Biogenesis of Organelles (Part 2) – Membrane Trafficking
Beyond the ER – Proteins destined for various locations begin at the ER
Transport from the ER to the Golgi
apparatus and to other compartments of the
endomembrane system carried out by the
continual budding and fusion of transport
vesicles
Transport pathways mediated by these
vesicles extend outward from the ER to the
plasma membrane, and inward from the
plasma membrane to lysosomes – providing Secretion and the Secretory Pathway
routes of communication between the Endocytosis and the Endocytotic Pathway
interior of the cell and its surroundings Retrograde Transport/Recycling
As proteins and lipids are transported
outward along these pathways, many undergo
various types of chemical modification: addition of carbohydrate side chains (lipids
and proteins) and formation of disulphide bonds (polypeptides) – stabilise protein
structure
Vesicles that bud from membranes usually have a
distinctive protein coat on cytosolic surface –
budding from its parent organelle, vesicles sheds
its coat, allowing membrane to interact directly
with the membrane to which it will fuse
Coat serves 2 functions: shapes the membrane into a bud + helps capture
molecules for onward transport
The GTPases ARF and Sar1 control
coat recruitment
They also control disassembly of
COPI and II coated vesicles
Uncoating of clathrin-coated
vesicles requires an Hsp70 family
ATPase
,- Best studied vesicles have coats largely coated
with clathrin protein – other kinds of transport
vesicles form in a similar way, carrying own
characteristic set of molecules
- Clathrin coated vesicles bud from the Golgi
apparatus on the outward secretory pathway and
from the plasma membrane on the inward
endocytic pathway (starting off an a clathrin-coated pit)
- Clathrin molecules assemble into a basketlike network on
the cytosolic surface of the membrane – assembly
process that starts shaping membrane into a vesicle
- Small GTP-binding protein = dynamin assembles as a ring
around the neck of each deeply invaginated coated pit –
together with other proteins recruited to the neck of the
vesicle, the dynamin causes the ring to constrict, pinching
off the vesicle
Experimental Methods
- D. melanogaster heat-sensitive mutant of dynamin = shibire
Shibire flies become paralysed as temperature increased
- Clathrin has no role in capturing specific
molecules for transport – this function is reserved for
a second class of coat proteins adaptins which
both secure the clathrin coat to the vesicle
membrane and aid select cargo molecules for
transport
- Molecules for onward transport carry
specific transport signals and are recognised by cargo receptors in compartment
membrane
- Adaptins help capture specific cargo molecules by trapping cargo receptors that
bind them – in a way, selected set of cargo molecules bound to their specific
, receptors is incorporated into lumen of each newly formed
clathrin-coated vesicle
Adaptins have different types: adaptins that bind to the
cargo receptors of Golgi are not the same as those on the
plasma membrane Adaptor Complexes (Clathrin
Adaptors)
- Tetrameric: made up of 2
adaptins (one medium chain
Not all transport requires vesicles – some better and one small)
- Have “ear domains”
described as “direct fusion”
Vesicle Docking depends on Tethers and SNAREs
After a transport vesicle buds from a membrane, it must travel to its correct
destination and deliver its contents – in most cases, vesicle is actively transported by
motor proteins that move along cytoskeletal fibres
Once a transport vesicle has reached its target, it must recognise and dock with the
organelle, then the vesicle and fuse with the target membrane
2 types of membrane fusion: Homotypic fusion
(fusion of two similar membranes) + Heterotypic
fusion (fusion of two different membranes)
The specificity of vesicular transport involves
the recognition of molecular markers on the
vesicle by the complementary receptors of the
target membrane
1. Identification process depends on a family of proteins = Rab proteins
Beyond the ER – Proteins destined for various locations begin at the ER
Transport from the ER to the Golgi
apparatus and to other compartments of the
endomembrane system carried out by the
continual budding and fusion of transport
vesicles
Transport pathways mediated by these
vesicles extend outward from the ER to the
plasma membrane, and inward from the
plasma membrane to lysosomes – providing Secretion and the Secretory Pathway
routes of communication between the Endocytosis and the Endocytotic Pathway
interior of the cell and its surroundings Retrograde Transport/Recycling
As proteins and lipids are transported
outward along these pathways, many undergo
various types of chemical modification: addition of carbohydrate side chains (lipids
and proteins) and formation of disulphide bonds (polypeptides) – stabilise protein
structure
Vesicles that bud from membranes usually have a
distinctive protein coat on cytosolic surface –
budding from its parent organelle, vesicles sheds
its coat, allowing membrane to interact directly
with the membrane to which it will fuse
Coat serves 2 functions: shapes the membrane into a bud + helps capture
molecules for onward transport
The GTPases ARF and Sar1 control
coat recruitment
They also control disassembly of
COPI and II coated vesicles
Uncoating of clathrin-coated
vesicles requires an Hsp70 family
ATPase
,- Best studied vesicles have coats largely coated
with clathrin protein – other kinds of transport
vesicles form in a similar way, carrying own
characteristic set of molecules
- Clathrin coated vesicles bud from the Golgi
apparatus on the outward secretory pathway and
from the plasma membrane on the inward
endocytic pathway (starting off an a clathrin-coated pit)
- Clathrin molecules assemble into a basketlike network on
the cytosolic surface of the membrane – assembly
process that starts shaping membrane into a vesicle
- Small GTP-binding protein = dynamin assembles as a ring
around the neck of each deeply invaginated coated pit –
together with other proteins recruited to the neck of the
vesicle, the dynamin causes the ring to constrict, pinching
off the vesicle
Experimental Methods
- D. melanogaster heat-sensitive mutant of dynamin = shibire
Shibire flies become paralysed as temperature increased
- Clathrin has no role in capturing specific
molecules for transport – this function is reserved for
a second class of coat proteins adaptins which
both secure the clathrin coat to the vesicle
membrane and aid select cargo molecules for
transport
- Molecules for onward transport carry
specific transport signals and are recognised by cargo receptors in compartment
membrane
- Adaptins help capture specific cargo molecules by trapping cargo receptors that
bind them – in a way, selected set of cargo molecules bound to their specific
, receptors is incorporated into lumen of each newly formed
clathrin-coated vesicle
Adaptins have different types: adaptins that bind to the
cargo receptors of Golgi are not the same as those on the
plasma membrane Adaptor Complexes (Clathrin
Adaptors)
- Tetrameric: made up of 2
adaptins (one medium chain
Not all transport requires vesicles – some better and one small)
- Have “ear domains”
described as “direct fusion”
Vesicle Docking depends on Tethers and SNAREs
After a transport vesicle buds from a membrane, it must travel to its correct
destination and deliver its contents – in most cases, vesicle is actively transported by
motor proteins that move along cytoskeletal fibres
Once a transport vesicle has reached its target, it must recognise and dock with the
organelle, then the vesicle and fuse with the target membrane
2 types of membrane fusion: Homotypic fusion
(fusion of two similar membranes) + Heterotypic
fusion (fusion of two different membranes)
The specificity of vesicular transport involves
the recognition of molecular markers on the
vesicle by the complementary receptors of the
target membrane
1. Identification process depends on a family of proteins = Rab proteins
