Endoplasmic reticulm

1. ‘ WAREHOUSE’ or ‘MANFACTURING UNIT ’
JUSTIN JOHNSON
M.Sc LIFE SCIENCE Ist Semester
SCHOOL OF LIFE SCIENCES

2. HISTORY
 Discovered in 1902 by Italian Scientist Emilio Verrati.
 He was student of Golgi and used Golgi's staining procedures and found a
new subcellular structure.
 Despite of his careful observation he was not able to convince the scientific
community, that such an organelle existed.
 Therefore his work was disregarded and put away, while research on golgi
apparatus, other organelles went dashing forward, ER was left behind.
 In retrospect, it is difficult to explain how in an era of excitement and
interest in sub cellular structure this organelle was slow to be
acknowledged.

3. Rediscovery
 Technological advancement enabled for the rediscovery of ER.
 1953- Keith Porter developed electron microscopy techniques that allowed
him to observe net like (reticulum) structure within (endo) the cytoplasm
(plastic).
 Hence named Endoplastic Reticulum.
 In 1954 he teamed up with father of modern cell biology, George Palade
and together they obtained high resolution images and finally proved the
existence of this organelle.
 Over 50 years after its discovery the ER stepped into the limelight and, was
accepted as a bonafide organelle attracting much curiosity and becoming
the object of many investigation.

4. Discoverers
EMIL VERATTI
KEITH PORTER
GEORGE PALADE

5. LOCATION AND DETAILS

6. ENDOPLASMIC RETICULUM
 Make and package proteins and
lipids
 Much like an assembly line
 Found in eukaryotic organisms
 Forms an interconnected network of
flattened, membrane-enclosed sacs
or tubes known as cisternae.
 Largest organelle in eukaryotic cell.
 It provides separate chemical
environment which allows for correct
protein folding.
 Two types
Rough Endoplasmic Reticulum
Smooth Endoplasmic Reticulum

7. FUNCTIONS OF ER
 Translocation of proteins (such as secretory proteins) across the ER membrane.
 Integration of proteins into the plasma membrane.
 Folding and modification of proteins in the ER lumen.
 Synthesis of phospholipids and steroids on the cytosolic side of the ER membrane.
 Storage of calcium ion in the lumen and their regulated release into the cytosol.
 Carbohydrate metabolism.

8. How the studies done?
 A major contribution to the study of the ER came with a protocol published
by Palade outlining a method to isolate ER- derived microsomes.
 Microsomes represent small authentic versions of ER, still capable of protein
translocation, glycolysation, Ca2+ uptake and release.
 Studded with ribosomes – rough microsomes
 Ribosomes are always found on outside surface, so the interior of the
microsome is biochemically equivalent to the lumenal space of the ER.
 If mRNA encoding a secreted protein translated on free ribosomes, protein
produced was larger than the normal secreted protein, however when
microsomes were added now the translated protein cleaved in to correct
size.

10. SMOOTH ENDOPLASTIC RETICULUM
 It is highly curved and tubular.
 It forms an interconnecting system of pipelines which form a network
throughout the cytoplasm.
 It is involved in the production of steroid hormones in the adrenal cortex
and endocrine glands.
 Lipid composition of the ER membrane is different to that of other cell
compartments.
 Has large abundance of phosphatidyl choline and a very low concentration
of cholestrol.
 It is also very fluid and disordered due to its large proportion of
unsaturated fatty acids.
 It is more prominent in some cells than in others, depending on cell function.

12. Contd….
 Synthesizes nearly all major classes of lipids but mainly PC, enzymes are
membrane facing the cytosol where their substrates are present.
 Sequesters almost all of the Ca2+ from the cytosol.
 It is achieved by Ca2+ pump and high concentrations of calcium binding
protein within the lumen.
 Specialized smooth ER is dedicated to the release and reuptake of Ca2+.
 Most widely used example of this is the sarcoplasmic reticulum, found in
muscle cells.
 Calcium ion release and reuptake triggers myofibril contraction and
relaxation respectively in muscle.

