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

THE MOLECULAR MECHANISMS OF MEMBRANE TRANSPORT
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functional differences between its compartments. Finally, we consider the function
of the Golgi apparatus, lysosomes, secretory vesicles, and endosomes, as we
trace the pathways that connect these organelles.
The Molecular Mechanisms of Membrane
Transport and the Maintenance of
Compartmental Diversity
Vesicle transport mediates a continuous exchange of components between the ten
or more chemically distinct, membrane-enclosed compartments that collectively
comprise the secretory and endocytic pathways. With this massive exchange, how
can each compartment maintain its special identity? To answer this question, we
must first consider what defines the character of a compartment. Above all, it is
the composition of the enclosing membrane: molecular markers displayed on the
cytosolic surface of the membrane serve as guidance cues for incoming traffic to
ensure that transport vesicles fuse only with the correct compartment. Many of
these membrane markers, however, are found on more than one compartment,
and it is the specific combination of marker molecules that gives each compartment
its molecular address.
How are these membrane markers kept at high concentration on one compartment
and at low concentration on another? To answer this question, we need to
consider how patches of membrane, enriched or depleted in specific membrane
components, bud off from one compartment and transfer to another.
We begin by discussing how cells segregate proteins into separate membrane
domains by assembling a special protein coat on the membrane’s cytosolic face.
We consider how coats form, what they are made of, and how they are used to
extract specific cargo components from a membrane and compartment lumen for
delivery to another compartment. Finally, we discuss how transport vesicles dock
at the appropriate target membrane and then fuse with it to deliver their cargo.
There Are Various Types of Coated Vesicles
Most transport vesicles form from specialized, coated regions of membranes.
They bud off as coated vesicles, which have a distinctive cage of proteins covering
their cytosolic surface. Before the vesicles fuse with a target membrane, they discard
their coat, as is required for the two cytosolic membrane surfaces to interact
directly and fuse.
The coat performs two main functions that are reflected in a common two-layered
structure. First, an inner coat layer concentrates specific membrane proteins
in a specialized patch, which then gives rise to the vesicle membrane. In this way,
the inner layer selects the appropriate membrane molecules for transport. Second,
an outer coat layer assembles into a curved, basketlike lattice that deforms
the membrane patch and thereby shapes the vesicle.
There are three well-characterized types of coated vesicles, distinguished by
their major coat proteins: clathrin-coated, COPI-coated, and COPII-coated (Figure
13–4). Each type is used for different transport steps. Clathrin-coated vesicles,
for example, mediate transport from the Golgi apparatus and from the plasma
membrane, whereas COPI- and COPII-coated vesicles most commonly mediate
transport from the ER and from the Golgi cisternae (Figure 13–5). There is, however,
much more variety in coated vesicles and their functions than this short list
suggests. As we discuss below, there are several types of clathrin-coated vesicles,
each specialized for a different transport step, and the COPI- and COPII-coated
vesicles may be similarly diverse.
The Assembly of a Clathrin Coat Drives Vesicle Formation
Clathrin-coated vesicles, the first coated vesicles to be identified, transport material
from the plasma membrane and between endosomal and Golgi compartments.
COPI-coated vesicles and COPII-coated vesicles transport material early