Transcript Human-physiology-II-lecture-Endomembrane-system

Human Physiology:
Endomembrane system
BY
DR BOOMINATHAN Ph.D.
M.Sc.,(Med. Bio, JIPMER), M.Sc.,(FGSWI, Israel), Ph.D (NUS, SINGAPORE)
PONDICHERRY UNIVERSITY
III Lecture
9/August/2012
Source: Collected from different sources on the internet-http://koning.ecsu.ctstateu.edu/cell/cell.html
Learning objectives
1.The structure and function of
endoplasmic reticulum(ER);
2.The structure and function of
Golgi complex;
3.The structure and function of
lysosomes .
Introduction

The Compartmentalization in Eukaryotic Cells
Membranes divide the cytoplasm of eukaryotic cells
into distinct compartments.
Three categories in eukaryotic cells:
1.the endomembrane system: endoplasmic reticulum, Golgi complex, Lysosomes.
2.the cytosol.
3.mitochondria, peroxisomes and the nucleus.
Membrane-bound structures (organelles) are found
in all eukaryotic cells.
Introduce


Endomembrane System : The structural
and functional relationship organelles
include endoplasmic reticulum ,Golgi
complex, lysosome, secretory vesicles.
Membrane-bound structures (organelles)
are found in all eukaryotic cells.
Endomembrane System
endoplasmic reticulum(ER)
Golgi complex
lysosome
secretory vesicles
Endomembrane System
Section I Endoplasmic Reticulum
1.1 The Structure of ER
The endoplasmic reticulum is a network of
flattened sacs and branching tubules that
extends throughout the cytoplasm in plant and
animal cells. These sacs and tubules are all
interconnected by a single continuous
membrane so that the organelle has only one
large, highly convoluted and complexly
arranged lumen (internal space).
The Sructure of ER
1.2 The types of ER
The ER comes in two forms.
Rough endoplasmic reticulum(RER)
Smooth endoplasmic reticulum(SER)
1.2 The types of ER
1.2.1 The rough ER
The ER are covered with
ribosome .Their rough
appearance under
electron microscopy led to
their being called rough
ER . RER has ribosomes
on the cytosolic side of
continuous, flattened sacs.
1.2 The types of ER
1.2.1 The rough ER
The outer membrane of
the nucleus is always
studded with ribosomes
and is continuous with
rough ER membrane .
The lumen of RER is
connected to nuclear
envelope .
1.2 The types of ER
1.2.2 The smooth ER
The parts are free of
ribosomes and are
called smooth ER
(SER). SER is an
interconnecting
network of tubular
membrane elements.
1.2 The types of ER
Rough and smooth ER differ
not only in structure, but also in
function.
1.3 The functions of the RER
1.3.1 Proteins synthesized on ribosomes of RER ：
* The ribosomes assemble amino acids into protein
units, which are transported into the lumen of rough
endoplasmic reticulum for further processing.
* These proteins may be either transmembrane
proteins, which become embedded in the membrane
of the endoplasmic reticulum, or water-soluble
proteins, which are able to pass completely through
the membrane into the lumen.
1.3 The functions of the RER
Those proteins that reach the inside of the
endoplasmic reticulum are folded into the
correct three-dimensional conformation.
Chemicals, such as carbohydrates or sugars,
are added, then the ER either transports the
completed proteins to areas of the cell where
they are needed, or they are sent to the Golgi
apparatus for further processing and
modification.
1.3 The functions of the RER
1.3.1 Proteins
synthesized on
ribosomes of rER
include:
 Secretory proteins;
 integral membrane
proteins;
 soluble proteins of
organelles.
1.3 The functions of the RER
?
How do the ribosomes (a membranebound ribosome ) attach to the outer
surface of rER ,and then the newly
synthesized protein pass completely
through the membrane into the
lumen of rER?
1.3 The functions of the RER
The Signal Hypothesis would explain
completely the mechanism that the
ribosomes attach to the outer surface
of rER ,and then the newly protein
pass completely through the
membrane into the lumen of rER?
