Essential Cell Biology 5th edition

Vesicular Transport
513
(A)
(C)
25 nm
(B)
0.1 µm 0.2 µm
Figure 15–20 Clathrin molecules form basketlike cages that help shape membranes into vesicles.
(A) Electron micrographs showing the sequence of events in the formation of a clathrin-coated vesicle
from a clathrin-coated pit. The clathrin-coated pits and vesicles shown here are unusually large and are
being formed at the plasma membrane of a hen oocyte. They are involved in taking up particles made
of lipid and protein into the oocyte to form yolk. (B) Electron micrograph showing numerous clathrincoated
pits and vesicles budding from the inner surface of the plasma membrane of cultured skin cells.
(C) In a test tube, clathrin molecules sometimes self-assemble into basketlike cages. The structure
of one such clathrin cage was determined by cryoelectron microscopy. The positions of three of the
constituent clathrin molecules, each of which has a characteristic three-armed shape, are highlighted
in red, green, and brown. (A, from M.M. Perry and A.B. Gilbert, J. Cell Sci. 39:257–272, 1979. With
permission from The Company of Biologists Ltd; B, from J. Heuser, J. Cell Biol. 84:560–583, 1980.
With permission from Rockefeller University Press; C, from A. Fotin et al., Nature 432:573−579,
2004. With permission from Macmillan Publishers Ltd.)
between the endoplasmic reticulum, the Golgi apparatus, and the plasma
membrane. But how does a transport vesicle select its particular cargo?
The mechanism is best understood for clathrin-coated vesicles.
Clathrin itself plays no part in choosing specific molecules for transport.
This is the function of a second class of coat proteins called adaptins,
which both secure the clathrin coat to the vesicle membrane and help
select cargo molecules for transport. Molecules for onward transport
carry specific transport signals that are recognized by cargo receptors in
the Golgi or plasma membrane. Adaptins help capture specific cargo
molecules by trapping the cargo receptors that bind them. In this way,
a selected set of cargo molecules, bound to their specific receptors, is
incorporated into the lumen of each newly formed clathrin-coated vesicle
(Figure 15–21). There are different types of adaptins: the adaptins that
ECB5 E15.19/15.20
bind cargo receptors in the plasma membrane, for example, are not the
same as those that bind cargo receptors in the Golgi apparatus, reflecting
the differences in the cargo molecules to be transported from each of
these sources.
Other classes of coated vesicles, called COP-coated vesicles (COP being
shorthand for “coat protein”), are involved in transporting molecules
between the ER and the Golgi apparatus and from one part of the Golgi
apparatus to another (Table 15–4).
TABLE 15–4 SOME TYPES OF COATED VESICLES
Type of Coated Vesicle Coat Proteins Origin Destination
Clathrin-coated clathrin + adaptin 1 Golgi apparatus lysosome (via endosomes)
Clathrin-coated clathrin + adaptin 2 plasma membrane endosomes
COPII-coated COPII proteins ER Golgi cisterna
COPI-coated COPI proteins Golgi cisterna ER