Biogenesis, Structure and Function of Lysosomes
627
Microautophagy
Crinophagy
Endocytosis
E
N
G
SV
APh
Chaperone-mediated
autophagy
Macroautophagy
L
L
L
L
L
L
Fig. 5
Lysosomal pathways of protein degradation in mammalian
cells. N
=
nucleus; L
=
lysosome; E
=
endosome;
APh
=
autophagosome; G
=
Golgi; SV
=
secretory vesicle. Five
different lysosomal degradation pathways in mammalian cells are
listed in the ±gure and described in the text.
5.2
Exocytosis
Proteins can be secreted from mammalian
cells by constitutive vesicular pathways
or by regulated mechanisms in which
the protein is stored in secretory vesicles
(Fig. 5). When there is reduced physi-
ological need for the secreted proteins,
the secretory vesicles can fuse with lyso-
somes rather than the plasma membrane
in a process called
crinophagy
.F
o
re
x
-
ample,
when
blood
glucose
levels
are
low, insulin secretion by the pancreatic
islet
β
-cells is reduced. Under such con-
ditions, the secretory vesicles containing
insulin fuse with lysosomes and the pack-
aged insulin is degraded. A percentage
of
serum
albumin
made
by
the
liver
and
secreted
constitutively
is
also
de-
graded in this way. This degradation of
secretory proteins has not been described
in yeast.
5.3
Macroautophagy
Macroautophagy is a degradative path-
way that is found in yeast (Fig. 4) and
in mammalian cells (Fig. 5). Regions of
the cytoplasm are sequestered in double-
membrane vesicles called
autophagosomes
(APhs).
The
membranes
surrounding
APhs are lipid-rich and contain primarily
a single protein. APhs often contain rec-
ognizable material such as mitochondria,
peroxisomes, ribosomes, or glycogen. The
APh acidi±es, and then fuses with lyso-
somes after which the contents of the APh
are digested.
Macroautophagy is induced in yeast be-
cause of nitrogen or carbon starvation and
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