Chapter 12 intracellular compartments and protein sorting
Percentages/important numbers
Important info
Examples/Diseases
Mechanisms, modes of action
Titles
®Summary: Eukaryotic cells contain organelles that have their own specific enzymes and other specialized
molecules.
oProteins confer one ach compartment its characteristic structural and functional components;
they catalyze reactions; and serve as organelle-specialized surface markers that direct new
deliveries of protein and lipids to the appropriate organelle.
oProtein synthesis occurs in the cytosol and are then delivered specifically to the organelle that
requires it.
®All eukaryotic cells have the same basic set of membrane-enclosed organelles
oMembrane-bound enzymes catalyze lipid metabolism, oxidative phosphorylation, and
photosynthesis
oThe membrane of each organelle must contain membrane transport proteins to import and
export specific metabolites
oMajor intracellular compartments:
Nucleus = DNA and RNA synthesis
Cytoplasm = cytosol and cytoplasmic
organelles
Cytosol: protein synthesis and
degradation; performs cell’s
intermediary metabolism, the many
reactions that degrade small
molecules and synthesize others to
provide a building block of
macromolecules
Endoplasmic reticulum
*has a total
membrane surface
area that is 25x and
12x that of the
plasms membrane
oRough ER
Contains ribosomes; protein synthesis
Produces most of the lipid for the rest of the cell
A store for Ca2+
oSmooth ER
Golgi Apparatus
oContains golgi cisternae
oFunctions: receives lipids and proteins from the ER and dispatches them to
various destinations while modifying them en route.
Mitochondria and chloroplasts
oGenerate most of the ATP that cells use
oChloroplasts are specialized versions of plastids which can store food or
pigment molecules
Lysosomes
oContains digestive enzymes that degrade defunct intracellular organelles
and macromolecules taken in from the outside by endocytosis
Endosomes
oOn the way to lysosomes, endocytosed material must first pass through
these
Peroxisomes
oSmall vesicular compartments that contain enzymes used in various
oxidative reactions.
®Evolutionary origins may help explain the topological relationships of organelles.
oThe precursors of the first eukaryotic cells are thought to have been relatively simple cells that
like most bacterial and archaeal cells have a plasms membrane but no internal membranes. And
the plasms membrane completed all of the membrane-depending functions.
oTypically, eukaryotic cells today are 10-30x larger in linear dimension and 1000-10,000x greater
in volume than a typically bacterium.
The profusion of the internal membrane can be regarded as an adaption to increase in
size.
Mitochondria and plastids differ from other organelles because they contain their own
genome suggesting that mitochondria and plastids evolved from bacteria that were
engulfed by other cells (symbiosis).
The evolutionary schemes just described group the intracellular compartments in
eukaryotic cells into four distinct families: (1) the nucleus and the cytosol, which
communicate with each other through nuclear pore complexes and are thus topologically
continuous (although
functionally distinct); (2) all
organelles that function in the
secretory and endocytic
pathways—including the ER,
Golgi apparatus, endosomes,
and lysosomes, the numerous
classes of transport
intermediates such as
transport vesicles that move
between them, and
peroxisomes; (3) the
mitochondria; and (4) the
plastids (in plants only).
®Proteins can move between compartments in
different ways
oThe synthesis of all proteins begins on ribosomes in the cytosol, except for the few that are
synthesized on the ribosomes of mitochondria and plastids.
Their subsequent fate depends on their , which can contain sorting amino acid sequence
signals that direct their delivery to locations outside the cytosol or to organelle surfaces.
Some proteins do not have a sorting signal and consequently remain in the cytosol as
permanent residents.
Many others, however, have specific sorting signals that direct their transport from the
cytosol into the nucleus, the ER, mitochondria, plastids, or peroxisomes; sorting signals
can also direct the transport of proteins from the ER to other destinations in the cell.
Three mechanisms by which proteins move from one compartment to another.
