76
UNIT 1
Organization of the Body
3
caves”), tubular or flask-shaped inpocketings of the plasma
membrane seen in many cell types, are involved in a unique
kind of receptor-mediated endocytosis. Like clathrin-coated
pits, caveolae capture specific molecules (folic acid, tetanus
toxin) from the extracellular fluid in coated vesicles and par-
ticipate in some forms of transcytosis. However, caveolae are
smaller than clathrin-coated vesicles. Additionally, their cage-
like protein coat is thinner.
Caveolae are closely associated with lipid raFs that are plat-
forms for G proteins, receptors for hormones (for example, in-
sulin), and enzymes involved in cell regulation. Tese vesicles
appear to provide sites for cell signaling and cross talk between
signaling pathways. Teir precise role in the cell is still being
worked out.
Vesicles coated with still another coat protein (coatomer)
function in most types of intracellular vesicular trafficking.
Perhaps the most important thing to remember about the coat
Receptor-mediated endocytosis.
Te main mechanism for the
specific
endocytosis and transcytosis of most macromolecules
by body cells is
receptor-mediated endocytosis
(±igure 3.13c).
Tis exquisitely selective mechanism allows cells to concentrate
material that is present only in small amounts in the extracel-
lular fluid. Te receptors for this process are plasma membrane
proteins that bind only certain substances. Both the receptors
and attached molecules are internalized in a clathrin-coated
pit and then dealt with in one of the ways discussed above.
Substances taken up by receptor-mediated endocytosis include
enzymes, insulin (and some other hormones), low-density li-
poproteins (such as cholesterol attached to a transport protein),
and iron. Unfortunately, flu viruses, diphtheria, and cholera
toxins also use this route to enter our cells.
Different coat proteins are used for certain other types of
vesicular transport. ±or example,
caveolae
(ka
0
ve-o
9
le; “little
Coated pit
ingests substance.
Protein-coated
vesicle detaches.
Coat proteins are recycled
to plasma membrane.
Uncoated vesicle fuses with a
sorting vesicle called an endosome.
Transport vesicle
containing membrane
components moves to
the plasma membrane
for recycling.
Fused vesicle may (a) fuse with
lysosome for digestion of its contents,
or (b) deliver its contents to the
plasma membrane on the opposite
side of the cell (transcytosis).
Protein coat
(typically clathrin)
Extracellular fluid
Plasma membrane
Endosome
Lysosome
Transport
vesicle
(b)
(a)
Uncoated
endocytic
vesicle
Cytoplasm
6
5
4
3
2
1
Figure 3.12
Events of endocytosis mediated by protein-coated pits.
Note the three possible
fates for a vesicle and its contents, shown in
5
and
6
.
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