1.3 The functions of the RER
The basic structure of the
process is a Signal Recognition Particle(SRP),that
lead the ribosomes attach to
the outer surface of
rER ,and then lead the
newly protein pass
completely through the
membrane into the lumen
of rER.
SRP
1.3 The functions of the RER
•Signal-recognition particle(SRP)
1.synthesis:
Six different polypeptides complexed with a
300-nucleotide molecule of RNA.
1.3 The functions of the RER
2. SRP have three main active sites:
* One that recognizes and binds to ER signal
sequence;
* One that interacts with the ribosome to block
further translation;
* One that binds to the ER membrane (docking
The site to
protein:receptor protein)
recognize and
bind to ER
signal sequence
The site to
block further
translation
The site to
recognize
receptor protein
1.3 The functions of the RER
•ER signal sequence:
1. synthesis :Typically 15-30 amino acids.
2.consist of three domains:
a positively charged N-terminal region;
a central hydrophobic region;
a polar region adjoining the site where cleavage from
the mature protein will take place.
A signal sequence on nascent seretory proteins targets
them to the ER and is then cleaved off.
SRP receptor (a binding protein or docking
protein:receptor protein)
Signal peptides and Signal patches
Figure 6-8 Two ways that a sorting signal can be built into a protein. (A) The
signal resides in a single discrete stretch of amino acid sequence, called a signal
peptide, that is exposed in the folded protein. Signal peptides often occur at the end of
the polypeptide chain, but they can also be located elsewhere. (B) A signal patch can be
formed by the juxtaposition of amino acids from regions that are physically separated
before the protein folds; alternatively, separate patches on the surface of the folded
protein that are spaced a fixed distance apart could form the signal.
1.3 The functions of the RER
The basic structure of the
mechanism of Signal
Hypothesis :
1. Signal Recognition
Particle(SRP)
2. ER signal sequence
3.SRP receptor
1
3
SRP
The mechanism of the Signal Hypothesis
As the signal sequence emerge from the ribosome, they are
recognized and bound by a signal recognition particle (SRP), This
binding inhibits translation and target the complex to the RER by
binding to the SRP receptor on the ER membrane. SRP is then
released and the ribosome binds to a protein translocation
complex in the ER membrane; the signal sequence is inserted into
a membrane channel, translation is resumed and the unfolded
growing polypeptide chain is translocated across the membrane
into the ER. As translocation proceeds, the signal sequence is
cleaved by signal peptidase and the polypeptide is released into
the ER lumen.
Signal Hypothesis
(SRP)
mRNA
AP
A
ribosome
signal
sequence
tRNA
SRP receptor
cytoplasme
Channel protein
核糖体结合蛋白
The lumen of RER
This process shows the signal receptor particle that
associates with the large and small subunit of the
ribosome that allows binding to the receptor on the
rough endoplasmic reticulum.After the protein is
synthesized, the ribosome dissociates into large and
small subunits and the SRP also looses its
attachment to the receptor.
1.3.2 Modification/processing of newly
synthesized proteins: glycosylation in the RER
Glycosylation of newly synthesized proteins
•N-linked: oligosaccharide chain is linked
to the amide nitrogen of asparagine
(Asn) (in ER)
•O-linked: oligosaccharide chain is linked
to the hydroxyl group of serine or
threonine (in Golgi)
1.3.2 N-linked glycosylation in the RER
Lipid-linked
oligosaccharide chain
is added to the dolichol
by glycosyltransferase,
then be transferred to
linked to the N
terminus of
asparagines within
polypeptide by
oligosaccharine
protein transferase.
 1.3.2 Modification/processing of newly
synthesized proteins: the folding of proteins
The lumen of rER
contains:
These chaperones and
enzymes recognize
and bind to unfolded
or misfolded proteins
and give them correct
conformation;
Quality control: ensuring that misfolded proteins
do not leave ER.
 1.3.2 Modification/processing of newly
synthesized proteins : the formed of disulfide
bonds within polypeptide
The lumen of rER
contains:
Protein disulfide
isomerase ( PDI )
transfer incorrect
disulfide bonds to
correct disulfide
bonds within
polypeptide.