1. Gated transport
oNuclear pores in the nuclear envelope.
2. Protein translocation
oProtein translocators between the cytosol into a space that is topologically
distinct.
Sometimes the protein must unfold to pass through the
translocator
Ex. From cytosol to the ER lumen or mitochondria, plastids, and
peroxisomes.
3. Vesicular transport (covered in chapter 13)
oTransport vehicles that ferry proteins from one topologically equivalent
compartment to another.
Ex. ER to GA or peroxisomes.
oTransport vesicles bud from one compartment (donor) and duse with
another topologically equivalent (TargeT) compartment. In this process,
soluble compartments (Red dots) are transferred from lumen to lumen.
®Signal sequences and sorting receptors direct proteins to the correct cell address
oEach newly synthesized organelle protein must find its way from a ribosome in the cytosol, where
the protein is made, to the organelle where it functions. It does so by following a specific
pathway, guided by sorting signals in its amino acid sequence that function as either signal
sequences or signal patches. Sorting signals are recognized by complementary sorting receptors,
which deliver the protein to the appropriate target organelle. Proteins that function in the cytosol
do not contain sorting signals and therefore remain there after they are synthesized.
oSignal sequences are found in the N-terminus of the polypeptide chain and are an AA sequence of
15-60 residues long.
Signal peptidases remove the signals sequence from the finished protein once the sorting
process is complete.
Signals can be used as gated transport into the nucleus
Sorting signals can also be com- posed of multiple internal amino acid sequences that
form a specific three-dimensional arrangement of atoms on the protein’s surface; such
signal patches are sometimes used for nuclear import and in vesicular transport.
Each signals sequence specifies a particular destination in the cell.
Signal sequences are recognized by complementary sorting receptors that guide proteins
to their appropriate destination, where the receptors unload their cargo.
®Most organelles cannot be constructed : they require information in the organelle itself.de novo
oCells cannot make membranes from scratch and therefore through mitosis, the organelles double
and the daughter cells are able to have their needed material and a membrane.
oA new ER could not be made without an existing ER. The same is true for mitochondria and
plastids.
oInformation in the form of at least one distinct protein that preexists in the organelle membrane
is also required and this information is passed from a parent cell to a daughter cell in the form of
the organelle itself.
oHowever, some organelles can form from other organelles and don’t have to be inherited at cell
division
Ex. ER buds off transport vesicles
Ex. The plasma membrane constantly buds off various types of specialized endocytic
vesicles.
®GATED TRANSPORT: The transport of molecules between the nucleus and cytosol
oThe nuclear envelope encloses the DNA and defines the nuclear compartment. It is composed of
two parts
The inner nuclear membrane
Contains proteins that act as binding sites for chromosomes and for the nuclear
lamina, a protein meshwork that provides structural support.
Outer nuclear membrane
Is continuous with the ER
Contains ribosomes that are used for protein synthesis.
The proteins made in the ribosomes are transported
into the space between the inner and outer nuclear
membrane (perinuclear space) that is continuous with
the ER.
There is bidirectional movement between the nucleus and
cytosol
The nucleus provides RNA- rRNA, tRNA, miRNA, and
snRNA.
The cytosol produces histones, DNA polymerases, RNA
polymerases, transcriptional regulators, and RNA
processing proteins that are imported to the nucleus.
oNuclear pore complexes (NPCs )
Perforate the nuclear envelope in all eukaryotes. Each NPC has
30 different proteins called nucleoporins. Each nucleoporin has
multiple copies so ~500-1000 protein molecules are in the
NPC.
~3,000-4,000NPCs per cell but this number can be 100s in
glial cells to 1000 NPCs can transport up to
macromolecules/sec
However, proteins larger than daltons cannot enter by passive diffusion. 60,000
This size cut-off is what allows for different protein compositions between the
nucleus and cytosol.
