Lecture 14 Flashcards
Define “Exotoxins” (general):
Exotoxins: proteins deliberately secreted by pathogens
- they disrupt the host cell
Name a mechanism by which bacterial infections spread?
Exotoxins
What were the 3 groups of exotoxins listed in class - Name
- AB toxins
- Pore-forming toxins
- Superantigens
Describe the mechanism/ components of AB toxins
- AB toxins interfere with processes inside the host cell
- It is a complex composed of 2 components:
A : active component
B: binding component - B: binds to a receptor on the host cell, triggering endocytosis which helps the A component enter
- A: is an enzyme which is the cause of the toxic effect
“Diphtheria Toxin” was an example of?
Diphtheria Toxin = AB toxin
Describe the mechanism of Diphtheria Toxin infection.
What does this toxin damage?
- Note this is an AB toxin
1) The B chain attaches to a receptor on the host cell membrane
2) The B component activates Endocytosis
3) The A component is released from the vacuole into the cell
4) The A factor: - modified elongation factor
- blocks translation
note: 1 toxin can modify all EF-2 in cell
This toxin damages:
- cardiac and nerve tissue
Describe the mechanism and target of “Shiga Toxin”
- Shiga Toxin is an AB toxin
- Enters host cells using B component
- The A component targets the ribosome
- A component removes a nucleobase from rRNA, disabling the ribosome
- Causes damage to vascular endothelium, leads to inflammation and GI bleeding
Describe the mechanism of pore forming toxins
Pore forming toxins are a type of AB toxins
- they insert into the host cell membrane, forming channels
- this disrupts the ion gradient and cytoplasmic contents leak out
- water enters the cells and it swells
- the cell is lysed
- lysis of the host cell releases nutrients and helps bacteria escape from phagosomes
What bacterium was used as an example of lysis
Hemolysins: toxins that lyse RBCs
- bacteria then scavenge the released heme/iron
Listeria Monocytogenes - Describe
Listeria Monocytogenes
- pore-forming toxin section
- are intracellular pathogens
- After they escape (via lysis) they replication in immune cells
- they spread through blood causing bacterial meningitis
True or False:
Superantigens only bind cells together if an antigen is present
False - Superantigens bind the cells together even if no antigen is present
Describe the process/mechanism of ‘Superantigens’
- Phagocytes, like macrophages engulf bacterium and present the antigen on its surface
- DC’s present antigens to T cells
- Superantigens bind these cells together, even when no antigen is present
- this triggers over production of proinflammatory cytokines
- (over reaction of the immune system)
- Leads to fever, low bp, organ failure
- ex. TSS
What fraction of bacterial proteins leave the cytoplasm?
1 in 3
Define: Translocation
Translocation: transport across membrane
- also: insertion into membrane
Define: Secretion
Secretion: release into external environment
- requires translocation through 1 or 2 membranes
What is a ‘signal peptide’ and when is it important/used?
- Translocated/secreted proteins often have signal peptides
- These are specific amino acid sequences at the N-terminus
- they are recognized by transport systems
Proteins are translocated to the periplasm by: Name the 3 mechanisms
- Sec system
- Tat system
- ABC exporters
What/where does the sec system transport?
The sec system transports unfolded proteins into the periplasm
What proteins binds the signal peptide?
SecA binds the signal peptide
Describe - Sec System: Post-Translational Translocation
- The Sec system translocates unfolded proteins into the periplasm
- chaperones stabilize the proteins before translation
- SecA binds to signal peptide, escorts proteins to SecYEG
- SecA drives translocation of protein through SecYEG (uses ATP)
- Post-translational translocation occurs after translation is complete
Describe what happens in the Sec System once the protein is in the periplasm?
- Signal peptidase recognizes, and cuts signal peptide during translocation
- The translocated protein folds in the periplasm with the help of periplasmic chaperones
-Disulfide bonds are formed bc the periplasm is an oxidizing environment
Fill in the blank: The cytoplasm is a _______ environment and the periplasm is a _______ environment
The cytoplasm is a reducing environment and the periplasm is a oxidizing environment
Where are Tat system proteins folded?
The tat system translocates folded proteins from the cytoplasm to the periplasm
What 3 things may be required for the tat system?