1.3.3 The transport of the proteins
The transport of the
proteins contains:
1.The formation of transport
Vesicle (secretory proteins);
2.The transport of
integral membrane proteins;
3.The transport of
soluble proteins of organelles.
1.3 The functions of the RER
1.Proteins synthesized on ribosomes of RER.
2.Modification and processing of newly
synthesized proteins ;
A.glycosylation in the RER.
B. the folding of proteins.
C. the formation of disulfide bonds within
polypeptide.
3. The transport of the proteins .
1.4 Functions of the SER
The side of cytoplasm
1.It takes part in the
synthesis of various
lipids:
phospholipids
(building membranes )
fatty acids
steroids
(e.g.,hormones).
The lumen of SER
flipase
1.4 Functions of the SER
Transport by
phospholipid
exchange
proteins (PEP)：
SER→other
organelles.
phospholipid exchange proteins (PEP)
1.4 Functions of the SER
2.Detoxification of organic compounds in
liver cells. It contains enzymes needed to
detoxify drugs.
3.Metabolism of heparin.
4.The SER serve as a storage place for
calcium.
•The SER serve as a
storage place for calcium
In the case of smooth
endoplasmic reticulum in
muscle cells, the tubules
serve as a store of calcium
which is released as one
step in the contraction
process of muscle. Calcium
pumps serve to move the
calcium.
1.4 Functions of the SER
1.Synthesis of lipid;
2.Detoxification of organic
compounds in liver cells;
3.Metabolism of heparin;
4.The SER serve as a storage place
for calcium.
What are the functional differences between the
RER and the SER?
Functions of the RER
Functions of the SER
1.Synthesis of lipid;
1.Proteins synthesized on
ribosomes of RER.
2.Detoxification of
2.Modification and
organic
compounds
in
processing of newly
liver cells;
synthesized proteins ;
A.glycosylation in the RER. 3.Metabolism of
B. the folding of proteins.
heparin;
C. the formation of
4.The
SER
serve
as
a
disulfide bonds within
storage place for
polypeptide.
3. The transport of the
calcium.
proteins .
Summary
•ER is a network of folded membranes that extend
through the cytoplasm to the nuclear membrane.
•There are two kinds of ER, rough and smooth.
•The functions of RER include the synthesis of protein,
modification/processing and quality control of newly
synthesized proteins.
•The SER has functions in several metabolic processes.
•It takes part in the synthesis of various lipids , fatty
acids and steroids, and also plays an important role in
carbohydrate metabolism, detoxification of the cell and
calcium storage.
Self-Quiz :Choosed-question
1.Where does protein synthesis take place
in eucaryotic cells ?
A.granum
B.nucleus
C.endoplasmic reticulum
D.golgi apparatus
Self-Quiz
2 .The ribosomes of prokaryotic cells are
found
A.in the golgi apparatus
B.free-floating
C.in the nucleus
D.in the endoplasmic reticulum
Self-Quiz
3.The smooth endoplasmic reticulum is the
area in a cell where ___ are synthesized.
A.polysaccharides;
B.proteins;
C.lipids;
D.DNA
SectionⅡ The Golgi complex
The Golgi apparatus
is a polarized structure consisting of an
oriented stack of discshaped cisternae surrounded by a swarm
of small vesicles.
2.1 The structure of Golgi complex
The Golgi complex
consists of a stack of
flattened 、 vesicles
and tubules.
The Golgi apparatus
has two distinct faces:
•a cis ,or forming face
•a trans,or maturing
face
2.2 The polarity of Golgi complex
The cis face is closely
associated with a
transitional RER.
In secretory cells ,the trans
face is the closest to the
plasma membrane.The large
secretory vesicles are found
in association with the trans
face of a Golgi stack. So it is
called the polarity organelle.
Image of nucleus, endoplasmic reticulum and Golgi apparatus.