But how do large molecules like DNA/RNA polymerases and other molecules pass
through? Only through receptor proteins that act as a ferry for large molecules.
oNuclear localization signals direct nuclear proteins to the nucleus.
Nuclear localization signals (NLSs ) are responsible for the selectivity of active nuclear
import.
(Refer to chart) many of these signals consist of one or two short sequences that
are rich in the positively charged amino acids lysine and arginine, lys-lys-lys-arg-
lys.
Particles bind to the cytosolic fibrils and then proceed down the center.
Nuclear import receptors bind to both nuclear localization signals and NPC proteins
(import receptors bind to their cargo molecules
un the cytosol, release them in the nucleus,
and are exported to the cytosol for reuse).
To initiatie nuclear import, most nuclear
localization signals must be recognized
by nuclear import receptors
(importins), encoded by a family of
related genes.
Each family member encodes a
receptor protein that can bind and
transport the subset of cargo proteins
containing the appropriate nuclear
localization signal.
oNuclear import receptors do not
always bind to nuclear proteins
directly. Additional adaptor
proteins can form a bridge
between the import receptors
and the nuclear localization
signals on the proteins to be
transported
o* By using a variety of import
receptors and adaptors, cells are able to recognize the broad repertoire of
nuclear localization signals that are displayed on nuclear proteins.
oThe import receptors are soluble cytosolic proteins that bind both to the
nuclear localization signal on the cargo protein and to the phenylalanine-
glycine (FG) repeats in the unstructured domains of the channel
nucleoporins that line the central pore. According to one model of nuclear
transport, the receptor–cargo complexes move along the trans- port path
by repeatedly binding, dissociating, and then re-binding to adjacent FG-
repeat sequences.
oNuclear export works like nuclear import, but in reverse.
Relies on nuclear export signals on the macromolecules to be exported and nuclear
export receptors (exportins).
These receptors bind to both the export signal and NPC proteins to guide their
cargo through the NPC to the cytosol.
Many nuclear export receptors are structurally related to nuclear import receptors, and
they are encoded by the same gene family of nuclear transport receptors, or
karyopherins. In yeast, there are 14 genes encoding karyopherins; in animal cells, the
number is significantly larger.
oGTPase Ran
Required for import and export
Like other GTPases, Ran is a molecular switch that can exist in two conformational states,
depending on whether GDP or GTP is bound. Two Ran-specific regulatory proteins trigger
the conversion between the two states: a cytosolic GTPase-activating protein (GAP)
triggers GTP hydrolysis and thus converts Ran-GTP to Ran-GDP, and a nuclear guanine
exchange factor (GEF) promotes the exchange of GDP for GTP and thus converts Ran-GDP
to Ran-GTP. Because Ran-GAP is located in the cytosol and Ran-GEF is located in the
nucleus where it is anchored to chromatin, the cytosol contains mainly Ran-GDP, and the
nucleus contains mainly Ran-GTP.
This gradient of the two conformational forms of Ran drives nuclear transport in the
appropriate direction. Docking of nuclear import receptors to FG-repeats on the cytosolic
side of the NPC, for example, occurs when receptors are loaded with appropriate cargo.
Import receptors, facilitated by FG-repeat binding, then enter the channel. If they reach
the nuclear side of the pore com- plex, Ran-GTP binds to them, and, if the receptors arrive
loaded with cargo molecules, the Ran-GTP binding causes the receptors to release their
cargo. Because the Ran-GDP in the cytosol does not bind to import (or export) receptors,
unloading occurs only on the nuclear side of the NPC. In this way, the nuclear localization
of Ran-GTP creates the directionality of the import process.
Nuclear export occurs by a similar mechanism, except that Ran-GTP in the nucleus
promotes cargo binding to the export receptor, rather than promoting cargo dissociation.
