Organization of the Body
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
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
Te main mechanism for the
endocytosis and transcytosis of most macromolecules
by body cells is
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,
Coat proteins are recycled
to plasma membrane.
Uncoated vesicle fuses with a
sorting vesicle called an endosome.
components moves to
the plasma membrane
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).
Events of endocytosis mediated by protein-coated pits.
Note the three possible
fates for a vesicle and its contents, shown in