(1) Nucleus. (2) Nuclear pore. (3) Rough endoplasmic reticulum
(RER). (4) Smooth endoplasmic reticulum (SER). (5) Ribosome on
the rough ER. (6) Proteins that are transported. (7) Transport
vesicle. (8) Golgi apparatus. (9) Cis face of the Golgi apparatus.
(10) Trans face of the Golgi apparatus. (11) Cisternae of the Golgi
apparatus.
2.2 The polarity of Golgi complex
The Golgi complex is
compartmentalized.
Phosphorylation occurs in
the Cis region. In other
regions, different types of
carbohydrates are added as
a glycoprotein passes
through the cisternae. This
figure illustrates the
different regions where
sugars like mannose ，
galactose , etc are added.
The final sorting is done in
the Trans Golgi complex. So
it is called the polarity
organelle
2.3 The functions of Golgi complex
2.3.1 Glycosylation in the Golgi complex
Golgi complex plays a key role in the
assembly of the carbohydrate component
of glycoproteins and glycolipids.
O-linked oligosaccharides takes place in
Golgi complex.
Oligosaccharide chain is linked to the
hydroxyl group of serine or threonine .
2.3.1 Glycosylation in the Golgi complex
The important role of Glycosylation :
1.One might suspect that they function to aid folding
and the transport process; for example,carbohydrate
as a marker during protein folding in ER and the use
of carbohydrate-binding lectins in guilding ER-toGolgi transport.
2.Limit the approach of other macromolecules to the
protein surface, more resist to digestion by proteases.
3.Regulatory roles in signaling through the cellsurface receptor notch, to allow these cells to respond
selectively to activating stimuli.
2.3.2 The processing、 sorting and
transport in the Golgi complex
The Golgi networks
are processing and
sorting stations
where proteins are
modified, segregated
and then shipped in
different directions.
Golgi complex and cell’s secretion
Constitutive secretion:
Continual,unregulated
discharge of material
from the cells
Regulated secretion :
The discharge of
products stored in
cytoplasmic granules,
require appropriate
stimulus(e.g.
neurotransmitter)
Can proteins be transported back to the RER ?
Sometimes vital proteins needed in the RER
are transported along with the other proteins to
the Golgi complex. The Golgi complex has a
mechanism for trapping them and sending them
back to the rough endoplasmic reticulum. This
cartoon shows the process.
The protein destined for secretion is red. The blue protein must
remain in the RER. The RER has inserted a receptor protein on the
membrane, it sends to the Golgi complex in the transitional vesicles
(shown in green). The ER protein receptor captures all of
the protein that carries the ER residency signal. Vesicles then bud
from the Golgi complex and move back to the RER. The receptor
can circulate and continue to return the proteins needed by the ER.
2.3 The functions of Golgi complex
2.3.1 Glycosylation in the Golgi complex
2.3.2 The processing、 sorting and
tran-sport in the Golgi complex
Summary
The Golgi complex consists of a stack of flattened、
vesicles and tubules.The Golgi apparatus has two
distinct faces:a cis face and a trans face. It is called
the polarity organelle .The Golgi complex receives
newly synthesized proteins and lipids from the ER
and then processing、sorting and distributes them
to the plasma membrane 、lysosomes and secretory
vesicles .Golgi is the site of O-linked glycosylation.
Self-Quiz
1. Which cell component consists of a stack
of smooth membrane elements, through
which newly synthesized proteins travel by
vesicles budding off and fusing while they
are being chemically modified and targeted
for export or other destinations?
A.cytoplasm
B.cell membrane
C.Golgi body
D.SER
E.RER
Self-Quiz
2.What is the function of
apparatus?
A.It produces DNA molecules
B.It propels the cell
C.It produces ribosomes
D.It secretes cell products
the
golgi
Self-Quiz
3. What is the correct sequence of
membrane compartments through which a
secretory protein moves from synthesis to
release from the cell?
A.SER → Golgi → RER → cell membrane
B.cell membrane → Golgi → RER → SER
C.RER → Golgi → cell membrane → SER
D.Golgi → RER → SER → cell membrane
E.RER → Golgi → cell membrane
Section Ⅲ The Lysosomes
3.1 The structure of the
lysosome:
Discovered in 1950 by Rene .
