Chapter 25
Protein trafficking
25.1 Introduction
25.2 Oligosaccharides are added to proteins in the ER and Golgi
25.3 The Golgi stacks are polarized
25.4 Coated vesicles transport both exported and imported proteins
25.5 Different types of coated vesicles exist in each pathway
25.6 Cisternal progression occurs more slowly than vesicle movement
25.7 Vesicles can bud and fuse with membranes
25.8 SNAREs control targeting
25.9 The synapse is a model system for exocytosis
25.10 Protein localization depends on specific signals
25.11 ER proteins are retrieved from the Golgi
25.12 Brefeldin A reveals retrograde transport
25.13 Receptors recycle via endocytosis
25.14 Internalization signals are short and contain tyrosine
Sorting signal is a motif in a protein
(either a short sequence of amino acids
or a covalent modification) that is
required for it to be incorporated into
vesicles that carry it to a specific
destination.
25.1 Introduction
Figure 25.1 Proteins that enter
the endoplasmic reticulum are
transported to the Golgi and
towards the plasma membrane,
Specific signals cause proteins
to be returned from the Golgi
to the ER,to be retained in the
Golgi,to be retained in the
plasma membrane,or to be
transported to endosomes and
lysosomes,Proteins may be
transported between the plasma
membrane and endosomes.
25.1 Introduction
Figure 25.2 Vesicles are
released when they bud from
a donor compartment and
are surrounded by coat
proteins (left),During fusion,
the coated vesicle binds to a
target compartment,is
uncoated,and fuses with the
target membrane,releasing
its contents (right).
25.1 Introduction
Figure 25.3 An
oligosaccharide is
formed on dolichol and
transferred by glycosyl
transferase to asparagine
of a target protein.
25.2 Oligosaccharides
are added to proteins in
the ER and Golgi
Figure 25.4 Sugars are
removed in the ER in a fixed
order,initially comprising 3
glucose and 1-4 mannose
residues,This trimming
generates a high mannose
oligosaccharide.
25.2 Oligosaccharides are
added to proteins in the
ER and Golgi
Figure 25.5 Processing for a complex
oligosaccharide occurs in the Golgi and
trims the original preformed unit to the
inner core consisting of 2 N-acetyl-
glucosamine and 3 mannose residues,
Then further sugars can be added,in the
order in which the transfer enzymes are
encountered,to generate a terminal
region containing N-acetyl-glucosamine,
galactose,and sialic acid.
25.2 Oligosaccharides are
added to proteins in the ER
and Golgi
Figure 25.6 The Golgi
apparatus consists of a
series of individual
membrane stacks,
Photograph kindly
provided by Alain
Rambourg.
25.2 Oligosaccharides
are added to proteins
in the ER and Golgi
Figure 25.7 A Golgi
stack consists of a series
of cisternae,organized
with cis to trans polarity,
Protein modifications
occur in order as a
protein moves from the
cis face to the trans face.
25.2 Oligosaccharides are added to
proteins in the ER and Golgi
Coated vesicles are vesicles whose membrane has on its surface a
layer of a protein such as clathrin,cop-I or COP-II.
Endocytosis is process by which proteins at the surface of the cell
are internalized,being transported into the cell within membranous
vesicles.
Exocytosis is the process of secreting proteins from a cell into the
medium,by transport in membranous vesicles from the endoplasmic
reticulum,through the Golgi,to storage vesicles,and finally (upon a
regulatory signal) through the plasma membrane.
Retrograde transport describes movement of proteins in the reverse
direction in the reticuloendothelial system,typically from Golgi to
endoplasmic reticulum.
25.3 Coated vesicles transport both
exported and imported proteins
Figure 25.8 Proteins are transported in coated vesicles,Constitutive (bulk flow)
transport from ER through the Golgi takes place by COP-coated vesicles,
Clathrin-coated vesicles are used for both regulated exocytosis and endocytosis.
25.3 Coated vesicles transport both
exported and imported proteins
Figure 25.9 Coated
vesicles are released
from the trans face of
the Golgi,The diameter
of a vesicle is ~70 nm,
Photograph kindly
provided by Lelio Orci.
