Protein Trafficking & Secretion Flashcards
Learning Outcomes
- 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.
You already know some of this…
- 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)
You already know some of this…
- Biochemical messengers are
secreted from cells - Nuclear receptors translocate
from cytoplasm → nucleus
And, very briefly, …
- 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
So……
How do the»_space;>30,000
proteins find their
correct cellular locations?
Proteins are directed to their destinations
via signals in the amino acid sequence
How can proteins be delivered to their
appropriate destinations?
Proteins are produced by ribosomes and
function in many different locations
Protein Destinations → nucleus
- Nuclear localization signal (NLS) anywhere in protein
- Translation in cytoplasm, NLS not removed
- import via nuclear pore complex (NPC)
Protein Destinations → nucleus
Import & Export via Nuclear Pore Complexes (NPCs)
- NPCs span both layers of nuclear envelope
- Protein NLS binds a nuclear import receptor
(importin) - Complex binds NPC proteins → import
Import & Export via Nuclear Pore Complexes (NPCs)
- 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)
Import & Export via Nuclear Pore Complexes (NPCs)
Import & Export via Nuclear Pore Complexes (NPCs)
- 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!
Import & Export via Nuclear Pore Complexes (NPCs)
Import & Export via Nuclear Pore Complexes (NPCs)
Import & Export via Nuclear Pore Complexes (NPCs)
- 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)
Protein Destinations: mitochondria (or plastid)
- 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
Translocation into Mitochondria is via TOM/TIM
Protein Translocators
- 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
Import & Export via Nuclear Pore Complexes (NPCs)
Translocation into Mitochondria is via TOM/TIM
Protein Translocators
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
Translocation into Mitochondria is via TOM/TIM
Protein Translocators
Translocation into Mitochondria is via TOM/TIM
Protein Translocators
Protein Destinations: secretion or membrane
- 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
Vesicles are
not simply
passive sacks!
ER translation/translocation
- 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
ER translation/translocation
Proteins are packed into vesicles for
trafficking & secretion
Vesicles are assembled by coat proteins
- Required for formation
- Discarded before fusion
- Clathrin
- Golgi plasma membrane
- COPI (coat protein 1)
- Golgi → Golgi
- Golgi → ER
- COPII (coat protein 2)
- ER → Golgi
Vesicles are assembled by coat proteins
Rabs and SNAREs mediate docking & fusion
Rabs and SNAREs mediate docking & fusion
- Rabs give the address
- Rabs bind Rab effectors →
motor/tether to membrane - SNAREs mediate fusion
- v-SNAREs on vesicle
- t-SNAREs on target membrane
Golgi apparatus
Golgi apparatus
- 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
Exocytosis can be constitutive or regulated
Neurotransmission – regulated secretion
Summary
Summary
- 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.
