Learning Goal: I’m working on a biology exercise and need a sample draft to help me learn.
GENERAL INTRODUCTION
This drawing assignment is to draw out the process of co-translational translocation, glycosylation, and intracellular trafficking of a Golgi transmembrane protein. The full details are below, along with due dates, a checklist that you can use to make sure you include all details required to earn full credit.
____________________________
DRAWING DETAILS
For this drawing you will draw all of the steps involved for a polypeptide—that is destined to be a glycosylated transmembrane protein in the Golgi membrane—to be co-translationally translocated into the ER membrane, glycosylated, and then transported in a vesicle to the Golgi.
1. Draw a polypeptide undergoing co-translational translocation. Your drawing should include recruitment of the polypetide and ribosome to the ER membrane during translation. The polypeptide should have an N-terminal signal sequence and one or more subsequent start/stop transfer sequences (this question includes content from Lecture 9 & Learning Module 4/Lecture 12/13—there are images of all details in the slides).
This part of your drawing should include (make sure you label these components):
- the polypeptide (that is destined to become the glycosylated transmembrane protein)
- the signal sequence (on the polypeptide)
- the ribosome
- SRP
- the SRP receptor
- the protein translocator
2. Now draw the polypeptide undergoing N-linked glycosylation (as shown in Lecture 10, slide 9).
This part of your drawing should include (make sure you label these components):
- the transmembrane protein
- oligosaccharyl transferase
- the oligosaccharide (on the transmembrane protein)
3. Now draw your glycosylated transmembrane protein getting trafficked from the ER to the Golgi (you will need to refer to learning module 4 and Lecture 12/13 slides for this part—just do your best!).
This part of your drawing should include:
- budding (with the specific coat protein labeled),
- transport (molecular motor, motor binding protein and cytoskeleton filament track it is “walking” on),
- targeting/tethering (Rab),
- and fusion (SNARE) of the vesicle;
- the different leaflets of the membrane should be colored differently to show that you understand “once cytosolic, always cytosolic.”
Here is an example from a previous semester where the assignment was to draw a soluble protein (so, not membrane bound) being trafficked to the lysosome. Although the assignment was a bit different, this drawing should give you some ideas…especially for Part 3!
____________________________
CHECKING YOUR WORK
Use the checklist below to make sure your drawing includes all of the details described above.
Each item on the list is worth 1pt! Each item should be drawn AND CLEARLY LABELLED.
For Part 1:
- the polypeptide (that is destined to become the glycosylated transmembrane protein) – 1pt
- the signal sequence (on the polypeptide) – 1pt
- the ribosome – 1pt
- SRP – 1pt
- the SRP receptor – 1pt
- the protein translocator – 1pt
For Part 2:
- the transmembrane protein – 1pt
- oligosaccharyl transferase – 1pt
- the oligosaccharide (on the transmembrane protein)- 1pt
For Part 3:
- process of budding – 1pt
- the specific coat protein is labeled (meaning you should include either COPI, COPII or Clathrin) – 1pt
- process of transport – 1pt
- molecular motor – 1pt
- motor binding protein – 1pt
- cytoskeleton filament track it is “walking” on – 1pt
- process of targeting/tethering–the process of “targeting/tethering” should be labeled (1pt) and the Rab should be labelled (1pt)
- fusion–the process of “fusion should be labeled (1pt) and the SNAREs should be labelled (on both the vesicle and Golgi – 1pt)
the different leaflets of the membrane should be colored differently to show that you understand “once cytosolic, always cytosolic.”
Lecture 10
Protein Structure &
Function
Stereocilia (composed of cross-linked actin filaments) of an inner ear hair cell
Lecture 10 Review
Protein biochemistry determines protein structure. Protein structure
determines function. Functional amino acids can be widely distributed, but
brought together during protein folding. Even after a protein has folded into
its final conformation, that conformation can be altered when other molecules
interact with or bind to the protein (all proteins bind other molecules).
Cells can control protein activity 1. Covalent modification like phosphorylation,
glycosylation, and/or lipid modification, 2. Proteolysis 3. Allosteric regulation
(binding of a regulatory ligand changes protein conformation in a way that
alters activity in a positive or negative way), 4. Regulation of a critical binding
partner, 5. Blocking a critical binding site, 6. Regulating polymerization, 7.
Regulating scaffold interactions
Lecture 10 Outline
A. How protein structure determines function
1. Protein biochemistry influences conformation
2. Protein interactions with other molecules can
change protein conformation
B. How protein function can be altered
1.