25.3 Coated vesicles transport both
exported and imported proteins
Figure 25.2 Vesicles are
released when they bud from
a donor compartment and are
surrounded by coat proteins
(left),During fusion,the
coated vesicle binds to a
target compartment,is
uncoated,and fuses with the
target membrane,releasing
its contents (right).
25.3 Coated vesicles transport both
exported and imported proteins
Figure 25.10 Vesicle
formation results when coat
proteins bind to a membrane,
deform it,and ultimately
surround a membrane vesicle
that is pinched off.
25.3 Coated vesicles
transport both exported
and imported proteins
Endocytic vesicles are membranous
particles that transport proteins
through endocytosis; also known as
clathrin-coated vesicles.
25.4 Different types of coated
vesicles exist in each pathway
Figure 25.11 Coated
vesicles have a polyhedral
lattice on the surface,
created by triskelions of
clathrin,Photograph
kindly provided by Tom
Kirchhausen.
25.4 Different types
of coated vesicles
exist in each pathway
Figure 25.12 Clathrin-
coated vesicles have a coat
consisting of two layers,the
outer layer is formed by
clathrin,and the inner layer
is formed by adaptors,
which lie between clathrin
and the integral membrane
proteins.
25.4 Different types
of coated vesicles
exist in each pathway
Are coated vesicles responsible for all
transport between membranous systems?
There are conflicting models for the
nature of forward transport from the ER,
through Golgi cisternae,and then from the
TGN to the plasma membrane.
25.5 An alternative model for protein transport
Figure 25.13 ARF and
coatomer are sufficient
for the budding of
COP-I-coated vesicles.
25.6 Budding and
fusion reactions
Figure 25.14 Vesicle
uncoating is triggered
by hydrolysis of GTP
bound to ARF.
25.6 Budding and
fusion reactions
Figure 25.15 Specificity for docking
is provided by SNAREs,The v-
SNARE carried by the vesicle binds
to the t-SNARE on the plasma
membrane to form a SNAREpin,
NSF and SNAP remain bound to the
far end of the SNAREpin during
fusion,After fusion,ATP is
hydrolyzed and NSF and SNAP
dissociate to release the SNAREs.
25.6 Budding and
fusion reactions
Figure 25.16 A
SNAREpin forms
by a 4-helix bundle,
Photograph kindly
provided by Axel
Brunger.
25.6 Budding and
fusion reactions
Figure 25.17 A SNAREpin complex protrudes parallel to the plane of
the membrane,An electron micrograph of the complex is superimposed
on the model,Photograph kindly provided by James Rothman.
25.6 Budding
and fusion
reactions
Figure 25.18 Neurotransmitters
are released from a donor
(presynaptic) cell when an
impulse causes exocytosis,
Synaptic (coated) vesicles fuse
with the plasma membrane,and
release their contents into the
extracellular fluid.
25.6 Budding and
fusion reactions
Figure 25.19 The kiss and run model proposes that a
synaptic vesicle touches the plasma membrane transiently,
releases its contents through a pore,and then reforms.
25.6
Budding
and
fusion
reactions
Figure 25.20 When synaptic
vesicles fuse with the plasma
membrane,their components
are retrieved by endocytosis of
clathrin-coated vesicles.
25.6 Budding and
fusion reactions
Figure 25.21 Rab
proteins affect
particular stages of
vesicular transport.
25.6 Budding and
fusion reactions
Lysosomes are small bodies,enclosed
by membranes,that contain hydrolytic
enzymes in eukaryotic cells.
25.7 Protein localization depends on further signals
Figure 25.22 A
transport signal in a
trans- membrane cargo
protein interacts with
an adaptor protein.
25.7 Protein
localization depends
on further signals
Figure 25.23 A
transport signal
in a luminal
cargo protein
interacts with a
transmembrane
receptor that
interacts with an
adaptor protein.
25.7 Protein localization depends on further signals
Figure 25.5 Processing for a complex
oligosaccharide occurs in the Golgi and
trims the original preformed unit to the
inner core consisting of 2 N-acetyl-
glucosamine and 3 mannose residues,
Then further sugars can be added,in
the order in which the transfer enzymes
are encountered,to generate a terminal
region containing N-acetyl-
glucosamine,galactose,and sialic acid.
