Endoplasmic Reticulum

Question 1

what process do proteins enter the ER by?

Answer 1

cotranslational translocation (type of transmembrane transport)

Question 2

by what process do protein leave the ER?

Answer 2

vesicular transport or retrotranslocation (type of transmembrane transport)

Question 3

what is the general construction of the rough and smooth ER?

Answer 3

rough: consists of stacks of flattened cisternae with a luminal space 20-30 nm high.
smooth: consists of a network of fine tubules 30-60 nm in diameter

Question 4

What does the smooth ER do?

Answer 4

1. site of synthesis of new lipids. lipid synthesizing enzymes are found on the cytosolic surface of the smooth ER. lipids are made from precursor molecules present in cytoplasm
2. stores and release Ca@+ ions in too cytosol. bilayer is studded with Ca2+ pumps that pump ions into ER lumen while buffering proteins in lumen bind Ca2+. release channels allow ions to exit into cytosol

Question 5

how can you separate rough and smooth ER?

Answer 5

through sucrose gradient centrifugation. rough ER is heavier and floats below smooth

Question 6

what’s the quick definition of co-translational translocation?

Answer 6

the protein is moved (translocated) across the lipid bilayer at the same time as it is being translated from its mRNA

Question 7

the signal hypothesis

Answer 7

if a nascent polypeptide chain contains an ER targeting signal sequence, the ribosome translating that protein will dock onto a translator protein complex (receptor protein) located within the ER bilayer. The signal sequence is ultimately cleaved off and degraded.

Question 8

what are SRPs?

Answer 8

signal recognition particles are within the cytosol and bind to the emerging signal sequence and also the ribosome. they trigger a pause in translation until ribosome is docked. (may do so via a hinge which blocks translation)

Question 9

what are SRP receptors?

Answer 9

the signal recognition particle (SRP) docks onto a SRP receptor on the ER bilayer. docking brings the ribosome and signal sequence into contact with the translator channel complex

Question 10

steps in the docking of a ribosome to the ER bilayer (5)

Answer 10

1. SRP binds signal sequence
2. binding of SRP triggers a pause in translation
3. the pause allows ribosome with SRP to bind SRP receptor in the ER membrane, positioned near a protein translator channel complex
4. ribosome/SRP/receptor complex docks with protein tranlocator channel complex in ER membrane
5. nascent polypeptide is injected into ER lumen through an aqueous pore in the channel complex

Question 11

what opens the aqueous pore in the protein translator channel?

Answer 11

binding the signal sequence

Question 12

why is there a plug in the translocator channels?

Answer 12

the plug prevents Ca2+ ions from escaping the ER lumen through inactive channels.

Question 13

what is important about the seam of the translator complex?

Answer 13

the seam allows for lateral gating and release of peptide chain into the bilayer.

Question 14

how does lateral release occur?

Answer 14

after the polypeptide has been completely translocated, a peptidase residing in the ER membrane cleaves off the signal sequence, releasing the translocated soluble protein into the ER lumen. the signal sequence is laterally released into the bilayer

Question 15

what is a stop transfer sequence?

Answer 15

a hydrophobic amino acid sequence within the center of a protein that can signal and trigger lateral gating and release of the polypeptide into the ER bilayer.
creates transmembrane proteins

Question 16

How is a transmembrane protein oriented so it is inserted in the membrane correctly?

Answer 16

the region flanking the start transfer sequence can help orient the protein. charges orient it as the ER Lum favors neg and cytosol favors pos

Question 17

how are proteins stitched into membranes as multipass proteins?

Answer 17

multiple start transfer and stop transfer sequences. The start transfer sequence will get protein translocated into the ER lumen, but when stop transfer sequence is encountered it will halt and eject the protein into the membrane. a new start sequence is necessary to continue the stitch pattern

Question 18

what is post translational translocation?

Answer 18

the protein is moved across the lipid bilayer after it has been translated from its mRNA and released in to the cytosol. Involves free ribosomes and only occurs in eukaryotes and bacteria.

