Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

710 Chapter 13: Intracellular Membrane Traffic
Figure 13–21 The entry of enveloped
viruses into cells. Electron micrographs
showing how HIV enters a cell by fusing
its membrane with the plasma membrane
of the cell. (From B.S. Stein et al., Cell
49:659–668, 1987. With permission from
Elsevier.)
200 nm
Summary
Directed and selective transport of particular membrane components from one
membrane-enclosed compartment to another in a eukaryotic cell maintains the
differences between those compartments. Transport vesicles, which can be spherical,
tubular, or irregularly shaped, bud from specialized coated regions of the donor
membrane. The assembly of the coat helps to collect specific membrane and soluble
cargo molecules for transport and to drive the formation of the vesicle.
There are various types of coated vesicles. The best characterized are clathrin-coated
vesicles, which mediate transport from the plasma membrane and the
trans Golgi network, and COPI- and COPII-coated vesicles, which mediate transport
between Golgi cisternae and between the ER and the Golgi apparatus, respectively.
Coats have a common two-layered structure: an inner layer formed of adaptor
proteins links the outer layer (or cage) to the vesicle membrane and also traps
specific cargo molecules for packaging into the vesicle. The coat is shed before the
vesicle fuses with its appropriate target membrane.
Local synthesis of specific phosphoinositides creates binding sites that trigger
clathrin coat assembly and vesicle budding. In addition, monomeric GTPases help
regulate various steps in vesicle transport, including both vesicle budding and
docking. The coat-recruitment GTPases, including Sar1 and the ARF proteins, regulate
coat assembly and disassembly. A large family of Rab proteins functions as
vesicle-targeting GTPases. Rab proteins are recruited to both, forming transport
vesicles and target membranes. The assembly and disassembly of Rab proteins
and their effectors in specialized membrane domains are dynamically controlled
by GTP binding and hydrolysis. Active Rab proteins recruit Rab effectors, such as
motor proteins, which transport vesicles along actin filaments or microtubules, and
filamentous tethering proteins, which help ensure that the vesicles deliver their contents
only to the appropriate target membrane. Complementary MBoC6 m13.19/13.19.5
v-SNARE proteins
on transport vesicles and t-SNARE proteins on the target membrane form stable
trans-SNARE complexes, which force the two membranes into close apposition so
that their lipid bilayers can fuse.
ENDOPLASMIC RETICULUM
GOLGI
Transport from the ER Through the
Golgi Apparatus
As discussed in Chapter 12, newly synthesized proteins cross the ER membrane
from the cytosol to enter the secretory pathway. During their subsequent transport,
from the ER to the Golgi apparatus and from the Golgi apparatus to the cell surface
and elsewhere, these proteins are successively modified as they pass through a
series of compartments. Transfer from one compartment to the next involves a
delicate balance between forward and backward (retrieval) transport pathways.
Some transport vesicles select cargo molecules and move them to the next compartment
in the pathway, while others retrieve escaped proteins and return them
to a previous compartment where they normally function. Thus, the pathway from
the ER to the cell surface consists of many sorting steps, which continuously select
membrane and soluble lumenal proteins for packaging and transport.
LATE ENDOSOME
LYSOSOME
EARLY ENDOSOME
RECYCLING
ENDOSOME
CELL EXTERIOR
SECRETORY
VESICLES