? Its membrane typically constitutes more than half of
the total membrane of an average animal cell.
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? The ER is organized into a netlike labyrinth of
branching tubules and flattened sacs extending
throughout the cytosol , to interconnect
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? The ER has a central role in lipid and protein
biosynthesis
? Its membrane is the
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site of production of all the
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transmembrane proteins and lipids for most of the cell's organelles( the ER itself, the Golgi apparatus, lysosomes,
endosomes, secretory vesicles, and the plasma
membrane).
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? The ER membrane makes a major contribution tomitochondrial and peroxisomal membranes by
producing most of their lipids.
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? Almost all of the proteins that will be secreted to the cellexterior plus those destined for the lumen of the ER,
Golgi apparatus, or lysosomes are initially delivered to
the ER lumen
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TRANSLOCATION OF PROTIENS IN E.R? Translocation of the proteins into the ER is mainly
guided by the signal hypothesis
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? It was proposed by the Blobel and Sabatini
? Proteins synthesized on the membrane bound
polyribosomes contained a peptide extension called
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signal peptide? Responsible for mediating their attachment with the
ER membrane
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? In contrast the protein being synthesized on the free
polyribosomes lack this signal peptide .
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? Insertion of resident proteins into the ER is dependent
on the specific signal eg KDEL
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? But the membrane flow of certain proteins from theER to the cell membrane is designated as bulk flow as
this transport is non selective, occurs without any
targeting signal involved
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? But on the way back to the membrane if the proteins
are destined to the lysosme or the secretory vesicles ,
the movement is mediated by the targeting sequence
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Properties of Signal Peptides Directing Proteinsto the ER
? Usually, but not always, located at the amino terminal
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? Contain approximately 12-35 amino acids? Methionine is usually the amino terminal amino acid
? Contain a central cluster (6-12) of hydrophobic amino
acids
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? The region near the N-terminus usually carries a net
positive charge
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? The amino acid residue at the cleavage site is variable,but residues -1 and -3 relative to the cleavage site must
be small and neutral
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Cotranslational targeting of
secretory proteins to the ER
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Cotranslational targeting of secretory proteins to ER? Step 1: As the signal sequence emerges from the ribosome, it
is recognized and bound by the signal recognition particle
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(SRP).? Step 2: The SRP escorts the complex to the ER membrane
where it binds to the SRP receptor (SR).
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? Step 3: The SRP is released, the ribosome binds to the
translocon, and the signal sequence is inserted into the
membrane channel.
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? Step 4: The signal sequence opens the translocon. Translation
resumes and the growing polypeptide chain is translocated
across the membrane.
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? Step 5: Cleavage of the signal sequence by signal peptidase
releases the polypeptide into the lumen of the ER.
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Posttranslational translocation
of proteins into the ER
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BIP-Binding immunoglobulin protein (BiP)Posttranslational translocation of proteins into the ER
? Proteins synthesized in the cytosol are prevented from
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folding by chaperone proteins such as members of the Hsp70family.
? The N-terminal signal sequence inserts into the Sec61
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translocon complex and the cytosolic chaperones are released.? BiP interacts with the protein and the Sec62/63 complex and
its bound ATP is hydrolyzed to ADP.
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? The protein is prevented from moving back into the cytosol
by the bound BiP and successive binding of BiP and ATP
hydrolysis pulls the protein into the lumen.
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? When the whole protein is inside, ADP is exchanged for
ATP and BiP is released
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Insertion of a membrane protein with a cleavable signal
sequence and a single stop-transfer sequence
? The signal sequence is
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cleaved as the polypeptidechain crosses the membrane, so the amino terminus
of the polypeptide chain is exposed in the ER lumen.
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? However, translocation of the polypeptide chainacross the membrane is halted when the translocon
recognizes a transmembrane stop-transfer sequence.
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? This allows the protein to exit the channel via alateral gate and become anchored in the ER
membrane.
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? Continued translation results in a membrane-spanningprotein with its carboxy terminus on the cytosolic
side.
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Variations in the way in which proteins are inserted
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? The orientations form initially in the ER membrane, but areretained when vesicles bud off and fuse with the plasma
membrane so that the terminal initially facing the ER
lumen always faces the outside of the cell.
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? Type I transmembrane proteins (eg, the LDL receptor and
influenza hemagglutinin) cross the membrane once and
have their amino termini in the ER lumen/cell exterior.
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? Type
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II
transmembrane
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proteins(eg,
the
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asialoglycoprotein and transferrin receptors) also cross the
membrane once, but have their C-termini in the ER
lumen/cell exterior.
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? Type III transmembrane proteins (eg, cytochrome P450,an ER membrane protein) have a disposition similar to
type I proteins, but do not contain a cleavable signal
peptide.
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? Type IV transmembrane proteins (eg, G-protein-
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coupled receptors and glucose transporters) cross the
membrane a number of times (7 times for the former
and 12 times for the latter); they are also called
polytopic membrane proteins
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? Sequences that determine the structure of a protein in
a membrane are called topogenic sequences.
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? A number of proteins
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possess the amino acid sequence KDEL
(Lys-Asp-Glu-Leu) at their carboxyl terminal
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? KDEL-containing proteins first travel to the GA in vesiclescoated with coat protein II (COPII)
? This process is known as anterograde vesicular transport.
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? In the GA they interact with a specific KDEL receptorprotein, which retains them transiently.
? They then return to the ER in vesicles coated with COPI
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(retrograde vesicular transport), where they dissociate from
the receptor, and are thus retrieved
? Certain other non-KDEL-containing proteins also pass to the
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Golgi and then return, by retrograde vesicular transport, to the
ER to be inserted therein.