Cytology - Ch.6 - Endoplasmic Reticulum

what is the endomembrane system?

a group of membranous organelles comprising ER, Glogi, endosomes, lysosomes, nuclear membs, and the vacuoles

the endomembrane system is involved in many crucial processes such as

synthesis and maturation of carbs, lipids and proteins -- also involved in sorting, targeting ,packing of the synthesized material - play an imp role in bulk transport - intracellular digestion & processing

why are the mitochondria and chloroplasts not part of the endomembrane system ?

since their membs are independent of the other cytoplasmic membs

how are the RER & SER different?

structurally (organization) and functionally

are the RER & SER continuous?

yes they are physically continuous

give an ex of when RER & SER cooperate

lipid synthesis

what do the SER & RER consist of?

membranous system that encloses a lumen

RER & SER divide the cytoplasm into

cytosol and lumen (that they enclose)

is the RER extensive?

yes

what does the RER consist of ?

interconnected flattened sacks (cisternae) where ribosomes are attached at the cytosolic face

where are the ribosomes of the RER found?

attached to the outer cytosolic face

the RER memb is continuous with

the outer nuclear memb

are there ribosomes on the outer nuclear memb ?

yes attached to the cytosolic face

what is the SER made of ?

interconnected tubular structures without ribosomes

are there ribosomes in the SER?

no

diff amount of ER depending on the cells activity ..... elaborate (+ex)

-cells that involved in active protein synthesis (ex: salivary glands -pancreatic cells) have very extensive RER

- cells involved in active steroid synthesis (ex: Leydig cells) have very extensive SER

give ex of cells that have similar amount of ERs

hepatocyte

where is the SER abundant?

skeletal muscle fibers - kidney tubule cells - steroid-producing endocrine glands

what is the SER called in skeletal muscle fibers ?

sarcoplasmic reticulum

main functions of the SER

-synthesis of steroid hormones and lipids

-detoxification of xenobiotics

-dephosphorylation of glucose 6-phosphate

-sequestering calcium ions

-peroxisome memb production & proliferation

where does the synthesis of steroid hormones take place? (+ex)

endocrine cells (Leydig cells - adrenal cortex)

how does the SER play a role in steroid synthesis?

bc steroid synthesis by endocrine cells needs specific enzymes that are present in the SER memb & lumen

how is the SER involved in lipid synthesis?

integral proteins of the SER memb in hepatocytes are responsible for the production of lipids of the lipoproteins that circulate the blood( LDL -HDL ... ) -- also involved in phospholipids synthesis

what are xenobiotics ? give ex

xenobiotics are foreign chemicals such as polycyclic aromatic HC (PAH) - ethanol - barbiturates

what do PAH come from?

incomplete combustion smoke(ex: tobacco smoke)

what do ethanol and barbiturates do ?

promote the proliferation of SER in liver cells

in what organs does the xenobiotics detoxification occur?

mainly in the liver but the skin - lungs - intestines significantly contribute to this process

what is detoxification?

the transformation of toxic hydrophobic parent chemicals into a less toxic hydrophilic metabolite which is readily eliminated by the kidney with urine or by the liver with bile products

what enzymes carry out detoxification ? (type + name)

oxygen transferring enzymes named oxygenases having broad specificity for wide variety of substrates

why is the SER an imp organelle in detoxification process?

bc it has some of the monooxygenase enzymes - the cytochromes p450 that are mainly present in the SER

where is glucose stored ? which of the storages dephosphorylates it?

stored in the liver and muscles - dephosphrylation takes place in the liver

where does glucose-6-phosphate dephosphorylation take place ? elaborate on the sugar storage there

in the hepatocytes that store glucose as glycogen granules in the cytosol next to the SER membranes

if the body isn't supplied by glucose from the intestines .... what happens to supply it with enerygy ?

it uses the liver glycogen reserve by the process of glycogenolysis

how is glucose 6-phosphate obtained starting from glycogen

phosphorylase adds phosphate add converts glycogen into glucose 1-phosphate that it isomerized into glucose 6-phosphate

can glucose 6-phosphate be released from the cell?

no it is not released into blood and is kept inside the cytoplasm bc the cell memb is not permeable to phosphorylated sugar (no channel for them)

how is dephosphorylation of glucose 6-phosphate done ? leads to?