2.
3.
4.
5.
6.
7.
Covalent modification
Proteolysis
Allosteric effectors
Non-allosteric binding partners
Blocking critical binding sites
Polymer formation
Scaffold interactions
Striated muscle tissue
Protein Structure & Function
Key Concepts
• A protein’s biochemistry dictates its functional activities
(protein biochemistry determines structure, and structure determines function)
• Regulation of protein structure and function is one of the
most fundamental ways that cells control their own activities
Example: Muscle cell regulation of proteins like myosin II and actin controls
muscle contraction. (Sliding of myosin II and actin filaments causes muscles to
contract.)
Protein structure determines function!
Functional amino acids are often
widely distributed; folding can
create a crevice or cavity that brings
them in close proximity. Folding is
critical!!!
Protein activity can be regulated by amino
acid position; the clustering of polar amino
acid side chains can alter their reactivity
Key Point: How amino
acids are positioned
determines protein
activity
What would happen if the Ser in the catalytic
site was positioned elsewhere?
Protein Structure & Function
ALL proteins bind to other molecules
The interaction may be strong or weak, but it always shows
great specificity
A protein molecule’s
physical interaction
with other molecules
determines its
biological role
For example, antibodies bind to viruses or
bacteria to mark them for destruction; thus,
antibodies have a biological role within our
immune system to help us evade infection.
Lecture 10 Outline
A. How protein structure determines function
1. Protein biochemistry influences conformation
2. Protein interactions with other molecules can
change protein conformation
B. How protein function can be altered
1.
2.
3.
4.
5.
6.
7.
Covalent modification
Proteolysis
Allosteric effectors
Non-allosteric binding partners
Blocking critical binding sites
Polymer formation
Scaffold interactions
Striated muscle tissue
Protein Structure & Function
1. Covalent Modification
Protein activity can be changed when its
structure is affected by the enzymatic
addition or removal of a covalent
subgroup (e.g. phosphorylation,
glycosylation, lipid modification, etc)
(a) Phosphorylation
Many changes in proteins are driven by
protein phosphorylation (addition of a
phosphate group)
A eukaryotic cell contains a large
collection of protein kinases (for
phosphorylation) and protein
phosphatases (for dephosphoryation)
phosphorylation-dephosphorylation
Protein Structure & Function
(b) Glycosylation
Addition of a carbohydrate
group.
For example: N-linked
glycosylation occurs when a
polypeptide chain enters the
ER lumen, it is glycosylated
on target asparagine (Asn)
amino acids.
A precursor oligosaccharide
is transferred to the Asn in a
reaction catalyzed by a
membrane-bound
oligosaccharyl transferase
enzyme in the ER lumen.
Figure 12-51 Molecular Biology of the Cell (© Garland Science 2008)
Protein Structure & Function
(c) Lipid Modification
Addition of covalently linked lipids can allow a protein to have a tight
association with the membrane
Lipid anchors (pink) can tether
(link) proteins to membranes
Figure 10-20 Molecular Biology of the Cell (© Garland Science 2008)
Different combinations of covalent modifications
can cause a protein to have
multiple different functions!!!
Example: p53 is a critical tumor suppressor gene, with many important cellular
functions. It can be modified at 20 different sites, leading to a combination of
many different potential functions
Any combination of
these modifications
are possible.
Different
combinations of
modifications =
different structure =
different function.
Each protein’s set of covalent modifications creates a combinatorial regulatory
code; in other words, different combinations of modifications on the same
protein = different functions
Activity
Potential covalent modifications 1. When the protein NGN is phosphorylated at
of the protein NGN
both sites (A & B), it translocates to the
nucleus.
B
A
2. When NGN is phosphorylated at position A
only, it is degraded.
3. When NGN is only acetylated, it binds to
another protein in the cytosol
NGN
Given this combinatorial code for NGN modification
and function, draw the different possibilities of NGN
covalent modification described here.
1
= Phosphorylation
= Acetylation
2
3
Protein Structure & Function
2. Proteolysis
Cells can start or stop a protein’s
activity by proteolysis (cleavage)
Example: Insulin is synthesized as a
larger protein (proinsulin) that is
cleaved by a proteolytic enzyme after
the protein chain has folded into a
specific shape.
Once insulin has been denatured and
its two polypeptide chains have
separated, its ability to reassemble is
lost.
