MCB 6: Intracellular Transport and Membrane Trafficking I Flashcards
1. List the main membrane-enclosed organelles and summarise their functions 2. Classify the types of intracellular protein transport 3. Outline the mechanisms by which proteins enter the nucleus and mitochondria 4. Outline the pathway and cellular locations for synthesis, post-translational modification and exocytosis of a secreted protein (partially covered in Session 7)
1
Q
How can cells perform many reactions at the same time?
A
- all eukaryotic cells have membrane-enclosed organelles
- these internal membranes allow different metabolic processes to be carried out at the same time
2
Q
Describe the basic architecture of the nucleus
A
- it is surrounded by a double membrane, called the nuclear envelope
- it communicates with the cytosol through the nuclear pores
- the outer membrane of the nuclear envelope is continuous with the membrane of the RER
3
Q
Describe the basic architecture of the endoplasmic reticulum
A
- this is a highly dynamic system of interconnected, membranous sacs and tubes
- it is a major site of synthesis of new membrane
- some ER is studded on the surface with ribosomes
- ER is continuously organised: motor proteins moving along microtubules can pull out sections of ER to form extended tubules which fuse and form a network
4
Q
Describe the basic architecture of the golgi apparatus
A
- a stack of flattened membrane-enclosed sacs
- usually close to the nucleus
- modifies and packages lipids and proteins, then releases them to other parts of the cell
5
Q
Briefly describe the function of transport vesicles
A
- transport vesicles carry proteins and lipids made in the ER to the Golgi apparatus
- they also transport modified proteins and lipids from the Golgi apparatus to the plasma membrane
6
Q
Briefly describe the structure of mitochondria
A
- they have two highly specialised membranes
- called the outer and an inner membrane
- the space between the two membranes is called intermembrane space
- the space inside the the inner membrane is called the matrix
7
Q
Describe the orientation of membrane-closed organelles and vesicles
A
- membranes retain their orientation during transfer between cell compartments
- this is also the case with membrane proteins, with the cytosolic part always facing the cytosol
8
Q
What is the lumen?
A
-
9
Q
What is the cytosol?
A
- cytosol is the cytoplasm that fills the space around the organelles
10
Q
What are the three mechanisms proteins can be transported into organelles?
A
- gated transport (through nuclear pores)
- transport cross membranes
- vesicular transport
11
Q
Describe gated transport (through nuclear pores)
A
- e.g. nuclear import of proteins
- proteins move from the cytosol into the nucleus through nuclear pores
- nuclear pores act as selective gates as some specific proteins are actively transported, but smaller molecules can freely diffuse through
12
Q
Describe transport across membranes
A
- e.g. newly synthesised proteins into the ER
- during protein synthesis, proteins are transported from the cytosol into the ER by protein translocators
- note when protein is imported into mitochondria, chloroplasts or peroxisomes, they have to be unfolded to enter
13
Q
Describe vesicular transport
A
- e.g. inter-organelle transport
- proteins are transported by transport vesicles
- they pinch off from the membranes of one organelle, then fuse with the membrane of another
- vesicles deliver soluble proteins, as well as proteins and lipids that are part of the vesicle membrane
14
Q
How are proteins directed to the correct compartment?
A
- proteins usually have a short stretch of amino acids called signal sequences or import sequences
- this directs the protein to their correct location
- if it is not present, then the protein is usually found in the cytosol
15
Q
Describe how the nuclear pore complex forms a gate
A
- the pore is lined with many different proteins
- some of these fill the centre of the channels and form mesh-like structures
- these stop large molecules from passing in, but allow small, water-soluble molecules to freely pass into and out of the nucleus
16
Q
Describe this diagram of nuclear pore complexes
A
- mesh-like network of proteins in the centre of the pore
- cytosolic fibrils protruding out from both sides of the pore
- the fibrils sticking into the nucleus form a basket-like structure
17
Q
Describe the steps of how nuclear proteins important from the cytosol?
A
- nuclear proteins are first made in the cytosol
- there needs to be a sorting signal called the nuclear localisation signal to be imported into the nucleus
- this is on the nuclear protein (cargo)
- the newly synthesised protein forms the ‘cargo’ of the receptor
- the receptor interacts with the cytosolic fibrils, which guides it towards the nuclear pore
- the receptor and cargo move randomly in the gel-like meshwork until a local passageway is found and the receptor and cargo can enter the nucleus
- once inside, the cargo is released
- this required energy supplied by the hydrolysis of GTP
- the empty nuclear receptor returns to the cytosol for reuse
18
Q
How are proteins imported into the mitochondria?
A
- a lot of the mitochondrial proteins are encoded in the nucleus and translated into protein in the cytosol
- precursor proteins have a short section of amino acids called a signal sequence, that has an affinity for a specialised protein called an import receptor
- the import receptor is associated with a protein translocator in the outer membrane
- the complex of receptor, mitochondrial protein and translocator diffuses in the outer membrane until it encounters a second translocator in the inner membrane
- the two translocator proteins transport the mitochondrial protein across both membranes, unfolding it in the process
- inside the mitochondrion, chaperone proteins help pull the protein in
- they bind to the precursor protein and prevent the chain from backsliding through the translocation tunnel
- once inside, an enzyme called signal peptidase cleaves the signal sequences from the precursor
- the chaperone proteins are released as the protein folds into its 3D structure
20
Q
Describe the two populations of ribosomes
A
the only difference between the population of ribosomes is the protein they are making at any given time
Polyribosomes Free in the Cytosol:
- two ribosomal subunits that bind to mRNA to form the polypeptide chain
- several of these ribosomes form polyribosomes
- they make all other proteins encoded in nuclear DNA
Polyribosomes Bound to Cytosolic Face of the ER:
- they make all the proteins being translocated into the ER
21
Q
Do free ribosomes synthesise proteins with a signal sequence?
A
- all proteins they synthesise do not have a signal sequence
- but proteins that go to the ER have a signal sequence
22
Q
What are mitoribososmes?
A
23
Q
Explain the stages of protein import into the ER
A
24
Q
Describe the ER translocation of double-pass transmembrane proteins
A
- A double-pass transmembrane protein has an internal ER signal sequence
- this sequence acts as a start transfer sequence and helps to anchor the final membrane protein in the membrane - Like N-terminal signal sequences, the start transfer sequence directs the ribosome to the ER
- The polypeptide chain continues to be threaded through the translocation channel until a stop transfer sequence enter the channel, at which point the channel discharges both sequences into the lipid bilayer
- Neither start transfer nor stop transfer sequences are cleaved off and the entire polypeptide chain remains anchored as double-pass membrane protein