Once the export receptor moves through the pore to the cytosol, it encounters Ran-GAP,
which induces the receptor to hydrolyze its GTP to GDP.
o Transport through NPCs can be regulated by controlling access to the transport machinery
Some proteins contain both nuclear localization signals and nuclear export signals. These
proteins continually shuttle back and forth between the nucleus and the cytosol. The
relative rates of their import and export determine the steady- state localization of such
shuttling proteins: if the rate of import exceeds the rate of export, a protein will be
located mainly in the nucleus; conversely, if the rate of export exceeds the rate of import,
a protein will be located mainly in the cytosol.
oDuring mitosis the nuclear envelope disassembles
The nuclear lamina is the framework of the nucleus
They are made nuclear subunits called nuclear lamins
The lamina is located in the inner nuclear membrane
It is anchored by attachment or both the NPCs and transmembrane proteins
It interacts with chromatin which interacts with transmembrane proteins.
*with inner membrane proteins, the lamina can provide structural links between
the DNA and nuclear envelope.
Breakdown
When a nucleus is dismantled during mitosis, the NPCs and nuclear lamina
disassemble and the nuclear envelope fragments. The dismantling process is a
consequence of direct phosphorylation of nucleoporins and lamins by the cyclin-
dependent protein kinase (Cdk) that is activated at the onset of mitosis
During this process, some NPC proteins become bound to nuclear import
receptors, which play an important part in the reassembly of NPCs at the end of
mitosis. Nuclear envelope membrane proteins—no longer tethered to the pore
complexes, lamina, or chromatin—disperse throughout the ER membrane. The
dynein motor protein, which moves along microtubules, actively participates in
tearing the nuclear envelope off the chromatin.
Reassembly
Later in mitosis, the nuclear envelope reassembles on the surface of the daughter
chromosomes. In addition to its crucial role in nuclear transport, the Ran GTPase
also acts as a positional marker for chromatin during cell division, when the
nuclear and cytosolic components intermix. Because Ran-GEF remains bound to
chromatin when the nuclear envelope breaks down, Ran molecules close to
chromatin are mainly in their GTP-bound conformation. By contrast, Ran molecules
further away have a high likelihood of encountering Ran-GAP, which is distributed
throughout the cytosol; these Ran molecules are mainly in their GDP-bound
conformation. As a result, the chromosomes in mitotic cells are sur- rounded by a
cloud of Ran-GTP. Ran-GTP releases the NPC proteins in proximity to the
chromosomes from nuclear import receptors. The free NPC proteins attach
to the chromosome surface, where they assemble into new NPCs. At the same
time, inner nuclear membrane proteins and dephosphorylated lamins bind again
to chromatin. ER membranes wrap around groups of chromosomes until they form
a sealed nuclear envelope. During this process, the NPCs start actively re-
importing proteins that contain nuclear localization signals. Because the nuclear
envelope is initially closely applied to the surface of the chromosomes, the newly
formed nucleus excludes all proteins except those initially bound to the mitotic
chromosomes and those that are selectively imported through NPCs. In this way,
all other large proteins, including ribosomes, are kept out of the newly assembled
nucleus.
pg. 658 THE TRANSPORT OF PROTEINS INTO THE MITOCHONDRIA AND
CHLOROPLASTS
®both are double membrane organelles with their own DNA, ribosomes, and other component required for
protein synthesis but most of their proteins are encoded in the nucleus and imported from the cytosol.
®Mitochondria specialize in ATP synthesis using energy from electron transport and oxidative
phosphorylation. Chloroplasts specialized in ATP synthesis via photosynthesis.
o**each sub compartment listed below contain a distinct set of proteins.
oThe movement of protein across membranes is called protein translocation
oNew mitochondria and chloroplasts are produced by the growth preexisting organelles, followed
by fission.
®MITOCHONDRIA:
oAnatomy:
Matrix space
Inner membrane
Encloses the matrix space and forms invaginations called cristae.
