Flashcards in lecture 7 Deck (29)
What sorts of compartments are there within a cell?
- nucleus surrounded by nuclear membrane
- continuous with endoplasmic reticulum
- golgi apparatus
What are the topological relationships between compartments in a eukaryotic cell?
- the plasma membrane separates the inside of the cell from the outside
- but not quite this simple
- the inside of the organelles is more similar to the outside of the cell
- not completely true that e.g. the inside of the golgi apparatus is exactly the same as outside the cell but whatever was on the outer surface of the plasma membrane will be on the inner surface of the organelles
What are the three basic types of protein trafficking?
- gated transport: in and out of the nucleus
- transmembrane transport: transports proteins into peroxisomes, mitochondria, plastids, and the ER
- vesicular transport: transport between vesicles (ER, Golgi, secretory, late endosome, lysosome, early endosome, cell exterior)
- the first forms of transport are very individual while vesicular transport might involve the movement of many hundreds/thousands of proteins at once
What directs proteins to their correct 'address' in the cell?
- signal sequences are like Post codes for proteins
e.g. import into nucleus= pro-pro-lys-lys-lys-arg-lys-val
while export from nucleus = leu- ala-leu-lys-leu-ala-gly-leu-asp-ile
Describe the nuclear envelope
- the nucleus is surrounded by a double-membrane envelope that is penetrated by nuclear pores and is continuous with the endoplasmic reticulum
What is the nuclear pore complex?
- the nuclear pore is not just a simple hole
- an intricate structure of various proteins and fibrils
- like a basketball hoop
- fibrils important for recognising proteins that enter the nucleus
- it is a gated diffusion barrier
- the limit size for free diffusion is around 60 kD (amino acids, ions, small organic molecules can freely diffuse through this pore)
- larger molecules can be actively transported through the NPC
What are the nuclear import receptors?
- proteins that need to go into the nucleus (cargo proteins) bind to specific nuclear import receptors via nuclear localisation signals (NLS)
- nuclear import receptors interact with NPC proteins to transfer cargo in/out of the nucleus
What gives directionality to nuclear transport?
- functions like a molecular switch
- ON in the nucleus, OFF in the cytosol
- Ran-GAP is located in the cytosol - assists the Ran-GTPase to hydrolyze the GTP back to GDP
- Ran-GEF is located in the nucleus - guanine exchange factor, assist in the exchange from GDP to GTP
Describe the model of nuclear transport.
1. Cargo with NLS bind NIR
2. NIR shuttles into the nucleus via the NPC
3. Ran GTP binds to NIR to discharge the cargo
4. NIR shuttles out of the nucleus via the NPC
5. Ran-GAP activates Ran-GTP hydrolysis to form Ran-GDP in the cytosol
6. Ran-GDO actually has its own NIR to shuttle it back into the nucleus where it is converted to Ran-GTP by Ran-GEF
- Export of Cargo from the nucleus is similar except that:
- Ran-GTP promotes binding of cargo (with nuclear export signal) to Nuclear Export Receptor (NER/exportin)
- Ran-GDP dissociates cargo from the NER
For both export and import, cycling of the NIR and NER through the pore is independent of Ran-GTP and cargo
Ran-GTP provides directionality.
What is the importance of the NLS?
Nuclear localisation requires an intact and functional NLS:
- normal Lys-rich sequence targets a protein to nucleus
- mutation of Lys->Thr causes cytoplasmic retention
- Add an NLS to a cytoplasmic protein -> nuclear localisation
What are the particular mechanisms/complexes of different organelles in regards to transmembrane transport of proteins?
- mitochondria: TOM/TIM complexes
- chloroplasts - SRP-like, Translocators
- peroxisomes - Peroxins
- Endoplasmic reticulum: Ribosome, SRP/Sec61 translocators
- all of these require energy: usually ATP/GTP hydrolysis
What does transmembrane transport mean?
- transport of a protein ACROSS a membrane
- NOT transport of 'transmembrane proteins'
How does transmembrane transport occur across a mitochondria?
- Cytosolic protein associated with small Hsp70 proteins keeping it in an unfolded state
- Binds to a receptor on the outer mitochondrial membrane that is part of the TOM complex
- With the addition of energy the Hsp70 proteins dissociate and the protein is fed through a translocator in the TOM complex
- then passess through the TIM complex on the inner mitochondrial membrane (this also requires energy)
How does transmembrane transport occur across the endoplasmic reticulum?
