‘ WAREHOUSE’ or ‘MANFACTURING UNIT ’

JUSTIN JOHNSON

M.Sc LIFE SCIENCE Ist Semester

SCHOOL OF LIFE SCIENCES

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.

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.

Discoverers

EMIL VERATTI

KEITH PORTER GEORGE PALADE

LOCATION AND DETAILS

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

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.

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.

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.

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.

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.

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.

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.

SIGNAL RECOGNITION PARTICLE

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.

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.

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.

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.

ALZHEIMERS

PARKINSON

BIPOLAR DISORDER

DIABETES

INFLAMMATION

DISEASES ASSOCIATED

THANK YOU