Chapter 13: Intracellular Membrane Traffic
exocytosis – transport vesicle fuses with the plasma membrane
endocytosis – plasma membrane patch is internalised, forming a transport vesicle –
content derives from extracellular space
o endosomes = internal
compartments
lumen = interior space of each membrane-
enclosed compartment
o proteins can travel in this space
without having to cross a membrane
transport vesicles = forms of containers
o bud off from one membrane and
fuse with another, carrying membrane components and soluble luminal
molecules, which are referred to as cargo
o donor compartment and target compartment
o must be selective
secretory pathway – ER Golgi apparatus
endocytic pathway – leads inward from the plasma membrane
retrieval transport transport in opposite direction; return to ER
The molecular mechanisms of membrane transport and the maintenance of
compartmental diversity
coated vesicles specialized, coated regions of membranes; have a distinctive
cage of proteins covering their cytosolic surface
o 1) concentrates specific membrane proteins in a specialised patch, which then
gives rise to the vesicle membrane
o 2) shapes vesicle
Three well-characterised types of
coated vesicles, distinguished by
their major coat proteins:
o 1) clathrin-coated –
mediate transport form the
Golgi apparatus and from
the plasma membrane
o 2) COPI-coated – mediate
transport from the ER and
from the Golgi cisternae
bud off from Golgi
apparatus
o 3) COPII-coated – mediate transport from the ER and from the Golgi cisternae
bud off from ER
Adaptor proteins select cargo into clathrin-coated vesicles
Transport from the ER through the Golgi apparatus
Golgi apparatus = Golgi complex major site of carbohydrate synthesis, as well
as a sorting and dispatching station for products of the ER
Proteins leave the ER at ER exit sites -whose membrane lacks bound ribosomes
o Proteins leave the ER in COPII-coated transport vesicles
Only proteins that are properly folded and assembled can leave the ER
o otherwise: chaperones / proteasomes
, Vesicular tubular clusters mediate
transport from the ER to the Golgi
apparatus
o heterotypic fusion
o homotypic fusion
retrieval pathway carrying back ER
resident proteins that have escaped,
as well as proteins such as cargo
receptors and SNAREs that
participated in the ER budding and
vesicle fusion reactions
o uses ER retrieval signals
The retrieval pathway to the ER uses sorting signals: KDEL
o on C-terminus!
o Lys-Asp-Glu-Leu
o KDEL receptor travel between ER and Golgi apparatus
can interact both with COPII coat and COPI coat
has high-affinity for KDEL sequence in vesicular tubular clusters and
the Golgi apparatus
has low-affinity for KDEL sequence in ER
Golgi apparatus
o cisternae membrane-
enclosed compartments
o cis face = entry face cis
Golgi network (CGN)
o trans face = exit face
trans Golgi network (TGN)
o N-linked oligosaccharide
serve to help proteins fold
and to help transport
misfolded proteins to the
cytosol for degradation in
proteasomes
controlling quality
of proteins exiting from the ER
are processes in the Golgi apparatus
Two broad classes of N-linked oligosaccharides:
o complex oligosaccharides
exocytosis – transport vesicle fuses with the plasma membrane
endocytosis – plasma membrane patch is internalised, forming a transport vesicle –
content derives from extracellular space
o endosomes = internal
compartments
lumen = interior space of each membrane-
enclosed compartment
o proteins can travel in this space
without having to cross a membrane
transport vesicles = forms of containers
o bud off from one membrane and
fuse with another, carrying membrane components and soluble luminal
molecules, which are referred to as cargo
o donor compartment and target compartment
o must be selective
secretory pathway – ER Golgi apparatus
endocytic pathway – leads inward from the plasma membrane
retrieval transport transport in opposite direction; return to ER
The molecular mechanisms of membrane transport and the maintenance of
compartmental diversity
coated vesicles specialized, coated regions of membranes; have a distinctive
cage of proteins covering their cytosolic surface
o 1) concentrates specific membrane proteins in a specialised patch, which then
gives rise to the vesicle membrane
o 2) shapes vesicle
Three well-characterised types of
coated vesicles, distinguished by
their major coat proteins:
o 1) clathrin-coated –
mediate transport form the
Golgi apparatus and from
the plasma membrane
o 2) COPI-coated – mediate
transport from the ER and
from the Golgi cisternae
bud off from Golgi
apparatus
o 3) COPII-coated – mediate transport from the ER and from the Golgi cisternae
bud off from ER
Adaptor proteins select cargo into clathrin-coated vesicles
Transport from the ER through the Golgi apparatus
Golgi apparatus = Golgi complex major site of carbohydrate synthesis, as well
as a sorting and dispatching station for products of the ER
Proteins leave the ER at ER exit sites -whose membrane lacks bound ribosomes
o Proteins leave the ER in COPII-coated transport vesicles
Only proteins that are properly folded and assembled can leave the ER
o otherwise: chaperones / proteasomes
, Vesicular tubular clusters mediate
transport from the ER to the Golgi
apparatus
o heterotypic fusion
o homotypic fusion
retrieval pathway carrying back ER
resident proteins that have escaped,
as well as proteins such as cargo
receptors and SNAREs that
participated in the ER budding and
vesicle fusion reactions
o uses ER retrieval signals
The retrieval pathway to the ER uses sorting signals: KDEL
o on C-terminus!
o Lys-Asp-Glu-Leu
o KDEL receptor travel between ER and Golgi apparatus
can interact both with COPII coat and COPI coat
has high-affinity for KDEL sequence in vesicular tubular clusters and
the Golgi apparatus
has low-affinity for KDEL sequence in ER
Golgi apparatus
o cisternae membrane-
enclosed compartments
o cis face = entry face cis
Golgi network (CGN)
o trans face = exit face
trans Golgi network (TGN)
o N-linked oligosaccharide
serve to help proteins fold
and to help transport
misfolded proteins to the
cytosol for degradation in
proteasomes
controlling quality
of proteins exiting from the ER
are processes in the Golgi apparatus
Two broad classes of N-linked oligosaccharides:
o complex oligosaccharides