25.7 Protein localization
depends on further signals
Figure 25.24 An (artificial)
protein containing both lysosome
and ER-targeting signals reveals a
pathway for ER-localization,The
protein becomes exposed to the
first but not to the second of the
enzymes that generates mannose-
6-phosphate in the Golgi,after
which the KDEL sequence causes
it to be returned to the ER.
25.8 ER proteins
are retrieved from
the Golgi
Figure 25.24 An (artificial)
protein containing both lysosome
and ER-targeting signals reveals a
pathway for ER-localization,The
protein becomes exposed to the
first but not to the second of the
enzymes that generates mannose-
6-phosphate in the Golgi,after
which the KDEL sequence causes
it to be returned to the ER.
25.8 ER proteins
are retrieved
from the Golgi
Figure 25.25 Endosomes sort proteins
that have been endocytosed and provide
one route to the lysosome,Proteins are
transported via clathrin-coated vesicles
from the plasma membrane to the early
endosome,and may then either return
to the plasma membrane or proceed
further to late endosomes and
lysosomes,Newly synthesized proteins
may be directed to late endosomes (and
then to lysosomes) from the Golgi
stacks,The common signal in
lysosomal targeting is the recognition
of mannose-6-phosphate by a specific
receptor.
25.9 Receptors recycle
via endocytosis
Figure 25.25 Endosomes sort proteins
that have been endocytosed and provide
one route to the lysosome,Proteins are
transported via clathrin-coated vesicles
from the plasma membrane to the early
endosome,and may then either return
to the plasma membrane or proceed
further to late endosomes and
lysosomes,Newly synthesized proteins
may be directed to late endosomes (and
then to lysosomes) from the Golgi
stacks,The common signal in
lysosomal targeting is the recognition
of mannose-6-phosphate by a specific
receptor.
25.9 Receptors recycle
via endocytosis
Figure 25.26 LDL receptor
transports apo-B (and apo-E)
into endosomes,where
receptor and ligand separate,
The receptor recycles to the
surface,apo-B (or apo-E)
continues to the lysosome
and is degraded,and
cholesterol is released.
25.9 Receptors recycle via
endocytosis
Figure 25.27 Transferrin
receptor bound to
transferrin carrying iron
releases the iron in the
endosome; the receptor
now bound to apo-
transferrin (lacking iron)
recycles to the surface,
where receptor and ligand
dissociate.
25.9 Receptors recycle
via endocytosis
Figure 25.28 EGF receptor
carries EGF to the lysosome
where both the receptor and
ligand are degraded.
25.9 Receptors recycle via
endocytosis
Figure 25.29 Ig receptor
transports immunoglobulin
across the cell from one
surface to the other.
25.9 Receptors recycle
via endocytosis
Figure 25.12 Clathrin-coated
vesicles have a coat
consisting of two layers,the
outer layer is formed by
clathrin,and the inner layer is
formed by adaptors,which lie
between clathrin and the
integral membrane proteins.
25.9 Receptors recycle via endocytosis
Figure 25.30 The
cytoplasmic
domain of an
internalized
receptor interacts
with proteins of
the inner layer of
a coated pit.
25.9 Receptors recycle via endocytosis
1,Proteins that reside within the reticuloendothelial system or that are
secreted from the plasma membrane enter the ER by cotranslational
transfer directly from the ribosome,
2,Proteins are transported between membranous surfaces as cargoes
in membrane-bound coated vesicles,
3,Modification of proteins by addition of a preformed oligosaccharide
starts in the endoplasmic reticulum,
4,Different types of vesicles are responsible for transport to and from
different membrane systems,
5,COP-I-coated vesicles are responsible for retrograde transport from
the Golgi to the ER,
6,COP-II vesicles undertake forward movement from the ER to Golgi,
25.10 Summary
7,In the pathway for regulated secretion of proteins,proteins are
sorted into clathrin-coated vesicles at the Golgi trans face,
8,Budding and fusion of all types of vesicles is controlled by a small
GTP-binding protein,
9,Vesicles recognize appropriate target membranes because a
vSNARE on the vesicle pairs specifically with a tSNARE on the target
membrane,
10,Receptors may be internalized either continuously or as the result
of binding to an extracellular ligand,
11,The acid environment of the endosome causes some receptors to
release their ligands; the ligand are carried to lysosomes,where they
are degraded,and the receptors are recycled back to the plasma
membrane by means of coated vesicles,
25.10 Summary
Protein trafficking
25.1 Introduction
25.2 Oligosaccharides are added to proteins in the ER and Golgi
25.3 The Golgi stacks are polarized
25.4 Coated vesicles transport both exported and imported proteins
25.5 Different types of coated vesicles exist in each pathway
25.6 Cisternal progression occurs more slowly than vesicle movement
25.7 Vesicles can bud and fuse with membranes
25.8 SNAREs control targeting
25.9 The synapse is a model system for exocytosis
25.10 Protein localization depends on specific signals
25.11 ER proteins are retrieved from the Golgi
25.12 Brefeldin A reveals retrograde transport
25.13 Receptors recycle via endocytosis
25.14 Internalization signals are short and contain tyrosine
Sorting signal is a motif in a protein
(either a short sequence of amino acids
or a covalent modification) that is
required for it to be incorporated into
vesicles that carry it to a specific
destination.