- the act of getting proteins into the ER couples translation with their transport
- in the cytosol there are free ribosomes/polyribosomes which will translate proteins
- proteins that are being transported into the ER end up being translated by ribosomes bound to the ER
- before it is even its fully folded state it is fed through the translocator into the ER, where it then completes folding
- transport into the ER is the first step in getting proteins into other organelles - from here it goes via vesicular transport
How do ER signal peptides and SRP direct ribosomes to the ER membrane?
- peptide synthesis is initiated on FREE ribosomes
- signal sequence of nascent polypeptide emerges from ribosome, binds an SRP (Signal Recognition Particle) and pauses translation
- SRP binds SRP receptor and directs ribosomes to the ER membrane
- Ribosome pore docks with translocator and SRP released
- Translation contintues into the ER lumen
Describe the translocation of a soluble protein across the ER membrane.
- translocator consists of 4x Sec61 complexes
- pore opens when the ribosome (+nascent peptide) binds translocator and polypeptide feeds through
- the ER signal peptide acts as a "start-transfer" signal
- ER signal cleaved by peptidase, exists translocator and degraded
- mature protein deposited in the lumen
How is a single-pass transmembrane protein integrated into the ER?
Similar to secreted protein BUT:
- 2nd stop-transfer sequence (hydrophobic) stops transfer but not synthesis
- carboxyl tail ends in the cytoplasm not ER lumen
How is a double-pass membrane protein inserted into the plasma membrane?
- internal signal peptide binds translocator; the amino terminal is left in the cytoplasm and middle peptide region feeds through translocator
- 2nd stop-transfer signal stops translocation but not synthesis
- carboxyl tail ends in the cytoplasm but not ER lumen
- start-transfer not cleaved since internal
Give an example of a multipass membrane protein
- some proteins have multiple transmembrane domains
- can be mapped by amino acid hydrophobicity plots
- rhodopsin - g protein - has 7 transmembrane domains
What happens when a protein enters the ER?
- protein glycosylation occurs in rough ER as soon as proteins enter the lumen
- transfer of precursor oligosaccharide (glucose, mannose, N-acetylglucosamine) by transferase enzyme associated with translocator (oligosaccharyl transferase)
What is vesicular transport?
Transport vesicles bud off from one compartment and fuse with another
What is conversation of topology?
Vesicle contents are always separated from cytosol (inside vesicle = extracellular space)
What are the three kinds of vesicular transport?
- biosynthetic secretory pathway
- retrieval pathways
What are the intracellular compartments in the eukaryotic cell involved in the biosynthetic-secretory and endocytic pathways?
(- nuclear envelope continuous with)
- endoplasmic reticulum
- golgi apparatus (cisternae)
- secretory vesicle
- late endosome
- early endosome
- plasma membrane
What are coat proteins?
- transport vesicles bud off from coated regions of membranes
- the coat assists in concentrating specific membrane proteins and forms the vesicle by causing curvature of the membrane
- there are three coat proteins: clathrin (cell membrane, trans Golgi network), COPI (Golgi apparatus), and COPII (ER)
Describe clathrin and its function
- clathrin forms 'clathrin-coated pits' on the plasma membrane (and trans golgi) which then forms the vesicles
- three clathrin heavy chains and three clathrin light chains combine to form a structure called a triskelion
- has a 3D strucutre which is what causes budding to occur
- tips of the heavy chains bind to adaptor proteins which are bound to cargo membrane protein
- forms a cage
- the assembly of the coat induces curvature to the membrane
- adaptins bind both clathrin triskelions and cargo receptors
- once the vesicle is fully formed the clathrin molecules are released to form the naked transport vesicle
- other proteins in the naked transport vesicle tell it where to go
What is dynamin?
Dynamin is a GTPase that destabilises the bud neck so that the lipid bilayers flow together
- blocked by some dynamin mutations
How does a naked vesicle know where to go?
Rab proteins guide vesicle targeting and SNAREs mediate membrane fusion.
Complementary sets of vesicle snare (v-snares) and target snares (t-snares) determine the selectivity of transport-vesicle docking
Together these proteins cause the vesicle to dock at the correct location and then fuse with the membrane.
Rab proteins associate with Rab effector (tethering protein) - specific Rab/effector for specific locations
v-snares and t-snares tightly associate and assist in the fusion of the membrane