Intermembrane space
Outer membrane
Contact with cytosol
o Protein translocation
Protein imported in the mitochondria are taken up from the cytosol seconds or minutes of
their release from ribosomes. These proteins are synthesized as “mitochondrial precursor
proteins” in the cytosol and then translocated into the mitochondria via post-translation
mechanism.
PRECUROSR PROTEINS
oAre imported as unfolded polypeptide chains
oMitochondrial precursor proteins do not fold into their native structures
after they are synthesized; instead, they remain unfolded in the cytosol
through interactions with other proteins. Some of these interacting
proteins are general chaperone proteins of the hsp70 family, whereas
others are dedicated to mitochondrial precursor proteins and bind directly
to their signal sequences. All the interacting proteins help to prevent the
precursor proteins from aggregating or folding up spontaneously before
they engage with the TOM complex in the outer mitochondrial membrane.
One or more signal sequences direct all mitochondrial precursor proteins to their
appropriate mitochondrial subcompartment. Many proteins entering the matrix space
contain a signal sequence at their N-terminus (N=NUCLEUS) that a signal peptidase
rapidly removes after import. Other imported proteins, including all outer membrane and
many inner membrane and intermembrane space proteins, have internal signal
sequences that are not removed.
SIGNAL SEQUENCES
OCCURS POST-TRANSLATIONALLY
Amphiphilic alpha helix with positive AA on one side and uncharged hydrophobic
AAs on the other side. NO PRECISE AA SEQUENCE IS NEEDED FOR ENTRANCE
ONLY THIS MAKEUP.
PROTEIN TRANSLOCATORS
Mediate protein movement across mitochondria membranes.
TOM Complex
oTransfers proteins across the OUTER MEMBRANE
oThe TOM complex is required for the import of all nucleus-encoded
mitochondrial proteins. It initially transports their signal sequences into the
intermembrane space and helps to insert transmembrane proteins into the
outer membrane. β-barrel proteins, which are particularly abundant in the
outer membrane, are then passed on to an additional translocator, the
SAM complex, which helps them to fold properly in the outer membrane.
oTRANSLOCATION: As a first step in the import process, the import receptors
of the TOM complex bind the signal sequence of the mitochondrial
precursor protein. The interacting proteins are then stripped off, and the
unfolded polypeptide chain is fed—signal sequence first—into the
translocation channel.
2 TIM Complexes (TIM23 and TIM22)
Function: transfer proteins across the INNER MEMBRANE.
*The TOM complex first transports the signal sequence across the
outer membrane to the intermembrane space, where it binds to a
TIM complex, opening the channel in the complex.
oTIM23
The TIM23 complex transports some soluble proteins into the
matrix space and helps to insert transmembrane proteins into the
inner membrane.
The mitochondrial hsp70 is part of a multisubunit protein assembly
that is bound to the matrix side of the TIM23 complex and acts as a
motor to pull the precursor protein into the matrix space. Like its
cytosolic cousin, mitochondrial hsp70 has a high affinity for
unfolded polypeptide chains, and it binds tightly to an imported
protein chain as soon as the chain emerges from the TIM
translocator in the matrix space. The hsp70 then undergoes a
conformational change and releases the protein chain in an ATP-
dependent step, exerting a ratcheting/pulling force on the protein
being imported.
oTIM22
The TIM22 complex mediates the insertion of a subclass of inner
membrane proteins, including the transporter that moves ADP,
ATP, and phosphate in and out of mitochondria.
OXA Complex
oMediates the insertion of those inner membrane proteins that are
synthesized within mitochondria and helps insert some imported inner
membrane proteins that are initially transported into the matric space by
other complexes.
**the complexes contain receptors for mitochondrial precursor proteins or
translocation channels.
o ENERGY
ATP Hydrolysis
One outside the mitochondria and one in the matrix space
Membrane potential
Across the inner mitochondrial membrane.
