COPII Vesicle formation
A) Inactive, soluble Sar1-GDP binds to a Sar1-GEF in the ERmembrane, causing
the Sar1 to release its GDP and bind GTP. A GTP-triggered conformational
change in Sar1 exposes an amphiphilic helix, which inserts into the cytoplasmic
leaflet of the ER membrane, initiating membrane bending.
B) GTP-bound Sar1 binds to a complex of two COPII adaptor coat proteins,
called Sec23 and Sec24, which form the inner coat. Sec24 has several different
binding sites for the cytosolic tails of cargo receptors. The entire surface of the
complex that attaches to the membrane is gently curved, matching the
diameter of COPII-coated vesicles.
C) A complex of two additional COPII coat proteins, called Sec13 and Sec 31,
forms the outer shell of the coat. Like clathrin, they can assemble on their own
into symmetrical cages with appropriate dimensions to enclose a COPII -coated
vesicle.
D) Membrane-bound, activeSar1-GTP recruits COPII adaptor proteins to the
membrane. They select certain transmembrane proteins and cause the
membrane to deform. The adaptor proteins then recruit the outer coat
proteins which help form a bud. A subsequent membrane fusion event pinches
off the coated vesicle. Other coated vesicles are thought to formin a similar
way.
COP-I Vesicle Formation
Clathrin Coat formation and trafficing.
The major protein component of clathrin-coated vesicles is clathrin itself,which
forms the outer layer of the coat. Clathrin triskelions assemble into a
basketlike framework of hexagons and pentagons to form coated pits (buds)
on the cytosolic surface of membranes.
The stages of vesicle formation-
1.Inititaion & Early invagination- A clathrin coat pit is formed and select cargo
specific adaptor protein, positioned between the clathrin cage and the
membrane.They bind the clathrin coat to the membrane and trap various
transmembrane proteins, including transmembrane receptors that capture
soluble cargo molecules inside the vesicle—so-called cargo receptors.
The adaptor protein AP2 serves as a well-understood example.
2. Late Invagination- Polymerise in hexagon &pentagon to form clathrin
coat.When it binds to a specific phosphorylated phosphatidylinositol lipid (a
phosphoinositide),it alters its conformation, exposing binding sites for cargo
receptors in the membrane.
Upon binding, they induce membrane curvature.
3. Constriction-Coat-recruitment GTPases, for example, control the assembly
of clathrin coats. Dynamin is recruited to the neck of vesicle,which have
GTPase activity to pinch off vesicle from membrane.
Remark-hsp70 chaperone protein (see Figure 6–80) functions as an uncoating
ATPase. Auxilin, another vesicle protein, is thought to activate the ATPase. The
release of the coat,
Local control-
PI and PIP kinases and PIP phosphatases can therefore be used to rapidly
control the binding of proteins to a membrane or membrane domain. The PIP-
binding proteins then help regulate vesicle formation and other steps in the
control of vesicle traffic.BAR-domain proteins are thought to help AP2 nucleate
clathrin-mediatedendocytosis by shaping the plasma membrane to allow a
clathrin-coated bud to form.
Clathrin Tri-skelion structure
The vesicles that bud from the trans golgi network have two layered coat, an
outer layer composed of fibrous protein called, Clathrin. Which have three
limbed shape triskelion structure.
Structure-
Each triskelion is composed of three clathrin heavy chains and three
clathrin light chains.
Two types of light chains are α and β. The light chains link to the actin
cytoskeleton, which helps generate force for membrane budding and
vesicle movement, and their phosphorylation regulates clathrin coat
assembly. The interwoven legs of the clathrin triskelions form an outer
shell from which the N-terminal domains of the triskelions protrude
inward.
When clathrin polymerizes, it forms a polygonal lattice with an intrinsic
curvature.
Even in the absence of membrane vesicles, clathrin triskelions can
polymerize to form the cage-like structure that is found around a coated
vesicle.
A typical clathrin-coated vesicle comprises a membrane-bounded vesicle
(tan) about 40 nm in diameter surrounded by a fibrous network of 12
pentagons and 8 hexagons. The fibrous coat is constructed of 36 clathrin
triskelions, one of which is shown here in red. One clathrin triskelion is
centered on each of the 36 vertices of the coat. Coated vesicles having
other sizes and shapes are believed to be constructed similarly: each
vesicle contains 12 pentagons but a variable number of hexagons.
Each of the three clathrin heavy chains has aspecific bent structure.
A clathrin light chain is attached to each heavy chain near the center; a
globular domain is at each distal (outer) tip.
An intermediate in the assembly of a clathrin coat, containing 10 of the
final 36 triskelions, illustrates the intrinsic curvature and the packing of
the clathrin triskelions.
