Ridge
February 12, 2016, 01:42 PM
BaConCell Lecture Notes
Page 1
The endomembrane system, part 1.
(compartments; protein sorting)
Alberts et al. read Chapters 12 and 13
1.
All eucaryotic cells have the same basic set of membrane-bounded
organelles, and many vital biochemical processes take place in or on
membrane surfaces (such as we have seen with mitochondria and
chloroplasts).
2.
The major intracellular compartments common to all cells are: the
cytoplasm itself (consisting of the organelles and the cytosol) the nucleus,
the
endoplasmic
chloroplasts
(in
reticulum,
plants),
the
Golgi
lysosomes,
apparatus,
mitochondria,
endosomes,
peroxisomes
(microbodies). The relative volumes of these organelles can vary but an
example is given in Table 12-1 for the hepatocyte, and a comparison for
relative amounts of membrane types in hepatocytes and exocrine cells are
compared in Table 12-2.
3.
The organelles are not randomly distributed in the cell but are positioned
according to function and the special purpose of the cell itself.
For
example, the Golgi apparatus is usually found close to the nucleus,
whereas the ER extends from the nucleus far out into the cytosol.
Organelle positioning is highly dependent on the cytoskeleton, and
disruption of the cytoskeleton leads to breakdown of the internal
organisation of the organelles.
How proteins move between compartments.
4.
Proteins are synthesized on ribosomes in the cytosol (a few are also made
in mitochondria and chloroplasts).
The subsequent fate of the protein
depends on their amino acid sequence, in particular the presence of
specific sequences of amino acids called sorting signals. Proteins that do
not have signal regions remain in the cytosol.
5.
To understand how sorting signals work, we must consider first how
proteins can move from one compartment to another. This movement can
Ridge
February 12, 2016, 01:42 PM
BaConCell Lecture Notes
Page 2
be between compartments of topological equivalence or difference. See
Figure 12-7.
6.
Between the nucleus and cytosol, proteins move through the nuclear pore
complexes, in a process called gated transport, where the complexes
function as selective gates. Because of the nuclear pore, the nucleus is
considered to be topologically equivalent to the cytosol.
7.
In movement through membranes (trans-membrane transport) special
protein translocators directly transport proteins through the membrane
from
the
cytosol
to
a
topologically
different
compartment.
The
transported protein must unfold to snake through the membrane.
8.
In
vesicular
transport,
transport-vesicles
ferry
proteins
from
compartment to another, such as between the ER and Golgi.
one
Because
vesicles pinch off one compartment and fuse with another, then the
contents are considered to be topologically equivalent.
9.
Sorting signals come in two basic flavours (see Figure 12-8 and Table 123). Signal peptides usually come at the end of the amino acid sequence
as a special set of amino acids, although they can be located elsewhere.
Signal patches are sequences located at different places on the protein
that can come together when the final conformation of the protein is
made.
10. Most
proteins
destined
for
the
ER,
mitochondria,
chloroplasts,
peroxisomes, and nucleus are the signal peptide type. Much less is known
about signal patches, although there is some evidence that signal patches
are used to direct traffic from Golgi to lysosomes.
11. Proteins that enter the ER, for example, will not leave that compartment
again, and often the signal peptide is cleaved by a signal peptidase.
However, proteins that enter the nucleus are often transported out again,
and they retain their signal sequence. Perhaps this is why nuclear signal
sequences are not at the end of the protein but within it.