The need for protein targeting/ trafficking, general principles of protein transport across cellular membranes, nuclear import, mitochondrial import Flashcards Preview

BIOC2003 > The need for protein targeting/ trafficking, general principles of protein transport across cellular membranes, nuclear import, mitochondrial import > Flashcards

Flashcards in The need for protein targeting/ trafficking, general principles of protein transport across cellular membranes, nuclear import, mitochondrial import Deck (24)
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1
Q

Into what kinds of compartments are cells divided ?

A

Eukaryotic cells are divided into membrane-bound compartments called organelles ?

2
Q

What is the difference between the cytoplasm and the cytosol ?

A
Cell = cytoplasm + nucleus
Cytoplasm = cytosol + organelles
3
Q

Between which organelles/compartments can we observe gated transport ?

A

The cytosol and the nucleus.

4
Q

Between which organelles/compartments can we observe transmembrane transport ?

A

The cytosol and :

  • mitochondria
  • endoplasmic reticulum
  • plastids
  • peroxisomes
5
Q

Between which organelles/compartments can we observe vesicular transport ?

A
The ER and the golgi
The golgi and : 
- late endosomes
- lysosomes
- early endosomes
- the cell exterior
- secretory vesicles
... etc.
6
Q

Where do nuclear proteins begin synthesis ?

Where can these proteins be transported thereafter ?

A

• Nuclear-encoded proteins begin synthesis in the cytosol
• From the cytosol they can be transported to:
- nucleus
- mitochondria
- ER
- plastids e.g. chloroplasts, peroxisomes

7
Q

How does a protein know where to go in the cell ?

A

Through sorting signals :

  • signal sequences : exposed, single stretch of AAs often at the end of the polypeptide chain
  • signal patch : AAs contributing to the signal are separate until after the protein folds
8
Q

What are signals for :

  • nuclear import ?
  • mitochondrial import ?
  • ER import ?
A

Nuclear import : –PPKKKRKV– (lysine and arginine-rich sequences)
Mitochondrial import : +H3N-MLSLRQSIRFFKPATRT LCSSRYLL– (amphipathic α helix)
ER import : +H3N-MMSFVSLLLVGILFWATEAEQLTKCEVFQ– (hydrophobic amino acids)

9
Q

Are signals cleaved or uncleaved after targeting ?

A

Signal sequences can be cleaved or uncleaved after targeting.

10
Q

How does a protein (even with a signal) cross a lipid bilayer ?

A
  • A receptor recognizes signal
  • A Channel through which to guide protein
  • Receptors and channels are made of proteins
11
Q

What are nuclear localization signals (NLSs) ?

A

A NLS is an AA sequence that “tags” a protein for import into the cell nucleus by nuclear transport. Typically, this signal consists of one or more short sequences of positively charged lysines (K) or arginines (R) exposed on the protein surface.

12
Q

How do proteins enter the nucleus ?

A
  • Proteins enter the nucleus via NUCLEAR PORES •Small molecules (< 5000 daltons) can freely diffuse through nuclear pores
  • Larger molecules are imported via active transport
13
Q

What are nucleoporins ?

A

Nucleoporins = proteins that make up the nuclear pore complex :

  • cytosolic fibrils
  • annular subunits
  • lumenal subunits
  • column subunits
  • ring subunits
  • nuclear fibrils
  • nuclear baskets
14
Q

How big are nuclear pores ?

A

About 0.1um (100nm) wide

15
Q

How does nuclear protein import occur ?

A
  • the folded protein along w/ its NLS is recognized by a nuclear import receptor (karyopherin family) = importin
  • nucleoporins have “FG repeats” (Phe + Gly) that serve as binding sites for the import receptors
16
Q

How does nuclear protein export occur ?

A
  • the folded protein along w/ its NLS is recognized by a nuclear import receptor (karyopherin family) = exportin
  • nucleoporins have binding sites for the export receptors
17
Q

What is the role of the Ran-GTPase ?

A

The Ran-GTPase drives directional transport through nuclear pores :
- for nuclear import, Ran-GTP binds the nuclear import receptor in the nucleus whilst the cargo is delivered, and after export, Ran-GTP is hydrolyzed by Ran-GAP (Ran-GTPase Activating Protein) to Ran-GDP + Pi and releases the nuclear import receptor
- for nuclear export, Ran-GTP binds the nuclear export receptor (caring a protein w/ a nuclear export signal) in the nucleus and transports it out of the nucleus, where it is hydrolyzed by by Ran-GAP to Ran-GDP + Pi
Every time Ran-GDP re-enters the cell, Ran-GEF (situated on the chromatin in the nucleus) triggers the exchange of GDP for GTP

18
Q

Is nuclear protein import pre or post-translational ?

A

Post-translational, proteins are folded before import

19
Q

What is the signal for the transport of protein into the mitochondria (e.g. cytochrome oxidase, subunit IV) ?

A

An amphipathic α helix : positively charged residues on one side of helix, uncharged residues on the other

20
Q

Mitochondria are double-membrane- enclosed organelles.

Where does translocation occur ?

A

At special sites where the inner and outer membranes are close together.

21
Q

Are proteins imported in the mitochondria as folded or unfolded polypeptide chains ?

A

Unlike nuclear import, proteins are imported as unfolded polypeptide chains.

22
Q

How are matrix proteins imported ?

A
  • The signal sequence is recognized by a receptor (part of the translocon of the outer membrane [TOM] complex)
  • The unfolded polypeptide is translocated through TOM and transferred to the translocon of the inner membrane [TIM], specifically TIM23
  • The protein to be translocated is kept unfolded by the binding of cytosolic Hsp70 (ATP hydrolysis necessary). and as the polypeptide travels through TOM, Hsp70 molecules dissociate)
  • Translocation across the inner membrane requires the electrochemical gradient (+ve outside Vs -ve inside)
  • The signal sequence is cleaved by a protease and a mitochondrial Hsp70 binds to drive further translocation
  • Hsp60 helps the translocated protein to fold correctly
23
Q

How are proteins inserted into the outer mitochondrial membranes ?

A
  • The polypeptide is translocated through TOM, kept unfolded in the intermembrane space by cheprones
  • The polypeptide is then inserted by the SAM (Sorting and Assembly Machinery) complex into the outer membrane, where it fully folds
24
Q

How are proteins transported to :

  • the inner membrane (anchored) ?
  • the inter-membrane space ?
  • the inner membrane (integral) ?
A

Inner membrane (anchored) :
–> form the cytosol :
- polypeptide in translocated through TOM and TIM
- when the stop-transfer (hydrophobic region) reaches the inner membrane, the signal sequences (now inside the matrix) is cleaved
–> from the matrix (or form the cytosol and already cleaved but possesses a second signal) :
- polypeptide is integrated in the inner membrane via the OXA complex
Inter-membrane space :
- when strop transfer sequence reaches the inner membrane, the polypeptide is cleaved and free in the inter-membrane space
Inner membrane (intergal) :
- polypeptide translocated through TOM
- kept unfolded by inter-membrane space chaperones
- integrated into the inner membrane by the TIM 22 complex (requires electrochemical gradient)