Figure 3-35 Molecular Biology of the Cell (© Garland Science 2008)
proteolysis
Protein Structure & Function
3. Allostery: Change in protein’s activity at one site caused
by the binding of a regulatory ligand at a different site on
the protein; ligand binding alters the protein conformation,
and conformational change alters protein activity.
First identified in enzymes
active
inactive
Now thought to apply to
most proteins
Key point: allosteric regulation always involves two different
binding sites on the protein being regulated
Protein Structure & Function
Allostery: Positive Regulation
Caused by positive conformational coupling between two distant binding sites
on an allosteric protein. (Binding of one molecule increases binding of the other)
Example: Both
glucose & molecule X
bind best to the closed
conformation of a
protein. Because both
glucose and molecule
X drive the protein
toward its closed
conformation, each
ligand helps the other
to bind (cooperative
binding).
Protein Structure & Function
Allostery: Negative Regulation
Caused by a negative conformational coupling between two distant binding sites
on an allosteric protein. (Binding one molecule decreases binding of the other)
Example: Molecule X
prefers open
conformation, while
glucose prefers closed.
Because glucose &
molecule X drive the
protein toward
opposite
conformations, the
presence of either
ligand interferes with
the binding of the
other.
Protein Structure & Function
Example of positive allosteric modulation: Hemoglobin/Oxygen
The allosteric site is the active site of an
adjoining protein subunit (hemoglobin
is a tetramer).
Oxygen binding to one subunit induces
a conformational change that causes the
remaining active sites in the other
subunits to increase their oxygen
affinity.
So, oxygen is both the substrate and the
effector.
Red blood cells ensure proper oxygen delivery using this mechanism: Hemoglobin binds
oxygen with greater affinity when there is lots of oxygen (and releases oxygen when
oxygen levels are low) – enables flow of oxygen to de-oxygenated tissues and not away.
Protein Structure & Function
Cells can control expression or release of
allosteric ligands/effector molecules
Example: Actinmyosin activity
requires calcium.
Therefore, muscle
cells “hide” calcium
in the sarcoplasmic
reticulum until they
need actin-myosin
activity (for muscle
contraction).
Ca2+ sequestered here until
muscle needs to contract,
then released into cytosol!
Wiki.bio.purdue.edu
Protein Structure & Function
4. Cells can regulate the presence of a critical binding partner
Binding partners that
activate or repress
protein activity
Example: signaling
molecule that
causes receptor
dimerization, which
leads to activation
Protein Structure & Function
5. Blocking a critical binding site
Cells can indirectly start and stop a protein’s activity by
blocking a critical binding site required for protein activity
Example: Tropomyosin blocks the myosin-binding site on actin in muscle cells;
myosin needs to bind actin for the proteins to function together
Guess what causes the release of tropomyosin?
Figure 16-78a Molecular Biology of the Cell (© Garland Science 2008)
Protein Structure & Function
6. Affecting Polymerization: Cells can start and stop a protein’s
activity by regulating the processes that make active polymers from
inactive subunits
Example: Actin and
myosin make active
polymers during cell
division to form the
contractile ring.
Polymerization requires
the help of many other
proteins. If a cell alters
expression of one of
those proteins,
polymerization will be
affected.
Figure 17-49a Molecular Biology of the Cell (© Garland Science 2008)
Protein Structure & Function
7. Affecting scaffold
complex components
Cells can start and stop a
protein’s activity by
regulating interactions of
proteins within a scaffold
complex
Example: Ubiquitin
Ligase complex is a 5protein scaffold complex.
If any single protein is
prevented from associating
with the complex, it cannot
function properly (designed
to work together).
Figure 3-79 Molecular Biology of the Cell (© Garland Science 2008)
Protein Structure & Function
Key Point: Scaffold proteins usually hold the scaffold complex together.
Manipulating just one of the proteins in a scaffold complex will alter the function
of the complex.