SER membs contains glucose 6-phosphatase enzyme which removes phosphate from glucose 6-phosphate and converts it into glucose that can be transported out of the cell through its specific transporter

the enzyme for glucose 6-phosphate is named ...... found in ??? what family does it belong to?

glucose 6-phosphatase found in the SER membranes ...... it's an integral protein

does the glycogen found in muscles also undergo dephosphorylation ?

no - only the one in the liver

where is the SER involved in sequestering calcium ions?

in the cisternal space (lumen)

other than sequestering, what does the SER do to calcium ions? explain

it stores them and releases them upon stimulation -- there is a high C of calcium-binding protein in the SER which helps store this cation

where is there a higher C of Ca2+ .... lumen or cytosol?

cytosol bc it is stored in the lumen mostly

what are some processes that are triggered by Ca2+ delivery into the cytosol?

fusion of secretory vesicles and muscle contraction

in muscle fibers, how do action potentials arrive by the motor neuron to the SER ?

via transverse tubules (transverse extensions of the plasma memb that touch the SER memb)

what happens at the level of SER in muscle contraction?

AP reaches SER memb - calcium ions in the SER memb open and calcium ions go out and bind to troponin triggering interaction btwn actin/myosin/tropomyosin/troponin => triggering contraction

how does muscle relaxation occur?

no more AP - Ca2+ channels close - Ca2+ pumps of SER transport it actively to the lumen - cytosolic Ca2+ C decreases and the muscles relax

calcium binds to what to trigger interactions that trigger contraction?

troponin

is it true to say that calcium concentration in the cytosol controls cytoskeleton functions ?

yes

RER functions

-synthesis of proteins by memb bound ribosomes

-glycosylation of proteins and maturation

-biosynthesis of lipids and memb proliferation

-export of vesicles to Golgi apparatus

what is the site of protein synthesis? why ?

the RER bc ribosomes are attached to its memb

what is different btwn free and attached ribosomes?

the proteins they produce are destined to different cell compartments

the proteins produced by free ribosomes are exported to ??? name the delivery

their specific compartments after translation (post-translational delivery) due to their specific signals -address codes

ex of proteins produced by free ribosomes

-cytosolic proteins

-peroxisomal proteins

-most mitochondrial & most chloroplast proteins

-proteins exported to the nucleoplasm

-extrinsic plasma memb proteins that are at the cytosolic face (ex: ankyrin)

what proteins are produced by attached ribosomes?

-integral plasma memb proteins

-integral proteins of the endomemb system membs

-proteins that are found inside the endomemb system

-proteins that are secreted outside the cell

name the type of delivery of proteins produced by attached ribosomes.... explain it

co-translational delivery .... i.e: sent to their destinations parallel to translation so the ribosomes are brought close to the RER memb during translation in order to release the aa chain (partially of totally) into the RER lumen to mature

is the ribosome attached to the memb during initiation and early elongation ?

no

when does the N -terminus of the polypeptide chain emerge from the ribosome?

when the ribosome has polymerized about 20 to 30 aa of the nascent polypeptide chain

when will the ribosome be attracted to the RER memb?

if the N -terminus includes a signal peptide

what is the signal peptide that must be found in the N - terminus of the polypeptide chain for the ribosome to be attracted to the RER memb?

a stretch of 6 to 15 non polar aa

bacterial ribosomes attach to what side of the plasma memb ? how ?

to the inner face of the bacterial plasma memb - in a process similar to what happens in eukaryotes

what are the steps of ribosome attraction to the RER memb ?

the emerged signal peptide is recognized and bound by a complex ribonucleoprotein named SRP - SRP binds to its receptor that is coupled to a translocon where the ribosome binds

what does SRP stand for ? what is its type ? what is it made of ?