Question 19

how is post translational translocations different from co-translational translocation?

Answer 19

the complex is the same (Sec61), however post translational translocation uses BiP, a chaperone protein, to pull the protein into the ER lumen and ensure no premature folding.

Question 20

how can post translational translocated proteins be inserted into the membrane?

Answer 20

a pre targeting complex brings the protein to Get3, an ATPase that will bring the protein to another Get complex in the membrane, bringing the protein into the membrane

Question 21

What is glycosylation? describe the process

Answer 21

the covalent attachment of oligosaccharides to create glycoproteins. N type is the most frequently occurring glycosylation, it involves a preformed oligosaccharide tree anchored in the membrane which is attached to a protein’s internal consensus sequence by oligosaccharyl transferase as it recognizes the protein sequence.

Question 22

how is the oligosaccharide tree synthesized?

Answer 22

they are created in the ER membrane. Dolichol is anchored in the membrane. First 2 N-acetylglucosamine are added, then 5 mannose. then the entire structure is flipped into the lumen where it is finished and ready to be used. (flipping is key thing here)

Question 23

How is an incompletely folded protein prevented from leaving the ER? (roles of calnexin, glucosidase, and glucose transferase)

Answer 23

Calnexin binds to unfolded proteins having terminal glucose residues on oligosaccharide attachment. A glucosidase removes these glucose residues, however if protein fails to completely fold, glucose residues are re-added by glucose transferase. This ensures continued binding of the unfolded protein to Calnexin, retaining in the ER. correctly folded proteins are not recognized by glucose transferase and can exit ER free of glucose residues.

Question 24

what happens to terminally misfolded proteins? (roles of AAA-ATPase, N-glycanase, ubiquitin, and proteasome)

Answer 24

Retrotranslocation. AAA-ATPase pulls the protein out into cytosol where their oligosaccharide attachments are removed by N-glycanase, they are tagged with a chain of ubiquitin proteins, and digested within a proteasome.

Question 25

what do misfolded proteins activate and what is the response of the cell?

Answer 25

misfolded proteins activate sensors like IRE1, causing a regulator to travel to the nucleus and activate production of more chaperone proteins to promote correct folding

Question 26

how are proteins attached to GPI anchors?

Answer 26

ER enzymes attach a GPI anchor to proteins having appropriate consensus sequence

Question 27

what causes Chagas Disease?

Answer 27

it is caused by kissing bugs carrying infectious trypanosomes (unicellular parasitic protozoans). parasites activate a phospholipase enzyme that cuts off the GPI anchor and allows the previously GPI anchored protein to be shed from cells surface. parasite avoids attack by immune system

Question 28

what causes African sleeping disease?

Answer 28

Tsetse fly takes blood meal and infects host with infectious trypanosome. parasites activate a phospholipase enzyme that cuts off the GPI anchor and allows the previously GPI anchored protein to be shed from cells surface. parasite avoids attack by immune system

Question 29

where are phospholipids made?

Answer 29

All are made in the cytosolic monolayer of the ER

Question 30

how are phospholipids made? don’t need details

Answer 30

fatty acids are brought in from cytosol and components are added to make phospholipid. involves fatty acid binding protein, acyl transferase, phosphatase, choline phosphotransferase

Question 31

how are phospholipids distributed in the ER membrane and PM?

Answer 31

ER membrane has phospholipids evenly distributed on both sides.
PM has uneven distribution with specific phospholipids in each monolayer.

Question 32

what do scramblase and flippase do in the ER and PM?

Answer 32

ER scramblase flips all phospholipids indiscriminately to achieve even distribution in the membrane. it is constitutively active (always flipping)
PM flippase is specific for PE and PS, it flips them unidirectionally into the cytosolic monolayer.
PM scramblase is not constitutively active. It’s regulated by cell-signaling mechanisms and is only active during specific events such as apoptosis