Essential Cell Biology 5th edition

Secretory Pathways
515
receptor
cargo protein
Rab TETHERING
tethering protein
CYTOSOL
v-SNARE
DOCKING
FUSION
Figure 15–22 Rab proteins, tethering
proteins, and SNAREs help direct
transport vesicles to their target
membranes. A filamentous tethering
protein (green) on a membrane binds to
a Rab protein (yellow) on the surface of a
vesicle. This interaction allows the vesicle
to dock on its particular target membrane.
A v-SNARE (red) on the vesicle then binds
to a complementary t-SNARE (blue) on
the target membrane. Whereas Rab and
tethering proteins provide the initial
recognition between a vesicle and its target
membrane, complementary SNARE proteins
ensure that transport vesicles dock at their
appropriate target membranes. These
SNARE proteins also catalyze the final fusion
of the two membranes (see Figure 15–23).
target membrane
t-SNARE
CARGO PROTEIN
DELIVERED
interact, fusion requires a much closer approach: the two bilayers must
come within 1.5 nanometers (nm) of each other so that their lipids can
intermix. For this close approach, water must be displaced from the
ECB5 e15.21/15.22
hydrophilic surfaces of the membranes—a process that is energetically
highly unfavorable and thus prevents membranes from fusing randomly.
All membrane fusions in cells must therefore be catalyzed by specialized
proteins that assemble to form a fusion complex that provides the
means to cross this energy barrier. For vesicle fusion, the SNARE proteins
themselves catalyze the process: when fusion is triggered, the
v-SNAREs and t-SNAREs wrap around each other tightly, thereby acting
like a winch that pulls the two lipid bilayers into close proximity
(Figure 15–23).
SECRETORY PATHWAYS
Vesicular traffic is not confined to the interior of the cell. It extends to
and from the plasma membrane. Newly made proteins, lipids, and carbohydrates
are delivered from the ER, via the Golgi apparatus, to the cell
surface by transport vesicles that fuse with the plasma membrane in the
process of exocytosis (see Figure 15−19). Each molecule that travels
along this secretory pathway passes through a fixed sequence of membrane-enclosed
compartments and is often chemically modified en route.
transport
vesicle
CYTOSOL
target membrane
v-SNARE
t-SNARE
TRANSPORT
VESICLE
DOCKS
MEMBRANES
COALESCE
LIPID BILAYERS
FUSE
QUESTION 15–5
The budding of clathrin-coated
vesicles from eukaryotic plasma
membrane fragments can be
observed when adaptins, clathrin,
and dynamin-GTP are added to
the membrane preparation. What
would you observe if you omitted
(A) adaptins, (B) clathrin, or
(C) dynamin? (D) What would you
observe if the plasma membrane
fragments were from a prokaryotic
cell?
Figure 15–23 Following vesicle
docking, SNARE proteins can catalyze
the fusion of the vesicle and target
membranes. Once appropriately
triggered, the tight pairing of v-SNAREs
and t-SNAREs draws the two lipid bilayers
into close apposition. The force of the
SNAREs winding together squeezes
out any water molecules that remain
trapped between the two membranes,
allowing their lipids to flow together to
form a continuous bilayer. In a cell, other
proteins recruited to the fusion site help
to complete the fusion process. After
fusion, the SNAREs are pried apart so
that they can be used again.