Synaptic
scaffold
If any single
protein is
prevented from
associating with
the synaptic
scaffold complex,
it cannot
function properly
(will lead to
improper
neuronal
transmission)
Write your own summary for Lecture 10 here
Lecture 9
Regulation of
Protein Synthesis &
Processing
Immunofluorescent
staining of tRNA (green) in
cultured cells
Lecture 9 Review
Protein Synthesis & Processing
o Translation is regulated by various mechanisms, including control at the 5’ and 3’ UTRs.
o Translation is initiated when the CBC recruits eIFS to the 5’UTR, which then bind PABPs to
circularize the mRNA. The initiator tRNA and small ribosomal complex are then recruited to the
5’UTR—they slide along the 5’UTR until they reach a start codon, at which point the large ribosomal
complex is recruited and translation begins.
o mRNA stability directly affects the amount of protein translated—because once mRNA
degrades, it cannot be translated. mRNA stability is influenced by the length of the polyA tail and by
extracellular cues.
o Proteins are folded with the help of chaperones (Hsp70, Hsp60, calnexin, calreticulin).
o The insertion of transmembrane proteins into the ER membrane either during or after translation
is directed by internal “stop-start” signal sequences and utilizes a protein translocator that is
embedded in the ER membrane.
o Covalent modifications of proteins involve the enzyme-mediated addition of a chemical
group, which can affect protein structure and, therefore, function.
o Some proteins function as polymers (macromolecules composed of many repeated protein
subunits), and others must be proteolytically cleaved in order to be functional (proteolysis is always
irreversible).
Lecture 9 Outline
Cell Specialization via
Protein Synthesis & Processing
1. Regulation of Translation
a. Preparing for translation: mRNA is circularized
b. Control of translation at the 5’ & 3’ UTRs
c. microRNAs (miRNAs)
1. Co-/Post-translational protein regulation
a. Folding/membrane insertion
b. Covalent modifications
c. Polymer assembly
d. Proteolytic modifications
Selective Degradation of RNA
a.
b.
Prevention of export of incomplete or
unprocessed RNA from the nucleus (unexported
RNA is degraded in the nucleus)
Prevention of translation of damaged or
unwanted RNA in the cytosol (untranslated RNA
is degraded in the cytosol)
X
NOT TRANSLATED
Nonsense Mediated
Decay!!!
(in Learning Module 3)
Preparing for Translation: mRNA circularization
1. Export ready
mRNA molecule is
transported
through the
nuclear pore into
the cytosol
2. Cap Binding
Complex (CBC)
recruits eukaryotic
initiation factors
eIF4E & eIF4G to the
5’ cap; CBC is then
released
3. eIFs bind poly-Abinding proteins (PABPs)
thereby joining the 5’ and
3’ UTRs, creating a circular
mRNA, which is ready to be
translated once eIFs recruit
ribosomal subunits
Regulation of Translation
Translation initiation occurs at at the 5’UTR
A normal 5’-cap allows proper assembly
of the translation initiation complex:
eIFs, initiator tRNA, ribosomal subunits,
mRNA.
The small ribosomal subunit binds eIFs
at the 5’ cap. After the initiator tRNA
is recruited, the complex slides
along until it recognizes an
AUG codon. Then, the large
Initiator
ribsomal subunit binds
tRNA
and translation begins.
Translation can continue
as long as the mRNA
remains stable.
eIF2
small ribosomal subunit
Nonsense Mediated Decay (NMD)
•
•
•
•
•
•
Nonsense mutations cause premature termination codons (PTDs), which destroy
protein structure and function.
NMD is a mechanism that ensures mRNA transcripts with PTDs are degraded.
In the nucleus, EJCs are deposited at each splice junction.
Stop codons should always be downstream of EJCs because splicing occurs in
the ORF.
If a stop codon is upstream of an EJC, it will be detected by molecular machinery
that will quickly degrade the defective mRNA.
NMD requires the help of the ribosome, so this only occurs in the cytosol.
Regulation of Translation
The amount of peptide translated from mRNA is controlled by
mRNA stability
In lecture 8 you learned that the length of the poly-A tail helps determine how
long the mRNA survives
Once the tail is degraded by
the process of deadenylation
(by exonucleases), the
circular structure of the
mRNA is destroyed, the 5’
cap is removed, and
exonucleases degrade the
mRNA from both exposed
ends.
(Note: this image does not show the
circular structure, which would be
intact prior to complete
deadenylation)
Regulation of Translation
The length of the Poly A tail can be affected by
extracellular signals
Studies have shown that extracellular signaling molecules, including
hormones & growth factors, can affect the longevity of mRNA within a cell
by affecting the length of the Poly A tail
Degradation of casein mRNA
in the presence and absence
of prolactin.
Prolactin is a hormone that
induces an intracellular
signal transduction cascade
that ultimately causes an
extension of the casein
mRNA PolyA tail.