25.1 Introduction
Figure 25.1 Proteins that enter
the endoplasmic reticulum are
transported to the Golgi and
towards the plasma membrane,
Specific signals cause proteins
to be returned from the Golgi
to the ER,to be retained in the
Golgi,to be retained in the
plasma membrane,or to be
transported to endosomes and
lysosomes,Proteins may be
transported between the plasma
membrane and endosomes.
25.1 Introduction
Figure 25.2 Vesicles are
released when they bud from
a donor compartment and
are surrounded by coat
proteins (left),During fusion,
the coated vesicle binds to a
target compartment,is
uncoated,and fuses with the
target membrane,releasing
its contents (right).
25.1 Introduction
Figure 25.3 An
oligosaccharide is
formed on dolichol and
transferred by glycosyl
transferase to asparagine
of a target protein.
25.2 Oligosaccharides
are added to proteins in
the ER and Golgi
Figure 25.4 Sugars are
removed in the ER in a fixed
order,initially comprising 3
glucose and 1-4 mannose
residues,This trimming
generates a high mannose
oligosaccharide.
25.2 Oligosaccharides are
added to proteins in the
ER and Golgi
Figure 25.5 Processing for a complex
oligosaccharide occurs in the Golgi and
trims the original preformed unit to the
inner core consisting of 2 N-acetyl-
glucosamine and 3 mannose residues,
Then further sugars can be added,in the
order in which the transfer enzymes are
encountered,to generate a terminal
region containing N-acetyl-glucosamine,
galactose,and sialic acid.
25.2 Oligosaccharides are
added to proteins in the ER
and Golgi
Figure 25.6 The Golgi
apparatus consists of a
series of individual
membrane stacks,
Photograph kindly
provided by Alain
Rambourg.
25.2 Oligosaccharides
are added to proteins
in the ER and Golgi
Figure 25.7 A Golgi
stack consists of a series
of cisternae,organized
with cis to trans polarity,
Protein modifications
occur in order as a
protein moves from the
cis face to the trans face.
25.2 Oligosaccharides are added to
proteins in the ER and Golgi
Coated vesicles are vesicles whose membrane has on its surface a
layer of a protein such as clathrin,cop-I or COP-II.
Endocytosis is process by which proteins at the surface of the cell
are internalized,being transported into the cell within membranous
vesicles.
Exocytosis is the process of secreting proteins from a cell into the
medium,by transport in membranous vesicles from the endoplasmic
reticulum,through the Golgi,to storage vesicles,and finally (upon a
regulatory signal) through the plasma membrane.
Retrograde transport describes movement of proteins in the reverse
direction in the reticuloendothelial system,typically from Golgi to
endoplasmic reticulum.
25.3 Coated vesicles transport both
exported and imported proteins
Figure 25.8 Proteins are transported in coated vesicles,Constitutive (bulk flow)
transport from ER through the Golgi takes place by COP-coated vesicles,
Clathrin-coated vesicles are used for both regulated exocytosis and endocytosis.
25.3 Coated vesicles transport both
exported and imported proteins
Figure 25.9 Coated
vesicles are released
from the trans face of
the Golgi,The diameter
of a vesicle is ~70 nm,
Photograph kindly
provided by Lelio Orci.
