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Summary Intracellular membrane traffic
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Intracellular membrane traffic(BOOK)
Chapter 13 pp 695-742
Intracellular membrane traffic
Exocytosis is the secretory pathway that delivers newly formed molecules to the outside and
endocytosis is the process in which cells remove plasma membrane components and deliver them to
the internal compartments called endosomes.
Cargo are the membrane components and soluble lumenal molecules that the transport vesicles
carry.
The molecular mechanisms of membrane transport and the
maintenance of compartmental diversity
The composition of the enclosing membrane is important for defining the character of a
compartment in the cell.
Coated vesicles
Most transport vesicles form specialised coated regions of membranes. These vesicles are called
coated vesicles and they have a distinctive cage of proteins covering their cytosolic surface. The coat
has two main functions, it concentrates specific membrane proteins in a specialised patch and they
deform the membrane patch which shapes the vesicle.
There are three types of coated vesicles: clathrin-coated, COPI-coated and COPII-coated.
Clathrin-coated vesicles transport material from the plasma membrane and between endosomal and
Golgi compartments. COPI- and COPII-coated vesicles transport material in de secretory pathway,
COPI buds from Golgi, and COPII buds from ER.
Clathrin-coated
Clathrin is the major protein component of clathrin-
coated vesicles. Each clathrin subunit consists of three
large and three small polypeptide chains that form a
triskelion. These clathrin triskelions determine the
geometric of the clathrin cage.
, Adaptor proteins in clathrin-coated vesicles form a discrete inner layer of the coat. They bind the
clathrin coat to the membrane and trap various transmembrane proteins including cargo receptors.
There are several types of adaptor proteins, each specific for a different set of cargo receptors.
Phosphoinositides
Although inositol phospholipids are not abundant in a membrane, they have important regulatory
features. They can produce various types of phosphoinositides (phosphatidylinositol phosphates or
PIPs). The distribution of PIPs varies from organelle to organelle and often within a continuous
membrane from region to region, thereby defining specialised membrane domains.
Vesicles transformation
The forces generated by clathrin coat assembly are alone not enough to shape and pinch off a
vesicle. Membrane-bending proteins that contain crescent-shaped domains called BAR domains,
which bind to and impose their shape to the membrane.
As a clathrin coated bud grows, soluble cytoplasmic proteins (such as dynamin) assemble at the neck
of each bud. Together with dynamin, the other proteins help to bend the patch membrane. Once
released from the membrane, the vesicles loses its coat.
Coat assembly
Coat-recruitment GTPases control the assembly of clathrin coats on endosomes and the COPI and
COPII coats on Golgi and ER membranes. They include the ARF protein (assembly of COPI and
clathrin) and Sar1 protein (assembly of COPII coats). GTP binding proteins function in regulation by
acting as switches, which flip between an active state (GTP bound) and an inactive state (GDP
bound). This flipping is regulated by two proteins: guanine nulcleotide exchange factors (GEFs)
catalyzes the reaction from GDP —> GTP and GTPase-activating proteins (GAPs) trigger hydrolysis of
GTP —> GDP.
!!! NOTE that not all vesicles are spherical! In the ER there are packaging proteins which drive the
assembly of much larger COPII vesicles for oversized cargo and can form tubular COPII-coated
vesicles in this way. !!!
Rab proteins
Rab proteins and Rab effects direct the vesicles to
specific sports on the correct target membrane.
Rab proteins cycle between a membrane and the
cytosol and regulate the reversible assembly of
protein complexes on the membrane. In their
inactive state (GDP-bound) they are bound to
another protein called a Rab-GDP dissociation
inhibitor or GDI that keeps them soluble.
In their active state (GTP-bound), Rab-GEFs
activate Rab proteins on transport vesicles and
target membrane. In this state, they can bound to
Rab effectors on the target membrane and fuse.
A Rab domain can be disassembled and replaced by a different Rab domain, changing the identity of
an organelle, this is called an Rab cascade.
All cargo contained in the endosomes that has not been recycled to the plasma membrane becomes
part of a late endosome, this is called endosome maturation.
SNAREs
Rab interactors can also interact with SNARE proteins, or SNAREs to couple membrane tethering
fusion. SNAREs consist of a complementary set with a v-SNARE (on the vesicle) and a t-SNARE (on
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