628
Biogenesis, Structure and Function of Lysosomes
in liver, kidney, and other tissues dur-
ing fasting. The cause for the increased
macroautophagy in mammalian tissues
is a reduction in circulating amino acids
combined with reduced insulin and ele-
vated glucagon. Certain cancer cells are
able to grow at least in part because they
contain mutations that suppress macroau-
tophagy.
Yeast mutants that fail to carry out
macroautophagy under starvation condi-
tions have been isolated and placed into ap-
proximately 20 complementation groups.
These macro
a
uto
p
ha
g
y(Apg)mu
tan
tsre
-
sult in the inhibition of different steps of
macroautophagy including formation of
APhs, growth of the APhs, docking and
fusion with lysosomes, and digestion of
the APh contents. The genes required for
APh formation include a protein conjuga-
tion system reminiscent of ubiquitination.
Apg12 is covalently linked to Apg5 through
the actions of Apg10 and Apg7. Apg7 is a
homolog of the ubiquitin activating pro-
tein, E1, and Apg10 is a homolog of a
ubiquitin-conjugating protein, E2. Many
of the yeast
APG
genes have homologs in
mammals, so macroautophagic processes
appear to be highly conserved.
Many
apg
mutants are also defective in
the Cvt pathway used for delivery of API
and
α
-mannosidase to the yeast vacuole.
OthergenesareuniquefortheCvtpathway
or for macroautophagy. For example, the
Cvt19 protein acts as a receptor for API
and
α
-mannosidase, and this protein is
not required for macroautophagy.
5.4
Microautophagy
Microautophagy
is
the
invagination
of
vacuolar
membrane
such
that
cytosol,
organelles, or pieces of organelles are in-
ternalized within vesicles in the vacuole
(Figs 4 and 5). Vacuolar lipases cause the
break down of the internalized membrane.
Microautophagy can also operate in modes
that are selective for particular organelles
such as peroxisomes or the nucleus. Mi-
croautophagy of peroxisomes (pexophagy)
is stimulated when yeast is switched from
methanol to glucose as an energy source
(Fig. 4). A form of microautophagy called
piecemeal autophagy
of the nucleus is a con-
stitutive process that is accelerated during
starvation. It probably accounts for the
degradation of nuclear preribosomal par-
ticles under conditions in which ribosome
numbers in the cytosol decline. Microau-
tophagy and macroautophagy have par-
tially overlapping genetic requirements,
but they also have distinct elements.
Microautophagy has been reproduced
using isolated yeast vacuoles. The invagi-
nation of the vacuolar membrane requires
ATP, GTP, and cytosol. In addition, a
membrane potential across the vacuolar
membrane is required.
5.5
Vacuolar Import and Degradation Pathway
Proteins
such
as
fructose-1,6-bisphos-
phatase (FBPase) are synthesized in yeast
when gluconeogenesis is required. These
proteins are rapidly degraded within vac-
uoles in response to addition of glucose by
a process called
vacuolar import and degra-
dation
(Vid). FBPase may also be degraded
by the ubiquitin/proteasome pathway. FB-
Pase traf±cking to the vacuole occurs in
two steps: (1) import of the protein into an
intermediate vesicle, the Vid vesicle, and
(2) delivery of the Vid vesicle to the vacuole.
TheV
idves
ic
lesa
reun
iqueandcon
ta
in
one prominent membrane protein. Import
of the protein into Vid vesicles requires
hsc70 while delivery of the Vid vesicles to
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