25.3 Coated vesicles transport both
exported and imported proteins
Figure 25.2 Vesicles are
released when they bud from
a donor compartment and are
surrounded by coat proteins
(left),During fusion,the
coated vesicle binds to a
target compartment,is
uncoated,and fuses with the
target membrane,releasing
its contents (right).
25.3 Coated vesicles transport both
exported and imported proteins
Figure 25.10 Vesicle
formation results when coat
proteins bind to a membrane,
deform it,and ultimately
surround a membrane vesicle
that is pinched off.
25.3 Coated vesicles
transport both exported
and imported proteins
Endocytic vesicles are membranous
particles that transport proteins
through endocytosis; also known as
clathrin-coated vesicles.
25.4 Different types of coated
vesicles exist in each pathway
Figure 25.11 Coated
vesicles have a polyhedral
lattice on the surface,
created by triskelions of
clathrin,Photograph
kindly provided by Tom
Kirchhausen.
25.4 Different types
of coated vesicles
exist in each pathway
Figure 25.12 Clathrin-
coated vesicles have a coat
consisting of two layers,the
outer layer is formed by
clathrin,and the inner layer
is formed by adaptors,
which lie between clathrin
and the integral membrane
proteins.
25.4 Different types
of coated vesicles
exist in each pathway
Are coated vesicles responsible for all
transport between membranous systems?
There are conflicting models for the
nature of forward transport from the ER,
through Golgi cisternae,and then from the
TGN to the plasma membrane.
25.5 An alternative model for protein transport
Figure 25.13 ARF and
coatomer are sufficient
for the budding of
COP-I-coated vesicles.
25.6 Budding and
fusion reactions
Figure 25.14 Vesicle
uncoating is triggered
by hydrolysis of GTP
bound to ARF.
25.6 Budding and
fusion reactions
Figure 25.15 Specificity for docking
is provided by SNAREs,The v-
SNARE carried by the vesicle binds
to the t-SNARE on the plasma
membrane to form a SNAREpin,
NSF and SNAP remain bound to the
far end of the SNAREpin during
fusion,After fusion,ATP is
hydrolyzed and NSF and SNAP
dissociate to release the SNAREs.
25.6 Budding and
fusion reactions
Figure 25.16 A
SNAREpin forms
by a 4-helix bundle,
Photograph kindly
provided by Axel
Brunger.
25.6 Budding and
fusion reactions
Figure 25.17 A SNAREpin complex protrudes parallel to the plane of
the membrane,An electron micrograph of the complex is superimposed
on the model,Photograph kindly provided by James Rothman.
25.6 Budding
and fusion
reactions
Figure 25.18 Neurotransmitters
are released from a donor
(presynaptic) cell when an
impulse causes exocytosis,
Synaptic (coated) vesicles fuse
with the plasma membrane,and
release their contents into the
extracellular fluid.
25.6 Budding and
fusion reactions
Figure 25.19 The kiss and run model proposes that a
synaptic vesicle touches the plasma membrane transiently,
releases its contents through a pore,and then reforms.
25.6
Budding
and
fusion
reactions
Figure 25.20 When synaptic
vesicles fuse with the plasma
membrane,their components
are retrieved by endocytosis of
clathrin-coated vesicles.
25.6 Budding and
fusion reactions
Figure 25.21 Rab
proteins affect
particular stages of
vesicular transport.
25.6 Budding and
fusion reactions
Lysosomes are small bodies,enclosed
by membranes,that contain hydrolytic
enzymes in eukaryotic cells.
25.7 Protein localization depends on further signals
Figure 25.22 A
transport signal in a
trans- membrane cargo
protein interacts with
an adaptor protein.
25.7 Protein
localization depends
on further signals
Figure 25.23 A
transport signal
in a luminal
cargo protein
interacts with a
transmembrane
receptor that
interacts with an
adaptor protein.
25.7 Protein localization depends on further signals
Figure 25.5 Processing for a complex
oligosaccharide occurs in the Golgi and
trims the original preformed unit to the
inner core consisting of 2 N-acetyl-
glucosamine and 3 mannose residues,
Then further sugars can be added,in
the order in which the transfer enzymes
are encountered,to generate a terminal
region containing N-acetyl-
glucosamine,galactose,and sialic acid.