Once the signal sequence has passed through the TOM complex and is bound to a
TIM complex, further translocation through the TIM translocation channel requires
the membrane potential, which is the electrical component of the electrochemical
H+ gradient across the inner membrane. Pumping of H+ from the matrix space to
the intermembrane space, driven by electron trans- port processes in the inner
membrane (discussed in Chapter 14), maintains the electrochemical gradient
oTransport into the inner mitochondrial membrane and intermembrane space occurs via several
routes
oA. Most common: only the
N-terminal signal
sequence of the
transported protein
actually enters the matrix
space.
Stop-transfer
sequence
oB. In another transport
route to the inner
membrane or
intermembrane space, the
TIM23 complex initially
translocates the entire
protein into the matrix
space
oC.
®CHLOROPLASTS:
oAnatomy:
Outer membrane
Intermembrane space
Inner membrane
Stroma ( )equivalent to mitochondrial matrix space
Thylakoid membrane
Derived from the inner membrane during development and then is pinched off
Thylakoid space
oSIGNAL SEQUENCES
Occurs POST-TRANSLATIONALLY, USES SEPARATE TRANSLOCATION COMPLEXES IN EACH
MEMBRANE, REQUIRE ENERGY, AND USE AMPHIPHILIC N-TERMINAL SIGNAL SEQUENCES
THAT ARE REMOVED AFTER USE… LIKE IN MITOCHONDRIA!
ENERGY
Have an electrochemical H+ gradient in the thylakoid membrane but not inner
membrane
Use GTP and ATP hydrolysis to power import across their membranes.
SIGNAL SEQUENCES
Import receptors on each organelle distinguish between the different signals
sequences between mitochondria and chloroplasts.
THYLAKOID
Many chloroplast proteins, including the protein subunits of the photosynthetic
system and of the ATP synthase (discussed in Chapter 14), are located in the
thylakoid membrane
TRANSLOCATION
Like the precursors of some mitochondrial proteins, the precursors of these
proteins are translocated from the cytosol to their final destination in two steps.
First, they pass across the double membrane into the matrix space (called the
stroma in chloroplasts), and then they either integrate into the thylakoid
membrane or translocate into the thylakoid space
The precursors of these proteins have a hydrophobic thylakoid signal sequence
following the N-terminal chloroplast signal sequence. After the N-terminal signal
sequence has been used to import the protein into the stroma, a stromal signal
peptidase removes it, unmasking the thylakoid signal sequence that initiates
transport.
**There are at least by which proteins cross or become integrated into four routes
the thylakoid membrane, distinguished by their need for different stromal
chaperones and energy sources
®Pg. 666 PEROXISOMES
oSingle membrane and don’t contain their own DNA or ribosomes. So, all their proteins are
encoded in the nucleus.
oMost of their proteins is imported from the cytosol and the rest by t
ER.
oAll cells have peroxisomes.
oFUNCTION:
they contain oxidative enzymes like catalase and urate
oxidase at high concentrations.
OXIDATION REACTIONS
They are also a major site of oxygen utilization LIKE
MITOCHONDRIA
RH2 + O2R + H (hydrogen peroxide)
2O2
oIn turn catalase uses H2O2 generated by other
enzymes in the organelle to oxidize a variety of
other substrates in a peroxidation reaction:
H202 + R’H2 R’ + 2H 02
Important in the liver and kidneys where
peroxisomes detoxify various harmful
molecules.
oWhen there is too much H catalase converts
202
into water 2H 0 + O
2022H2 2
Breakdown of fatty acids
oThrough β -oxidation that shortens alkyl chains
of fatty acids sequentially in blocks of two carbon atoms at a time, thereby
converting . The peroxisomes then export the the fatty acids to acetyl CoA
acetyl CoA to the cytosol for use in biosynthetic reactions. In mammalian
cells, β oxidation occurs in both mitochondria and peroxisomes; in yeast
and plant cells, however, this essential reaction occurs exclusively in
peroxisomes.
oThey also catalyze plasmalogens which are the most abundant class of
phospholipids in myelin. Plasmalogen deficiencies cause profound
abnormalities in the myelination of nerve-cell axons, which is one reason
why many peroxisomal disorder lead to neurological disease.