Signal recognition particle - ribonucleoprotein - 6 diff protein subunits and 7S RNA

other than the signal peptide, the SRP has affinity to

the ribosome

what happens to elongation during the attachment of ribosome to RER memb ?

elongation is temp arrested after SRP binds to the signal peptide - arrested until the SRP binds to its specific receptor in the RER memb

when does translation resume (in the process of ribosome attachment to RER memb) ? How?

when the SRP and ribosome have met their respective receptors , SRP leaves - the signal peptide binds to an inner site in the translocon and elongation continues

what is a translocon?

an aqueous channel made for proteins

when does protein maturation begin ?

immediately after the growing chain enters the RER lumen

polypeptides lose their signal peptides due to ?

the activity of a proteolytic enzyme named signal peptidase

where is signal peptidase found?

attached to the translocon on the lumen site

polypeptides lose their signal peptide before or after translation finishes ?

most lose it before the end of translation

carb motifs are branched at diff positions of the aa chain by ?

oligo-saccharyl-transferases

what are oligosaccharyl-transferases?

integral proteins of the RER memb

what catalyzes the formation of disulfide bridges btwn cysteine residues in the polypeptide?

an specific enzyme named protein disulfide isomerase (PDI)

where does proteins processing take place?

starts in the RER but is completed in the Golgi cisternae

what is protein processing imp for?

the proteins' activities

why do we say "if the N-terminus includes signal peptide " ?

bc the signal peptide may be located internally (far from the N-terminus)

what does the mechanism of ribosome attached apply to ? what doesn't it apply to?

applies to all the proteins in the lumen of the endomemb system - secreted proteins - plasma memb proteins (except the extrinsic ones of the cytosolic face)

what about the insertion of the polypeptides of the integral plasma memb + EM proteins ?

their insertion though the translocon is stopped due to the presence of a stop-transfer sequence

what is the stop-transfer sequence made of ? what will it form ?

a stretch of about 15 hydrophobic aa which will form the spanning helix that fit in the lipid bilayer

talking of the depart of the stop-transfer sequence from the translocon?

before or after the end of translation towards the lipid bilayer by lateral opening of the translocon along its side

due to insertion of the polypeptides in the RER lumen .... where are the parts of the polypeptide found ?

the N-terminus extremity is in the RER lumen , the C-terminus extremity is in the cytosol , third one is in the lipid bilayer

is inverted orientation (in inserted proteins in RER lumen ) common ? how is that ? ex?

yes for ex in multispanning proteins- N-terminus extremity is in the cytosol

the inversion of orientation in proteins insertion in RER requires

rotation of the polypeptide chain by 180 before the transmemb segment leaves the translocon .... or it requires the inversion of insertion direction by successive stop-transfer sequences

can we say that the translocon recognizes the diff parts of the polypeptides and orients them properly in the memb?

yes

what is glycosylation?

branching of sugar motifs on proteins & lipids

glycosylation occurs for most of the proteins produced by ..... ribosomes

attached ribosomes

the added carb motifs that are added during glycosylation to proteins are needed for

the maturation and activity of many proteins

a precise combination of about ...... monosaccharides composes the oligosacchride of a glycoprotein in the ER

14

the most commonly added motifs (to form the oligosaccharide of a glycoprotein) are

glucose - mannose - galactose - fucose - sialic acid - N-acetylglucosamine

do random aa serve a branching points for oligosaccharides in glycosylation?

no only specific one

what are the types of glycosylation ?

N-glycosylation and O-glycosylation

the glycosylation that occurs in the ER is of what type?

N-glycosylation

what is N-glycosylation?

linking of oligosacchride to the amino grp of a specifc asparagine aa

where does O-glycosylation take place ? what happens?

in the Golgi apparatus - the oligosaccharide recipient is the OH grp of a specific serine or threonine

what enzymes catalyze glycosylation ?what are they ?

glycosyl-transferases & oligosaccharyl-transferases ..... intergral proteins in the ER memb

is glycosylation one step?

no it is a multi step rxn

what is the acceptor of monosaccharides when preparing the oligosaccharide of glycosylation?

dolichol phosphate

what is dolichol?

ER memb lipid - consist of isoprene polymer CH2=C(CH3)-CH=CH2how

what do the enzymes that catalyze glycosylation do ?

glycosyl-transferases prepare the oligosaccharide entirely on a dolichol phosphate - oligosaccharyl-transferases transfer that oligosacchride to the growing polypeptide

glycosylation is parallel to protein folding ?

yes

what monitors protein folding?

specific protein called chaperons (calnexin & calreticulin)