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

750 Chapter 13: Intracellular Membrane Traffic
Having selected a unique set of cargo molecules, the rafts then bud from the TGN
into transport vesicles destined for the apical plasma membrane. This process is
similar to the selective partitioning of some membrane proteins into the specialized
lipid domains in caveolae at the plasma membrane discussed earlier.
Membrane proteins destined for delivery to the basolateral membrane contain
sorting signals in their cytosolic tail. When present in an appropriate structural
context, these signals are recognized by coat proteins that package them into
appropriate transport vesicles in the TGN. The same basolateral signals that are
recognized in the TGN also function in early endosomes to redirect the proteins
back to the basolateral plasma membrane after they have been endocytosed.
Summary
Cells can secrete molecules by exocytosis in either a constitutive or a regulated fashion.
Whereas the regulated pathways operate only in specialized secretory cells,
a constitutive secretory pathway operates in all eukaryotic cells, characterized by
continual vesicle transport from the TGN to the plasma membrane. In the regulated
pathways, the molecules are stored either in secretory vesicles or in synaptic vesicles,
which do not fuse with the plasma membrane to release their contents until they
receive an appropriate signal. Secretory vesicles containing proteins for secretion
bud from the TGN. The secretory proteins become concentrated during the formation
and maturation of the secretory vesicles. Synaptic vesicles, which are confined
to nerve cells and some endocrine cells, form from both endocytic vesicles and from
endosomes, and they mediate the regulated secretion of small-molecule neurotransmitters
at the axon terminals of nerve cells.
Proteins are delivered from the TGN to the plasma membrane by the constitutive
pathway unless they are diverted into other pathways or retained in the Golgi
apparatus. In polarized cells, the transport pathways from the TGN to the plasma
membrane operate selectively to ensure that different sets of membrane proteins,
secreted proteins, and lipids are delivered to the different domains of the plasma
membrane.
What we don’t know
• How are targeting and fusion
proteins such as SNAREs regulated,
so that they can be returned to their
respective donor compartments in an
inactive state?
• How does a cell balance exocytic
and endocytic events to keep its
plasma membrane a constant size?
• Can newly formed daughter cells
generate a Golgi apparatus de novo,
or do they have to inherit it?
• How do lysosomes avoid digesting
their own membranes?
• How does a cell maintain the
right amount of every component
(organelles, molecules), and how does
it change these amounts as needed
(for example, to greatly expand the
endoplasmic reticulum when the cell
needs to produce large amounts of
secreted proteins)?
Problems
Which statements are true? Explain why or why not.
13–1 In all events involving fusion of a vesicle to a target
membrane, the cytosolic leaflets of the vesicle and target
bilayers always fuse together, as do the leaflets that are not
in contact with the cytosol.
13–2 There is one strict requirement for the exit of a protein
from the ER: it must be correctly folded.
13–3 All the glycoproteins and glycolipids in intracellular
membranes have oligosaccharide chains facing the
lumenal side, and all those in the plasma membrane have
oligosaccharide chains facing the outside of the cell.
Discuss the following problems.
13–4 In a nondividing cell such as a liver cell, why must
the flow of membrane between compartments be balanced,
so that the retrieval pathways match the outward
flow? Would you expect the same balanced flow in a gut
epithelial cell, which is actively dividing?
13–5 Enveloped viruses, which have a membrane coat,
gain access to the cytosol by fusing with a cell membrane.
Why do you suppose that these viruses encode their own
special fusion protein, rather than making use of a cell's
SNAREs?
13–6 For fusion of a vesicle with its target membrane to
occur, the membranes have to be brought to within 1.5 nm
so that the two bilayers can join (Figure Q13–1). Assuming
that the relevant portions of the two membranes at the
fusion site are circular regions 1.5 nm in diameter, calculate
the number of water molecules that would remain
between the membranes. (Water is 55.5 M and the volume
of a cylinder is πr 2 h.) Given that an average phospholipid
target membrane
1.5 nm
vesicle
Figure Q13–1 Close approach of a vesicle and its target membrane
in preparation for fusion (Problem 13–6).