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Molecular Biology Flashcards > Protein Trafficking & Secretion > Flashcards

Protein Trafficking & Secretion Flashcards

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1
Q

Learning Outcomes

A
  • Outline how proteins with distinct localisation signals are
    delivered to the nucleus and mitochondrion.
  • Be able to list the steps involved producing a secreted protein.
  • Outline the functions of vesicles.
  • Identify the roles of coat proteins, Rabs, and SNAREs.
  • Describe the difference between regulated and unregulated
    exocytosis in secretion.
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2
Q

You already know some of this…

A
  • Cells have many membranous
    organelles that regulate
    protein transport (Yu Suk Choi’s
    lectures)
  • Translation produces
    peptides/proteins from
    ribosomes within the
    cytoplasm/Rough ER (Mitali
    Sarkar-Tyson’s lectures)
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2
Q

You already know some of this…

A
  • Biochemical messengers are
    secreted from cells
  • Nuclear receptors translocate
    from cytoplasm → nucleus
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3
Q

And, very briefly, …

A
  • Secreted proteins are
    transcribed in the nucleus,
    translated on ribosomes,
    often at the rough ER,
    modified by the Golgi
    apparatus, packaged into
    vesicles and exocytosed in
    response to stimulus
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4
Q

So……
How do the&raquo_space;>30,000
proteins find their
correct cellular locations?

A

Proteins are directed to their destinations
via signals in the amino acid sequence

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4
Q

How can proteins be delivered to their
appropriate destinations?

A
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5
Q

Proteins are produced by ribosomes and
function in many different locations

A
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6
Q

Protein Destinations → nucleus

A
  • Nuclear localization signal (NLS) anywhere in protein
  • Translation in cytoplasm, NLS not removed
  • import via nuclear pore complex (NPC)
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6
Q

Protein Destinations → nucleus

A
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7
Q

Import & Export via Nuclear Pore Complexes (NPCs)

A
  • NPCs span both layers of nuclear envelope
  • Protein NLS binds a nuclear import receptor
    (importin)
  • Complex binds NPC proteins → import
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8
Q

Import & Export via Nuclear Pore Complexes (NPCs)

A
  • NPCs span both layers of nuclear envelope
  • Protein NLS binds a nuclear import receptor
    (importin)
  • Complex binds NPC proteins → import
  • Ran-GTP binds complex → complex dissociates
    → protein delivered
  • Importin is recycled (control of import)
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9
Q

Import & Export via Nuclear Pore Complexes (NPCs)

A
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10
Q

Import & Export via Nuclear Pore Complexes (NPCs)

A
  • NPCs span both layers of nuclear envelope
  • Protein NLS binds a nuclear import receptor
    (importin)
  • Complex binds NPC proteins → import
  • Ran-GTP binds complex → complex dissociates
    → protein delivered
  • Importin is recycled (control of import)
  • Importin-Ran-GTP complex leaves nucleus →
    GAP hydrolises GTP→GDP → dissociates
  • Importin is now free to import proteins again!
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10
Q

Import & Export via Nuclear Pore Complexes (NPCs)

A
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11
Q

Import & Export via Nuclear Pore Complexes (NPCs)

A
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11
Q

Import & Export via Nuclear Pore Complexes (NPCs)

A
  • NPCs span both layers of nuclear envelope
  • Protein NLS binds a nuclear import receptor
    (importin)
  • Complex binds NPC proteins → import
  • Ran-GTP binds complex → complex dissociates
    → protein delivered
  • Importin is recycled (control of import)
  • Importin-Ran-GTP complex leaves nucleus →
    GAP hydrolises GTP→GDP → dissociates
  • Importin is now free to import proteins again!
  • Nuclear export involves a NES, nuclear export
    receptor, and Ran-GTP-binding (GTP→GDP
    hydrolysis)
12
Q

Protein Destinations: mitochondria (or plastid)

A
  • Produced as mitochondrial precursor proteins
  • N-terminal localization signal (translated first)
  • Translation in cytoplasm, signal removed after import
  • Folding structure blocked by chaperones (hsp70)
  • Import via TOM/TIM complex
12
Q

Translocation into Mitochondria is via TOM/TIM
Protein Translocators

A
  • TOM: transporter outer membrane complex
  • TIM: transporter inner membrane complex
  • SAM: sorting and assembly machinery
  • inserts proteins into outer membrane
  • OXA: oxidase activity
  • inserts proteins into inner membrane
12
Q

Import & Export via Nuclear Pore Complexes (NPCs)

A
13
Q

Translocation into Mitochondria is via TOM/TIM
Protein Translocators

A

Import to matrix space
* Protein signal binds receptor protein in TOM
* Signal region translocates to TIM
* ATP hydrolysis drives import, requires [H+
]
gradient (supplied by ETC)
* Signal peptide cleaved → mature protein

13
Q

Translocation into Mitochondria is via TOM/TIM
Protein Translocators

A
14
Q

Translocation into Mitochondria is via TOM/TIM
Protein Translocators

A
15
Q

Protein Destinations: secretion or membrane

A
  • N-terminal ER signal sequence (cleaved inside ER)
  • Co-translational import
  • begins in cytoplasm → signal peptide/ribosome → rough ER
  • Shuttled to Golgi apparatus
  • Shuttled to membrane → inserted/secreted
16
Q

Vesicles are
not simply
passive sacks!

A
16
Q

ER translation/translocation

A
  • Nascent signal peptide binds signal recognition particle (SRP)
  • SRP pauses translation, targets ribosome-growing peptide to SRP
    receptors on ER
  • Inserts ribosome-peptide into translocator, translation continues
17
Q

ER translation/translocation

A
18
Q

Proteins are packed into vesicles for
trafficking & secretion

A
19
Q

Vesicles are assembled by coat proteins

A
  • Required for formation
  • Discarded before fusion
  • Clathrin
  • Golgi plasma membrane
  • COPI (coat protein 1)
  • Golgi → Golgi
  • Golgi → ER
  • COPII (coat protein 2)
  • ER → Golgi
20
Q

Vesicles are assembled by coat proteins

A
20
Q

Rabs and SNAREs mediate docking & fusion

A
20
Q

Rabs and SNAREs mediate docking & fusion

A
  • Rabs give the address
  • Rabs bind Rab effectors →
    motor/tether to membrane
  • SNAREs mediate fusion
  • v-SNAREs on vesicle
  • t-SNAREs on target membrane
20
Q
A
21
Q

Golgi apparatus

A
21
Q

Golgi apparatus

A
  • cis-Golgi receives and sorts all ER proteins
  • Post-translational modification of proteins for secretion
    (glycosylation, sulfation)
  • Modified vesicles bud off from trans-Golgi surface
22
Q

Exocytosis can be constitutive or regulated

A
23
Q

Neurotransmission – regulated secretion

A
24
Q

Summary

A
25
Q

Summary

A
  • Proteins are targeted to their subcellular location by
    localisation signal peptides that interact with specific transport
    molecules.
  • Secreted proteins are translated at the RER, modified in the
    Golgi apparatus and assembled into vesicles.
  • Coat proteins enable packaging of vesicles, Rab proteins target
    vesicles to a specific destination, and SNAREs facilitate docking
    and fusion of vesicles.
  • Regulated exocytosis involves the production of secretory
    vesicles, their storage, and release in response to signal.

Molecular Biology Flashcards (28 decks)

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