Plants
o1. Leaves via photorespiration
o2. Germinating seeds via the glyoxylate cycles (peroxisomes are called
glyoxysomes) in which needed for fatty acids are converted into sugars
growth of the young plant.
The glyoxylate cycles occur in animals.DOES NOT
oSIGNAL SEQUENCES
A specific sequence of three amino acids (Ser–Lys–Leu) located at the C-terminus of many
peroxisomal proteins functions as an import signal. Other peroxisomal proteins contain
signal sequences near the N-terminus
1. Recognized by receptors in the cytosol
2. Peroxins participate in the import process, DRIVEN BY ATP hydrolysis
oProteins do not have to unfold to be imported because a group of at least 6
peroxins form a protein translocator. DIFFERENT THAN
MITO/CHLOROPLASTS
oPeroxin PEX5 recognizes the C-terminal peroxisomal signal. It accompanies
its cargo all the way into peroxisomes and, after cargo release, cycles back
to the cytosol. After delivering its cargo to the peroxisome lumen, Pex5
undergoes ubiquitylation. This modification is required to release Pex5
back into the cytosol, where the ubiquitin is removed. An ATPase
composed of Pex1 and Pex6 harnesses the energy of ATP hydrolysis to help
release Pex5 from peroxisomes.
The importance of this import process and of peroxisomes is
demonstrated by the inherited human disease Zellweger
syndrome, in which a defect in importing proteins into peroxisomes
leads to a profound peroxisomal deficiency. These individuals,
whose cells contain “empty” peroxisomes, have severe
abnormalities in their brain, liver, and kidneys, and they die soon
after birth. A mutation in the gene encoding peroxin Pex5 causes
one form of the disease. A defect in Pex7, the receptor for the N-
terminal import signal, causes a milder peroxisomal disease.
oFORMATION
Organelle growth or fission (LIKE IN MITO/CHLOROPLASTS) or is it a specialized
compartment from the ER?
BOTH.
oMost peroxisomal membrane proteins are made in the cytosol and insert
into the membrane of preexisting peroxisomes, but others are first
integrated into the ER membrane, where they are packaged into
specialized peroxisomal precursor vesicles. New precursor vesicles may
then fuse with one another and begin importing additional peroxisomal
proteins, using their own protein import machinery to grow into mature
peroxisomes, which can undergo cycles of growth and fission.
®Pg. 669 ER
o FACTS
ALL eukaryotic cells have an ER.
It constitutes more than ½ of the total membrane of an average animal cell
The ER lumen is continuous with the space between the inner and outer nucleus.
o FUNCTION
As mentioned at the beginning of this chapter, the ER has a central role in both lipid and
protein biosynthesis, and it also serves as an intracellular Ca2+ store that is used in many
cell signaling responses
The ER membrane is the site of production of all the transmembrane proteins and
lipids for most of the cell’s organelles, including the ER itself, the Golgi apparatus,
lysosomes, endosomes, secretory vesicles, and the plasma membrane.
The ER membrane is also the site at which most of the lipids for mitochondrial and
peroxisomal membranes are made. In addition, almost all of the proteins that will
be secreted to the cell exterior—plus those destined for the lumen of the ER, Golgi
apparatus, or lysosomes—are initially delivered to the ER lumen
Rough
Mammalian cells begin to import most proteins into the ER before complete
synthesis of the polypeptide chain—that is, import is a co-translational process
oIn co-translational transport, the ribosome that is synthesizing the protein
is attached directly to the ER membrane, enabling one end of the protein
to be translocated into the ER while the rest of the polypeptide chain is
being synthesized.
In contrast, the import of proteins into mitochondria, chloroplasts, nuclei, and
peroxisomes is a post-translational process
