesized

on

membrane-bound

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polyribosomes and are
destined for the GA or
PM reach these sites
inside transport vesicles

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? Each vesicle has its own

set of coat proteins.
Vesicular Transport

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? Vesicular transport is the predominant mechanism for

exchange of proteins and lipids between membrane-
bound organelles in eukaryotic cells

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? This form of transport involves the movement of various

elements with the aid of the bubble like vesicles created
from the cell membrane

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? It is fundamentally divided into endocytosis and

exocytosis

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? Endocytosis is divided into 3 distinct mechanisms

Phagocytosis

Pinocytosis and

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Receptor mediated endocytosis



Receptor Mediated Endocytosis

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? The major mechanism of vesicular transport between ER

and Golgi.

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? Takes place in the regions of the membranes known as

coated pits

? The coated pits has high concentration of protein

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

? However there is another method in which the receptor

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mediated endocytosis takes place without the clarthin
coated vesicles

? The SNARE proteins helps in the later type of the

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receptor
Some Types of Vesicles and Their Functions

Vesicle

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Function

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COPI

Involved in intra-GA transport and

retrograde transport from the GA to the

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ER

COPII

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Involved in export from the ER to either

ERGIC or the GA

Clathrin

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Involved in transport in post-GA locations

including the PM, TGN and endosomes

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Secretory vesicles

Involved in regulated secretion from

organs such as the pancreas (eg, secretion

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of insulin)

Vesicles from the TGN

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They carry proteins to the PM and are also

to the PM

involved in constitutive secretion

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