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Cell Bio > L8. Intracellular compartments & transport I > Flashcards

L8. Intracellular compartments & transport I Flashcards

1
Q

main function of the endoplasmic reticulum

A
  • synthesis of most lipids
  • synthesis of proteins for distribution to many organelles and to the plasma membrane
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2
Q

main function of the Golgi apparatus

A
  • modification, sorting, and packaging of proteins and lipids
  • for either secretion or delivery to another organelle
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3
Q

main function of lysosomes

A

break down different biomolecules

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4
Q

main function of peroxisomes

A

break down of toxic molecules using H2O2

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5
Q

explain the evolution of the eukaryotic endomembrane system

A

an ancient prokaryotic archaea with an engulfed bacteria invaginates and creates the nuclear envelope

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6
Q

what are the three mechanisms of protein transport

A
  1. transport through nuclear pores
  2. transport across membranes
  3. transport by vesicles
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7
Q

mechanisms of protein transport - nuclear pores

A
  • proteins moving from the cytosol into the nucleus
  • protein needs to be folded
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8
Q

mechanisms of protein transport - across membranes

A
  • moving from the cytosol into the ER, mitochondria, or chloroplast
  • transported by protein translocators
  • protein needs to be unfolded
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9
Q

mechanisms of protein transport - by vesicles

A
  • moving onward from the ER, from one compartment to another
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10
Q

what are signal sequences

A
  • it is a sorting signal on a protein
  • it is both necessary and sufficient to direct a protein to a particular destination
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11
Q

transport through nuclear pores - explain the structure of the outer nuclear membrane

A
  • it is continuous with the ER membrane
  • the nuclear membrane is penetrated by nuclear pores
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12
Q

transport through nuclear pores - what are nuclear pores

A
  • they form the gates through which molecules enter or leave the nucleus
  • only allows small, water-soluble molecules to pass freely
  • large molecules will need to have the appropriate sorting signal
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13
Q

transport through nuclear pores: nuclear pores - what is the appropriate sorting signal

A
  • nuclear localization signal (NLS)
  • NLS is recognized by nuclear import receptors
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14
Q

transport through nuclear pores - explain the import of proteins into the nucleus

A
  • nuclear import receptors recognize nuclear localization signal (NLS)
  • entry into nuclear pore causes the cargo to be released
  • the receptor then returns to the cytosol
  • this is all mediated by GTP hydrolysis
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15
Q

transport through nuclear pores - explain how nuclear transport is mediated by GTP hydrolysis

A
  • as the nuclear import receptor takes in a protein into the nucleus, it encounters Ran-GTP
  • Ran-GTP binds to the import receptor and causes it to release the nuclear protein
  • the receptor (still carrying Ran-GTP) is taken out of the nucleus
  • outside, an accessory protein hydrolyzes the GTP turning it to Ran-GDP
  • Ran-GDP then falls off the import receptor
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16
Q

transport through nuclear pores: nuclear transport is mediated by GTP hydrolysis - what is Ran-GAP

A
  • it is in the cytosol and causes Ran-GDP to predominate in the cytosol
  • GTPase-Activating Protein
17
Q

transport through nuclear pores: nuclear transport is mediated by GTP hydrolysis - what is Ran-GEF

A
  • it is in the nucleus and causes Ran-GTP to predominate in the nucleus
  • Guanine Exchange Factor
18
Q

transport through nuclear pores: nuclear transport is mediated by GTP hydrolysis - what is Ran

A

monomeric GTPase

19
Q

transport across membranes: mitochondria - explain how proteins are unfolded during import to the mitochondria

A
  • both outer and inner membranes must be crossed
  • receptor recognizes the mitochondrial signal sequence
  • the receptor then interacts with a protein translocator in the outer membrane
  • the protein/receptor/translocator complex diffuses in the outer membrane and encounters the inner membrane translocator
  • the two translocators then transport the protein across both membranes while unfolding the protein
  • once in the mitochondrial matrix, the signal sequence is cleaved off
20
Q

transport across membranes: ER- how is the ER network extensive

A
  • most proteins begin to be threaded across the ER membrane before the polypeptide chain has been fully synthesized
  • this means that the ribosome synthesizing the protein has to be attached to the ER membrane (rough ER)
21
Q

transport across membranes - how are common pools of ribosomes used to synthesize proteins

A
  • ribosomes remain in cytosol when they are translating proteins with no ER signal
  • ribosomes will be directed to the ER when they encounter a ER signal sequence
  • these ribosomes will bind to each mRNA to form a polyribosome
  • at the end of protein synthesis, the subunits are released back into the cytosol to rejoin the common pool
22
Q

transport across membranes: ER - what must happen for growing polypeptide chain to go into the ER

A
  • a signal recognition particle (SRP) binds to the ER signal sequence and ribosome
  • this causes protein synthesis to be slowed down
  • the ribosome/protein/SRP complex binds to the SRP receptor on the ER
  • this causes the SRP to be released and the ribosome will be passed to a translocator
  • protein synthesis will then resume in the ER
23
Q

transport across membranes: ER - what happens after protein synthesis is resumed

A
  • a translocator binds to the signal sequence and threads the protein through as a loop
  • the signal peptide will be cleaved by a signal peptidase and will be degraded
24
Q

transport across membranes: ER - what happens after protein synthesis is finished and the polypeptide becomes a soluble protein

A
  • the soluble protein is released into the ER lumen
  • the translocation complex pore closes
25
Q

transport across membranes: ER - explain how a single-pass transmembrane protein is retained in the bilayer

A
  • the N-terminal signal initiates translocation
  • a second hydrophobic sequence act as a stop-transfer sequence
  • as the stop-transfer sequence enters the translocation channel, the channel discharges the growing protein sideways
  • the N-terminal signal peptide is cleaved and protein synthesis continues in the cytolytic side
26
Q

transport across membranes: ER - explain how a double-pass transmembrane protein is retained in the bilayer

A
  • internal signal sequence acts as a start-transfer sequence and as an anchor
  • second hydrophobic sequence acts as a stop-transfer sequence
  • when stop-transfer sequence enters the translocation channel, the channel will discharge both the signal sequence and the stop-transfer sequence into the bilayer
  • this causes the protein to be grown sideways
  • neither sequences are cleaved and protein synthesis will continue on the cytolytic side
27
Q

transport across membranes: ER - single-pass N-C terminus orientation

A
  • in ER lumen: C in cytosol, N in organelle
  • in cytosol being transported to membrane: N will move towards the outside of the cell with C staying in the cytosol
28
Q

transport across membranes: ER - double-pass N-C terminus orientation

A

C and N always faces the cytosol

Cell Bio (21 decks)

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