8.4 - Bacterial Protein Secretion Flashcards
What two things does secretion do for cells?
(1) Puts things out there so you can communicate and change the environment (such as degrading enzymes to extract nutrients
(2) Important for building cells (key assembly processes)
What bacteria have protein export to the cell membrane (inner membrane)
Both gram positive and gram negative bacteria
What is the signal recognition particle (SRP)
You start with protein synthesis on ribosome and you make peptide which has sequence recognized by rNA protein complex. This is called signal recognition particle.
It is a protein and RNA (could have come from RNA world)
How does protein export to the inner membrane work?
(1) Ribosome is paralyzed by SRP and moves towards the membrane to interact with FtsY
(2) Some integral membrane proteins are inserted directly
(3) Periplasmic proteins are secreted via the Sec system
How does protein secretion in eukaryotes work?
Also uses SRP but has different channels. SRP binds and arrests translation. Often translation is on a membrane like the ER. They have channels and proteins which cleave signal peptides but these evolved independently
What is protein transprot to the periplasmic space also called?
SecA-dependent export
Which bacteria use SecA-dependent export
Gram negative bacteria
What does the trigger factor do in secA-dependent export?
It allows the ribosome to complete synthesizing the pre-secretion protein
When the pre-secretion protein is ejected from the ribosome where does it go?
The protein wraps around the pilot protein SecB
What does SecB do?
Delivers the protein to SecA and SecYEG
What happens when the protein is delivered to SecA and SecYEG?
The protein is pushed through the SecYEG channel to the periplasm.
What is required to push the protein through the SecYEG channel?
ATP hydrolysis
What does SecA do during this secretion process?
SecA repeatedly releases the protein, withdraws, and pushes more of the protein through SecYEG.
What is the rols of LepB
LepB cleaves the signal sequence so that periplasmic chaperones can fold the protein
Can you delete any of the genes used for SecA-dependent secretion?
No it will kill the bacteria
What type of bacteria use ExoPortal domain secretion?
Gram positive
What processes do bacteria combine in ExoPortal domains?
Septation/Pepdoglycan synthesis and secretion
What is an ExoPortal?
A cluster of secretion systems and accessory factors at the cytoplasmic membrane in an anionic phospholipid domain.
Why do gram positive bacteria need an ExoPortal?
Gram-positive bacteria must also export proteins
across the cell membrane and then fold and process
them once they are secreted. However, Grampositive
bacteria lack a periplasmic space that could
facilitate interactions between newly secreted proteins
and the accessory processing proteins.
What are the main proteins in the ExoPortal?
HtrA, sortase, sec system components and chaperones
What does HtrA do?
Assists in pili formation and covalent attachment of proteins to the cell wall
What do sortases do?
Aids in maturation of secreted proteins.
What happens to the proteins that pass through the ExoPortal but are not secreted?
They have a transmembrane domain that achors the protein to the gram-positive cytoplasmic membrane. This is missing in gram-negative homologs. The anchored protein is extracellular but will not float away.
What happens in export of pre-folded proteins to the periplasm?
The protein is first made in the cytoplasm then is secreted out.
Why would you want to export a whole protein?
Some things you can only do in cytoplasm you cannot do in periplasmic space
What do prefolded proteins contain?
The amino acid motif RRXFXK within their N-terminal signal sequence. This sequence is called the “twin arginine motif”
What does the twin arginine motif target?
Proteins to the membrane-embedded twin arginine translocase (TAT)
What is TAT?
A transport complex that ships fully folded proteins across the cell membrane to the periplasm
What happens after the protein is translated and folded in the cytoplasn in preforded protein export?
The signal sequence of the protein binds to TatB-TatC complex. The complex can now recruit TatA monomers.
What to TatA monomers assemble into?
They assemble into translocase, with help from the proton motive force (PMF)
How is the protein transported?
The protein is transported through the TatA translocase. The signal sequence is cleaved by signal peptidase
What happens after the protein is transported?
The TatA channel dissociates and the system resets.
How are OM proteins exported from the cytoplasm to the periplasm?
Using the SecA dependent secretion system
What prevents aggregation of OM proteins as they traverse the periplasm?
Periplasmic chaperones
What is the beta barrel assebly machine (BAM)?
A complex that facilitates OMP assembly in the outer membrane.
How do you get secretion out of a cell?
By getting all the compartments to work together.
How is hemolysin secreted out of a cell?
Lots of bacteria destroy other cells like hemolysin. It destroys red blood cells. You have ABC cassette membrane protein which selectively effluxes hemolysin( the whole thing is like a cannon)
How are proteins secreted to the periplasm to make pap pili stick to bladder cells?
Proteins from the cytoplasm are secreted by the Sec system to the periplasm, where they are chaperoned by PapD to the site of assembly
What does PapC do?
PapC (usher) assembles the individual proteins in the proper order. Assembly starts with the tip protein, PapG which binds to carbohydrates on host membranes
How many and what pathwaysn do pap proteins illustrate?
3
1) SecA (for PapD “chaperones”
(2) SecA + BAM (for PapC (usher))
(3) SecA + PapCD (for remaining Paps)
How does the pap operon illustrate transcription termination?
o At end of operon transcription stops because of terminator
o There is also a terminator which works sometimes but not all of the time
o Working on proteins but do not need all of them in the same amount
oTip in operon is located in second part
How do flagella assemble?
In stages.
First stage executed and then next stage is executed only until first stage is done
How are inner membrane proteins of flagella made?
With a signal recognition protein (so are secreted) FtsY and SecYEG
How are periplasmic proteins of flagella made?
SecAB and SecYEG pathway
How are L ring proteins of flagella made?
Theorized to be BAM
What are Type 3 secretory systesm (T3SS)?
Type three secretion system is a protein appendage found in several Gram-negative bacteria. In pathogenic bacteria, the needle-like structure is used as a sensory probe to detect the presence of eukaryotic organisms and secrete proteins that help the bacteria infect them
How are extra large amounts of flagellin (FliC) proteins synthesized only when t3SS is ready?
Stages, preparations, sigma-28, and checkpoint control
How is FliA inactive at the start?
Because it is bound by FlgM
What happens whilst FliA is inactive?
The hook and basal body are assembled, and FliA delivers FlgM to this secretion channel
WHat happens after FlgM is exported out of the cell ?
FliA is now free to bind to core RNA polymerase
What does FliA initiate?
Transcription and genes encoding flagellar motor, the flagellin subunits of the flagellum, and the chemotaxis machinery.