14. ROUGH ENDOPLASMIC RETICULUM
 Flattish sealed sac that is continuous with the nuclear membrane.
 Studded on its outer surface with ribosomes.
 Found throughout the cell but the density is higher near the nucleus and the
golgi apparatus.
 It has abundant translocon pores.
 Ribosomes are free to attach at these sites to synthesize proteins and
transport them directly into the ER lumen, after which the ribosomes can
detach.
 The presence of ribosomes studded on the membranes is what gives the
rough ER its name.

15. FUNCTIONS OF RER
 Protein folding
 Assembly of multi-subunit proteins
 Disulphide bond formation
 This requires an oxidising environment whereas the cytosol provides a
reducing environment. An oxidising environment is found within the ER lumen
which also contains disulphide isomerase, an enzyme which facilitates
disulphide bond formation.
 Glycosylation – the initial stages occur on specific asparagine residues
through the calnexin/calreticulin cycle.
 Degradation of misfolded proteins through the ubiquitin proteasome
pathway.

17. Targeting proteins to ER
 Proteins can be translocated into the ER either during their synthesis on
membrane bound ribosomes (co translational translocation) or after their
translation has been completed on free ribosomes in the cytosol
(posttranslational translocation).
 In mammalian cells mostly co translationally whereas both ways are used in
yeast.
 Ribosomes engaged in synthesis of proteins are targeted to ER by a signal
sequence at the amino terminus of the growing polypeptide chain.
 These signal sequences are short stretches of hydrophobic amino acids that
are usually cleaved from the pp chain during its transfer to ER lumen.

18. SIGNAL RECOGNITION PARTICLE

20. Formation of Tertiary Structure in Protein
 If a polypepetide is translocated into the ER from a membrane bound
ribosome it is known as co-translational translocation.
 The folding of a polypeptide into its correct three dimensional protein
structure is mediated by molecular chaperones.
 Chaperones bind to unfloded polypeptides, stabilising them to prevent
incorrect folding. One of the primary chaperone proteins is binding
immunoglobulin protein(BiP).
 BiP belongs to a heat sensitive protein family (hsp70) and has both a
protein binding and an ATPase domain.
 BiP is also used to seal the translocon pore when not occupied by a
ribosome.
 If a polypeptide is incorporated into the ER from a free ribosome it is
known as post-tranlational translocation.

22. Ubiquitin Proteasome Pathway
 If a protein fails to fold correctly, it is marked for degradation. This process
involves BiP, other chaperone proteins, protein disulphide isomerase and
other supporting proteins.
 If high levels of misfolded proteins are detectes, the competition for BiP
triggers a stress response (the unfolded protein response). This leads to
inhibition of protein synthesis, increased expression of chaperones and an
increase in the degradation of secretory pathway mRNAs.
 This responde halts protein production and synthesises more chaperones
which can help to destroy or revive the high volume of misfolded proteins.
 If these steps do not decrease the levels of misfolded proteins, apoptosis in
induces, destroying the affected cell.

24. Secretory Pathway
 The RER exports folded proteins within vesicles via the secretory pathway:
 RER Golgi apparatus Secretory vesicles Target.
 This transport is bidirectional; it allows for the recycling of valuable, soluble
proteins such as BiP. There are retention signals on BiP which signal the
Golgi to initiate retrograde transport back to the ER.
 This process is driven by a protein-protein interaction on the cytosolic face
of the ER membrane. Once a vesicle carrying BiP is formed, a coat of COPII
proteins cover the vesicle and guide it towards Golgi. A group of Proteins
called SNAREs then mediate the de-coating process, which is associated
with GTP hydrolysis. The de-coating process must occur in order for the
Golgi to recognise the vesicle membrane, ensuring that the subsequent
fusion of membranes I with the correct target membrane.

26. CURRENT RESEARCH
 Aberrant Lipid Metabolism disrupts calcium
homeostasis causing liver endoplasmic reticulum
stress in obesity.
 The ubiquitylation (a regulatory protein) machinery
of the endoplasmic reticulum.
 From endoplasmic reticulum stress to the
inflammatory response.

27. ALZHEIMERS
PARKINSON
BIPOLAR DISORDER
DIABETES
INFLAMMATION
DISEASES ASSOCIATED

28. THANK YOU