Regulation of Translation
Regulation by microRNAs
microRNA (miRNA) are small (~22nt) noncoding RNA transcripts that can
regulate translation via RNA interference – they interfere with the ability of
the transcript to be translated
miRNAs are functional
molecules! They regulate many
animal and plant genes (up to
60% of the human genome !) &
also serve as a defense
mechanism against viruses &
transposons
(parasitic RNA)
Often bind mRNA within the
3’ UTR
RNA interference: microRNAs can inhibit mRNA translation
miRNAs are small pieces of RNA that bind to
complimentary regions of mRNA transcripts
and influence their translation.
First miRNA identified was lin-4 in C.elegans,
which represses the translation of lin-14
Mechanisms. Bind to mRNA and:
1. Block binding to initiation factors or
ribosomes
2. Recruit RNA-digesting enzymes
(exonucleases) to degrade the RNA
3. Recruit protein-digesting enzymes
(proteases) to digest the nascent protein
(miRNP=microRNA ribonucleoprotein complex)
Cell Specific Regulation of Peptide/Protein Production
Co-/Post-translational protein regulation
a.
b.
c.
d.
e.
Protein Folding
Membrane insertion
Covalent modifications
Polymer assembly
Proteolytic modifications
Co/Post-translational Regulation
Protein Folding
(R group)
Energy dynamics
of protein
conformation
The final folded
structure, or
conformation, of any
polypeptide chain is
the one that
minimizes its free
energy
Most of this is driven
by the polar aqueous
and non-polar
membrane phases
Co/Post-translational Regulation
Protein Folding
Energy dynamics of protein conformation: In the aqueous phase, polar
side chains face out, but in the (hydrophobic) membrane they are hidden
Figure 3-5 Molecular Biology of the Cell (© Garland Science 2008)
Co/Post-translational Regulation
Protein Folding
Which part of this
protein do you think has
exposed polar side
chains?
Which part do you think
has exposed nonpolar
side chains?
ure 3-5 Molecular Biology of the Cell (© Garland Science 2008)
Co/Post-translational Regulation
Protein Folding
• Molecular chaperones help guide the folding of polypeptides
into most energetically favorable conformation during or after
synthesis; especially critical in times of cellular stress
– Examples of chaperones in the cytosol: Heat shock proteins (Hsp)
Hsp70, Hsp60
– Examples of chaperones in the ER: Calnexin, calreticulin
Nonpolar/hydrophobic
residues (yellow)
Polar/hydrophilic
residues (blue)
Co/Post-translational Regulation
Protein Folding
Mechanism of Hsp70: binds hydrophobic moieties as the
peptide is being translated in the cytosol (during synthesis)
Co/Post-translational Regulation
Protein Folding
Mechanism of Hsp60: aids in peptide folding after peptides have
been fully translated in the cytosol
Correctly
Polypeptide
folded
protein
Cap
Hollow
cylinder
Steps of Chaperonin
Chaperonin
(fully assembled) Action:
1 An unfolded poly-
peptide enters the
cylinder from one
end.
Fig. 5-24
2 The cap attaches, creating
3 The cap comes
an isolation chamber that
off, and the properly
prevents aggregation &
folded protein is
provides a favorable
released.
hydrophilic environment for
the folding of the polypeptide.
Co/Post-translational Regulation
Protein Folding
Proteins translated on bound ribosomes on the
rough ER are translated into the lumen of the
ER—they also often need help folding!
Calnexin and Calreticulin:
ER chaperone proteins that retain
incompletely folded proteins in the
ER to give them a chance to properly
fold
Improperly folded proteins are
exported from the ER, ubiquitylated,
and degraded
Figure 12-54 Molecular Biology of the Cell (© Garland Science 2008)
Co/Post-translational Regulation
Protein Folding
No quality control needed for
correctly folded proteins (some fold
without assistance by chaperones).
Incompletely folded proteins are
refolded by molecular chaperones
These “protein-rescue” processes
compete with destruction by the
proteasome.
These processes prevent protein
aggregation, which occurs when
misfolded proteins clump together
nonspecifically ! protein
aggregation can lead to devastating
diseases like Parkinson’s and
Huntington’s Disease.
Figure 6-85 Molecular Biology of the Cell (© Garland Science 2008)
Co/Post-translational Regulation
Protein Folding
Misfolded proteins are ubiquitylated, which targets them for regulated
destruction via the proteasome
Proteasome: composed of multiple protein subunits,
including multiple proteases with active sites facing the
central cylinder’s inner chamber (Fig 6-90)
Co/Post-translational Regulation
Membrane Insertion
Our cells have a lot of proteins
embedded in the membranes of
organelles and the plasma
membrane.
