Bacterial Protein Secretion and Assembly of Surface Structures Flashcards
Why is secretion/transport so important
In order to cause disease in the host, bacteria must interact with their host by:
- Sensing host physiological “cues”
- Delivering bioactive molecules (toxins/effectors) to host cells/tissues
How do bacteria sense the host cues or deliver bioactive molecules
Synthesising structures that allow toxin secretion/delivery in host tissues/cells
What are the identified mechanisms used in Gram positive to deliver toxins from the cytoplasm of the bacteria to the host
- Sec*
- Tat*
- ABC
- T7SS
What are the identified mechanisms used in Gram negative to deliver toxins from the cytoplasm of the bacteria to the host
Sec-dependent or periplasmic intermediate:
- T2SS
- T5SS*
Sec-independent or no periplasmic intermediate:
- T1SS
- T3SS*
- T4SS
- T6SS*
What does T5SS secrete
Adhesins
Proteases
Esterase
What does T6SS secrete
Hcp - inner tube of the secretion system
VgrG - spike that gets fired into the target cells
They are fired into the target and sometimes they can carry effector proteins or toxic domains
What does T3SS secrete
Adenylate Cyclase
Phospholipase A2
What are the 3 main systems involved in the transport of proteins in Gram positive bacteria
1) The Sec pathway (general secretory pathway)
2) Tat pathways (twin arginine translocation)
3) ATP-binding cassette (ABC) transporter
What is the Sec Pathway
This is a universal protein secretion system used to move proteins across membranes, especially from the cytoplasm to either the periplasm (in the bacteria) or into the organelles in eukaryotes
Where is the Sec pathway found
It is ubiquitous and essential - it is present in all domains of life, bacteria, archaea, and eukaryotes
- In eukaryotes, its used not just for secretion, but for also moving proteins across organelle membranes like the ER, mitochondria or chloroplasts
What is the basis of Sec-dependent secretion
1) Signal peptide/leader sequence
2) Translocation across the cytoplasmic membrane
3) Proteins must be unfolded
4) Energy source is ATP hydrolysis
5) Once the protein has been translocated, the signal peptide is cut off by a signal peptidase, leaving the mature protein.
What is the purpose of a signal peptide/leader sequence
Proteins that are destined for secretion via the Sec pathway need a special short amino sequence in the N-terminus
This ‘tag’ directs them to the Sec machinery
Why must the proteins be unfolded
The SecYEG channel is narrow and can only move liner polypeptide chains through it
This is contrast with the Tat pathway, which transports folded proteins
What is the process of the Sec pathway
1) the pre-protein is targeted to the cytoplasmic membrane surface with the assistance of the export chaperone SecB
2) SecA, an ATPase, drives the pre-protein chain across the membrane through the SecYEG channel, using the energy of ATP hydrolysis
3) Once the pre-protein is translocated into the periplasmic space, the signal peptide is cleaved off by the Type 1 signal peptidase
4) The protein then folds - periplasmic folding often involves periplasmic chaperones like DsbA/DsbC which help form disulfide bonds
What is SecB’s function
Keeps the pre-protein in an unfolded state
What is the function of the SecYEG
- The SecYEG complex is a core translocation channel in the membrane
- When SecA binds, it causes a conformational change that opens the channel wider
What is the signal sequence
This is about 20 amino acids long at the N-terminus
They are usually hydrophobic, allowing for the interaction with the membrane-bound Sec system
Co-translational vs post translational route in the Sec pathway
The Sec pathway is a default export route for many bacterial proteins
The studied route is post translational route
Co-translational export is where the ribosome docks on the SecYEG and pushes the protein through as its being, often uses the SRP (signal recognition pathway)
Co-translational pathway is more common for eukaryotes.
What is the Tat pathway
This is a protein translocation system that moves fully folded proteins across the cytoplasmic membrane
It is named after the Twin Arginine motif that is found in the signal peptide of the protein that it transports
What are the key features of the Tat pathway
It would transport fully folded proteins
The energy source is the proton motive force
Contains the twin arginine motif which is crucial for recognition
This pathway is mainly found in bacteria and archaea
It is able to transport multimeric proteins and those with metal cofactors already loaded
What is the process of the Tat pathway
1) Protein synthesis in the cytoplasm
2) Signal peptide with twin arginine motif
3) Recognition by the Tat machinery
4) TatA assembles to form a channel
5) Translocation across the membrane
6) Signal peptide cleavage
What does protein synthesis include in the cytoplasm in the Tat pathway
The protein is fully synthesised and folded in the cytoplasm with metal cofactors like the haem group, sulfur clusters
These are often enzymes or redox proteins that need to fold before export
What does the Tat machinery recognise
The folded protein and signal peptide is recognised by the TatBC complex, embedded in the inner membrane
How does TatA form a channel
After recognition, TatA oligomerizes (forming a ring/pore-like structure) to create a transient channel large enough for a folded protein
This channel is dynamic - it assembled only when needed and then disassembled
