Protein translocation and secretion systems Flashcards
What are translocated proteins destined for?
to move proteins into 4 areas in a gram negative cell, which are required to move hydrophilic proteins through hydrophobic membrane barriers:
4 things
1. inner membrane
2. periplasm
3. outer membrane
4. outside the cell (secreted)
What is the general secretion pathway and what does it do?
SecYEG, it inserts proteins into inner membrane and translocates proteins into periplasm
How does protein insertion work in the inner membrane
4 steps
1. hydrophobic N-terminal signal sequence of the protein emerges when the ribosome is translating
2. signal sequence is bound by signal reconginition particle (SRP) and paralyzes the ribosomes and moves it to FtsY
3. complex is relocated to FtsY protein
4. polypeptide co-translationally inserted and goes to the secYEG translocon (sercetion system)
* this is where membrane spanning region makes hydrophobic parts
* when the protein is being inserted into the membrane it is connected to the RNA which is connected to the ribosome and the ribosome is connected to the DNA
How does protein insertion work in the periplasm
Periplasm conatins important proteins that bind nutrients for transport into the cell and it gets delievered to the periplasm via general secretion pathway involving SecA protein
Steps:
1. the protein is translated in the cytoplasm and the presecretion protein is captured and bind by SecB which helps to keep the protein in an unfolded state and help make secretion through a membrane easier
2. SecB then delievers the protein to SecA, a structure peripherally associated with the membrane-spanning SecYEG translocon (the pore in which the protein will be passed
3. SecA is an ATPase and binds to ATP allowing it to insert deep into the SecYEG channel shoving about 20 amino acids of the target into the export channel and ATP hydrolysis casues SecA to release the protein and withdraw
4. Proteins have cleavable signal sequences at their amino-terminal ends and following translocation into the periplasm, the periplamsic signal peptidases (LepB) snips off the amino-terminal signal sequence of the protein
5. The mature protein is released into the periplasm and the periplasmic proteins devlievered by the Sec system arrive unfolded and inactive, and require periplasmic chaperones to guide their tertiary folding
How does protein insertion work in the outer membrane
steps:
1. translocation into periplasm via Sec-dependent pathway
2. periplasmic chaperones escort protein to outer membrane
3. beta barrel assembly machine (BAM) facilitates insertion into outer membrane (unknown mechanism)
How does protein insertion work in the outside the cell
The periplasmic chaperones escort proteins to outer membrane and a beta barrel assembly machine forms
What is the beta barrel assembly machine look like
top from bottom
beta barrel
alpha-helices
this is TolC
type I secretion system
what is type I secretion system?
has three parts top to bottom
Tol C
Hly D
Hly B
HlyA
the bacteria secretes HlyA and then HlyA binds to HlyB and causes a conformation change in HlyB and HlyD complex and makes it bind to TolC causing a channel where HlyA can travel through
It is done using ABC = ATP binding cassette
What is type III secretion system?
causes bacteria to be able to inject proteins directly into the host cell cytoplasm as they have tiny syringes and allows them to invavde a eukaryotic host cell and become an intracellular parasite
it has a needle and the end of the needle is made of single protein, and it also has an inner membrane ring, connector, and outer membrane ring
the tip of needle needs to come in contact with the host cell and they deliver effector protein
champerone proteins bind to them and delievers them loosely to the base of the needle and ATPase triggers the release of effector proteins and helps them to unfold and beginning their joruney to the host
How does type III secretion work? (injectosome)
it has LEE (locus enterocyte effacement) and it carries genes that will be turned on as long as the bacteria senses enough of its own for it to be productive excerise, quorum sensing
it releases Tir, intimin, T3SS components and others
Tir is a translocated intimin receptor
* It is injected into the host cell
* Only EPEC and EHEC E.coli can do this to humans
* Tir binds to intimin which is located on the bacteria cell (surface of the bacterial cell) and it injects Tir into the host cell which then causes the tir to bind to the intimin of the bacteria, and then there is a tight association and helps prevent the bacteria get washed away in the gut
Which bacteria does injectsome to humans?
EPEC and EHEC E.coli