De .Duve, a Lysosome is a tiny
membrane-bound organelle
found in the cytoplasm of all
eucaryotic cells containing
various acid hydrolytic
enzymes that can digest every
kind of biological molecule.
3.1 The structure of the lysosome
Lysosomes is
common in animal
cells but rare in
plant.
Marker enzyme:
acid phosphatase.
3.1 The structure of the lysosome
•Lysosome membrane:
1.H+-pumps:
internal proton is kept high H+concentration by H+-ATPase
2.Glycosylated proteins:
may protect the lysosome from
self-digestion.
3.Transport proteins:
transporting digested materials.
3.2 Biogenesis of Lysosomes
1. A phosphate attached to the mannose residue.
3.2 Biogenesis of Lysosomes
2.This mannose-6 phosphate forms a sorting signal that
moves through the cisternae to the trans region where it
binds to a specific receptor.
3.2 Biogenesis of Lysosomes
3.After it binds to the receptor, it begins to bud and a
“coat” made of clathrin forms around the bud (to
strengthen it).
3.2 Biogenesis of Lysosomes
4.It moves away to fuse with a late endosome .
3.2 Biogenesis of Lysosomes
5.The phosphate is removed and hydrolase is
dissociated from the receptor.
3.2 Biogenesis of Lysosomes
6.The receptor is then recycled back to the Golgi
complex .
Biogenesis of Lysosomes
Late endosome
Lysosomal hydrolase precursor
Addition of phosphate
Mannose-6-phosphate(M-6-P)
Receptor-dependent
transport
ATP
ADP+Pi
From
RER
Clathrin coat
Binding to
M6P receptor
Dissociation at
acidic pH
RER
Cis golgi
network
Golgi apparatus
H+
内吞体
H=5
P
Removal of
phosphate
Mature
lysosomal
hydrolase
Trans golgi
network
M6P receptor in
budding vesicle
Mature lysosomes
3.3 The types of lysosomes
•Primary lysosome
•Secondary lysosomes
Primary Lys.
•Phagosome
•Autophagosome
•residual bodies
(lipofuscin)
Second Lys
3.3 The types of lysosomes
Primary lysosome are
newly formed by budding
from the Golgi
complex,and therefore
have not yet encountered
substrate for digestion
and with acid Hydrolytic
enzymes inactive.
3.3 The types of lysosome
Secondary lysosomes result
from the repeated fusion of
primary lysosomes with a
variety of membrane
bounded substrates and
active hydrolytic enzymes
within the lysosomes. The
bounded substrates may be
food 、bacterium、or worn
organalles and so on.
3.3 The types of lysosome
Phagosome is a kind
of secondary
lysosomes licked up
food or bacterium.
Autophagosome is a
kind of secondary
lysosomes licked up
ageing organelles.
3.3 The types of lysosome
The secondary lysosomes
digest the contents of
phagocytic or autophagic
vesicles to form residual
bodies that either
undergo exocytosis or are
retained in the cell as
lipofuscin granules.
3.4 The functions of Lysosomes
Lysosomes are involved in five major
cell functions:
D. The Functions of Lysosomes
1.Heterophagy;
2.autophagy;
3.The extracellular digest;
4. Autocytolysis;
3.4 The functions of Lysosomes
1. Heterophagy
Digestion of materical
of extracellular origin.
Lysosomes pick up foreign
invaders such as bacteria,
food and break them into
small pieces that can
hopefully be used again. If
they pick up a really
harmful invader, they will
eat it up and expel what is
left of it out of the cell so
that the debris can be
removed from the body.
3.4 The functions of Lysosomes
2.Autophagy
Digestion of materical of
intracellular origin.
Lysosomes also play a
key role in destroying old
organelles within the cell
and thus allow them to be
replaced with fresher,
more effective ones.This
process is known as
autophagy and is
accomplished in two
stages.
3.4 The functions of Lysosomes
Firstly, a membrane is donated by
the endoplasmic reticulum. This
membrane then surrounds the old
organelle.