How do they get inserted into the
membrane?
These membrane proteins all have
sequences referred to as start- and stoptransfer sequences, that get recognized by a protein called the
protein translocator (the blue protein in the image above).
•
Co-translational translocation is when a protein is embedded in the
membrane by the protein translocator during translation.
•
Post-translational translocation is when a protein is embedded in the
membrane by the protein translocator after its translation is complete.
Co/Post-translational Regulation
ribosome
Membrane insertion
can be:
1. Co-translational =
occurs when proteins are
translated on bound
ribosomes on the ER; the
ribosome docks near the
translocator.
2. Post-translational =
peptide is fully translated
and released before
insertion into target
membrane.
(A) Transmembrane proteins of the
ER, Golgi, endosome, lysosome
and plasma membrane are all
translated on bound ribosomes at
the ER & are co-translationally
translocated into the ER membrane
first
(B) Transmembrane proteins of the mitochondria,
peroxisome and nucleus are translated on free
ribosomes & post-translationally translocated
directly into their destination membranes
Co/Post-translational Regulation
Membrane Insertion
The start transfer and
stop transfer sequences
are stretches of amino
acids recognized by the
protein translocator
These sequences can be
internal (as shown here)
or sometimes they are at
the N-terminus (as shown
on the next slide).
Figure 12-47 (part 2 of 2) Molecular Biology of the Cell (© Garland Science 2008)
The membrane-spanning
sequences (in red) are typically
highly hydrophobic stretches
of amino acids
Co/Post-translational Regulation
Membrane Insertion
When the starttransfer sequence is at
the N-terminus
(shown in red) it is
cleaved.
That means there
must be at least a
subsequent stoptransfer sequence
(shown in orange) to
cause the protein to be
embedded in the
membrane.
If the stop-transfer sequence was not there, this protein would have ended up
in the ER lumen as a soluble protein (meaning, not membrane embedded).
C. Peptide/Protein Production:
2. Co/Post-translational Regulation
Folding and Membrane Insertion
In multi-pass transmembrane proteins, the polypeptide chain
passes back and forth repeatedly across the lipid bilayer.
These proteins
must have
multiple
start- and stoptransfer
sequences
If this peptide had an N-terminal start transfer sequence, how many total
start/stop-transfer sequences did it have?
Figure Q12-5 Molecular Biology of the Cell (© Garland Science 2008)
C. Peptide/Protein Production:
2. Co/Post-translational Regulation
Covalent Modifications
• Glycosylation by cell-specific enzymes can
change the function of a shared protein
• Different kinases in different cells can lead to
the phosphorylation of proteins at different
sites
• Isomerization of disulfide linkages in different
cells can produce different functions
• Variability in methylase/acetylase proteins
can dramatically alter cell phenotype and
function
C. Peptide/Protein Production:
2. Co/Post-translational Regulation
Polymer Assembly
Polymers are
macromolecules
composed of
repeated subunits.
The protein collagen
is a triple helix
formed by 3
collagen
polypeptide chains
that form a helices,
and wrap around
one another.
“rope-like superhelix”
Figure 3-27a Molecular Biology of the Cell (© Garland Science 2008)
C. Peptide/Protein Production:
2. Co/Post-translational Regulation
Polymer Assembly
Many rodlike collagen molecules are
cross-linked together in the
extracellular space to form fibrils
that have the tensile strength of
steel.
42 genes in humans that code for
collagen a chains
(You need three to make a protein)
40 different collagen proteins have
been identified so far
Figure 19-62 Molecular Biology of the Cell (© Garland Science 2008)
C. Peptide/Protein Production:
2. Co/Post-translational Regulation
Proteolytic Modifications
Occur when a protein must be
proteolytically processed (cleaved)
to be functional.
Proteolysis is irreversible.
The polypeptide hormone insulin is
synthesized as a larger protein
(proinsulin) that is cleaved by a
proteolytic enzyme after the protein
chain has folded into a specific shape.
Figure 3-35 Molecular Biology of the Cell (© Garland Science 2008)
Write your own summary to
Lecture 9
Extra images of membrane insertion
N-terminal starttransfer sequence
Internal starttransfer sequence
Extra photos of membrane insertion
Post-translational Translocation
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