- Co-factor insertion
- Cytoplasmic chaperons
- Assembly into complexes
Describe the mechanism of the tat system
- the protein is folded in the cytoplasm
- folded protein is targeted to TatABC by signal peptide
- TatABC translocates the protein into the periplasm using the proton motor force for energy
- the signal protein is cleaved off
What are ABC transporters used for?
ABC
- primary active transport = use ATP
- specific
ABC importers
- substances in like nutrients
ABC exporters
- waste, antibiotics
- proteins (recognize signal peptide, cleave SP during export)
proteins in the membrane are usually ______
proteins in the membrane are usually hydrophobic
All transporters are inserted into the membrane using the ____ system
All transporters are inserted into the membrane using the sec system
How do Sec Systems insert proteins into the membrane?
- The signal peptidase is translated first, this is very hydrophobic for a membrane protein
- The signal peptide is recognized by signal recognition particle
- This halts translation
- The signal recognition particle delivers the ribosome to SecYEG
- Translation continues, inserting the protein into the membrane
-= Co-translational translocation
Describe the process of the Gram-Negative BAM complex
- B-barrel proteins are targeted to the outer membrane
- The unfolded protein is translocated by the Sec system
- It is stabilized by periplasmic chaperones (SurA, Skp)
- Delivered to B-barrel assembly machinery (BAM) complex
- inserts into membrane
Gram-Negative Secretion System - Proteins can be:
Gram-Negative Secretion System - Proteins can be:
- bound to cell surface
- released into the external environment
- injected into other cells
Describe Type V Secretion System
- Tat or Sec system translocates the protein into the periplasm
- The protein is translocated through B-barrel in the outer membrane
- The B-barrel can be: a separate protein or a part of the protein being translated
What are the 3 components and their functions in a T1SS
ABC transporter:
- powers transport
- determines specificity
B-barrel protein:
- forms a channel thru the outer membrane
Membrane Fusion Protein (MFP):
- connects ABC transporter to B-barrel
What is the purpose of T3SS
To inject proteins into eukaryotic cells
Which secretion system has effector proteins and what do they do?
T3SS have effector proteins
- Effectors manipulate host cell structure and function, facilitating colonization
How are T3SS’s assembled?
- they consist of a basal body and a needle/filament tip
- during assembly, subunits (needle, tip) travel through the hollow central channel
- After assembly is complete, a plug blocks the channel
Describe how an effector uses a T3SS to get into the target protein
- Once the T3SS binds the target, the channel opens
- Chaperones deliver unfolded effectors to T3SS
- The Effector travels through T3SS, and folds in the target
- The transport is powered by ATP and the PMF
- Note: a single T3SS secretes different effectors, they are recognized by N-terminal secretion signal
T3SS Effector Proteins
Target:
Function:
T3SS Effector Proteins
Target: effectors target host cytoskeleton, signal transduction
Function: they rearrange the cytoskeleton of epithelia cells, forming surface ‘ruffles’ which allows bacteria to enter cells that are normally non-phagocytic
What is the purpose of efflux pumps?
To transport molecules to the periplasm or outside the cell
Name 2 classes of efflux pumps
- ABC transporters
- RND tripartite system
Name a major contributor to multidrug resistance?
Efflux pumps
Define: Extracellular Vesicles
Extracellular Vesicles: spherical buds released from cell surface used by bacteria to export substances
“Outer membrane vesicles” are the term used to describe extracellular vesicles released by ____ ___
“Outer membrane vesicles” are the term used to describe extracellular vesicles released by gram negative bacteria
Name 3 things extracellular vesicles contribute to:
- HGT
- Virulence
- Antibiotic resistance
What example was given for Ev’s and Antibiotic Resistance
- Gram-Negative EV’s containing B-lactamases
- B-lactam enters EV through porins
- B-lactamases in EV degrade B-lactam
- Keeps B-lactams away from PBPs
What makes gram-positive protein secretion easier
- no outer membrane
- does not need elaborate secretion system
Which secretion systems are used by gram-positive bacteria
Tat and Sec systems
What does Sortase do?
Sortase - attaches proteins that have cell wall sorting signals to the cell’s surface
Name 4 functions of Gram-positive surface proteins
- often contribute to virulence
- used to attach to host cells extracellular matrix
- Nutrient acquisition in host
- Immune evasion