25.7 Protein localization
depends on further signals
Figure 25.24 An (artificial)
protein containing both lysosome
and ER-targeting signals reveals a
pathway for ER-localization,The
protein becomes exposed to the
first but not to the second of the
enzymes that generates mannose-
6-phosphate in the Golgi,after
which the KDEL sequence causes
it to be returned to the ER.
25.8 ER proteins
are retrieved from
the Golgi
Figure 25.24 An (artificial)
protein containing both lysosome
and ER-targeting signals reveals a
pathway for ER-localization,The
protein becomes exposed to the
first but not to the second of the
enzymes that generates mannose-
6-phosphate in the Golgi,after
which the KDEL sequence causes
it to be returned to the ER.
25.8 ER proteins
are retrieved
from the Golgi
Figure 25.25 Endosomes sort proteins
that have been endocytosed and provide
one route to the lysosome,Proteins are
transported via clathrin-coated vesicles
from the plasma membrane to the early
endosome,and may then either return
to the plasma membrane or proceed
further to late endosomes and
lysosomes,Newly synthesized proteins
may be directed to late endosomes (and
then to lysosomes) from the Golgi
stacks,The common signal in
lysosomal targeting is the recognition
of mannose-6-phosphate by a specific
receptor.
25.9 Receptors recycle
via endocytosis
Figure 25.25 Endosomes sort proteins
that have been endocytosed and provide
one route to the lysosome,Proteins are
transported via clathrin-coated vesicles
from the plasma membrane to the early
endosome,and may then either return
to the plasma membrane or proceed
further to late endosomes and
lysosomes,Newly synthesized proteins
may be directed to late endosomes (and
then to lysosomes) from the Golgi
stacks,The common signal in
lysosomal targeting is the recognition
of mannose-6-phosphate by a specific
receptor.
25.9 Receptors recycle
via endocytosis
Figure 25.26 LDL receptor
transports apo-B (and apo-E)
into endosomes,where
receptor and ligand separate,
The receptor recycles to the
surface,apo-B (or apo-E)
continues to the lysosome
and is degraded,and
cholesterol is released.
25.9 Receptors recycle via
endocytosis
Figure 25.27 Transferrin
receptor bound to
transferrin carrying iron
releases the iron in the
endosome; the receptor
now bound to apo-
transferrin (lacking iron)
recycles to the surface,
where receptor and ligand
dissociate.
25.9 Receptors recycle
via endocytosis
Figure 25.28 EGF receptor
carries EGF to the lysosome
where both the receptor and
ligand are degraded.
25.9 Receptors recycle via
endocytosis
Figure 25.29 Ig receptor
transports immunoglobulin
across the cell from one
surface to the other.
25.9 Receptors recycle
via endocytosis
Figure 25.12 Clathrin-coated
vesicles have a coat
consisting of two layers,the
outer layer is formed by
clathrin,and the inner layer is
formed by adaptors,which lie
between clathrin and the
integral membrane proteins.
25.9 Receptors recycle via endocytosis
Figure 25.30 The
cytoplasmic
domain of an
internalized
receptor interacts
with proteins of
the inner layer of
a coated pit.
25.9 Receptors recycle via endocytosis
1,Proteins that reside within the reticuloendothelial system or that are
secreted from the plasma membrane enter the ER by cotranslational
transfer directly from the ribosome,
2,Proteins are transported between membranous surfaces as cargoes
in membrane-bound coated vesicles,
3,Modification of proteins by addition of a preformed oligosaccharide
starts in the endoplasmic reticulum,
4,Different types of vesicles are responsible for transport to and from
different membrane systems,
5,COP-I-coated vesicles are responsible for retrograde transport from
the Golgi to the ER,
6,COP-II vesicles undertake forward movement from the ER to Golgi,
25.10 Summary
7,In the pathway for regulated secretion of proteins,proteins are
sorted into clathrin-coated vesicles at the Golgi trans face,
8,Budding and fusion of all types of vesicles is controlled by a small
GTP-binding protein,
9,Vesicles recognize appropriate target membranes because a
vSNARE on the vesicle pairs specifically with a tSNARE on the target
membrane,
10,Receptors may be internalized either continuously or as the result
of binding to an extracellular ligand,
11,The acid environment of the endosome causes some receptors to
release their ligands; the ligand are carried to lysosomes,where they
are degraded,and the receptors are recycled back to the plasma
membrane by means of coated vesicles,
25.10 Summary