Secondly ,a lysosome fuses with
this membrane to form an
autophagic vacuole. The lysosome
can safely enter it's enzyme
contents into this vacuole and
destroy the old organelle. The
electron micrograph shows a
lysosome in the process of
destroying a membrane bound
mitochondria.
3.The extracellular digest :Another function
of lysosome in the human occurs during
fertilization of the egg by the sperm. The
head of the sperm cell contains a package
of lysosomal material called the acrosome.
The enzymes from this are released
when the sperm makes contact with an
egg and they effectively bore a hole
through the cell membrane of the egg
allowing the sperm to enter.
3.4 The functions of Lysosomes
4.Autocytolysis :Lysosomes may also be
important in development.For instance , they
are responsible for the breakdown of a
tadpole’s tail as the tadpole develops into a frog.
In the process ,the lysosome releases hydrolases
to cytoplasm to digest the cell of oneself.
3.5 Lysosomes and Diseases
Autolysis: A break or leak in the membrane of lys
releases digestive enzymes into the cell which damages
the surrounding tissues.For example,The miner‘s disease
silicosis results from the uptake of silica fibres from the
dusty atmosphere of a coal mine by macrophages and
other phagocytic cells in the lungs. These fibres then
become enclosed in the lysosomes of these cells but they
cannot be digested by the enzymes. Instead they cause
the lysosome to leak it’s contents in quantities which
cannot be neutralized resulting in damage to the tissue of
the lungs.
5. Lysosomes and Diseases
Autolysis: The same process occurs in asbestos
workers resulting in the disease asbestosis. Both
conditions can be severely debilitating or even
fatal. It is also thought that as we age lysosomes
become “leaky” and can cause damage to our
own tissues. Rheumatoid Arthritis is thought to
occur partly due to damage caused to cartilage
cells in the joints by enzymes leaked from
lysosomes.
5. Lysosomes and Diseases
Lysosomal storage diseases are due to the
absence of one or more lysosomal enzymes,
and resulting in accumulation of material
in lysosomes as large inclusions.
1、Acid Maltase Deficiency (Lysosomal
Glycogen Storage Disease) which leads to
the accumulation of glycogen in muscle
tissue.
5. Lysosomes and Diseases
2、Tay-Sachs Disease is due to a deficiency in
one of the enzymes which breaks down certain
types of fat (lipid) called hexosaminidase A. As
a result of this deficiency， huge amounts of
lipids are deposited in neuronal (nerve) tissue
and leads to severe brain damage and nervous
degeneration. The disease is progressive and
terminal resulting in early death around 3
years of age.
5. Lysosomes and Diseases
3、Gaucher‘s Disease is due to the
deficiency of the lysosomal enzyme
glucocerebrosidase. The disease results in
liver and spleen enlargement and erosion
of the long bones such as the femur. If the
disease manifests itself in infancy there is
also brain damage causing learning
disability.
Summary
Lysosome is A membrane-bound organelle in the
cytoplasm of most cells containing various acid
hydrolases. Lysosomes are involved in four major
cell functions:Phagocytosis;Autophagy;
Extracellular digest;Autocytolysis. Primary lys fuse
with either phagocytic or autophagic vesicles,
forming residual bodies that either undergo
exocytosis or are retained in the cell as lipofuscin
granules.
Choosed-question
1.The cell organelle that contains acid
hydrolases is the
A.endoplasmic reticulum
B.lysosome
C.golgi
D.ribosome
Choosed-question
2.Lysosomes are found in ……
A.ribosomes
B.animal cells
C.enzymes
D.bacteria
Fill-blank questions
1.Endomembrane System include ER, _______, lysosome,
secretory vesicles.
2.There are two types of glycosylation :N-linked: linked
to the amide nitrogen of asparagine (Asn).These process
take place in _______;O-linked: linked to the hydroxyl
group serine or threonine via GalNac. These process take
place in RER and _________.
5. Primary lysosome are newly formed by budding from
__________.
The short-answer questions
1.what are the functions of the lysosome?
2.what are the differences between the
primary lysosome and secondary lysosome?