Lecture 14 -- Protein Transport and Secretion Flashcards
- SEC dependent pathway
- Twin-arginine pathway (TAT)
List the pathways only involved in translocation
- Types I, II, IV, VI
- Type VII
List the pathways responsible for translocation and secretion in one step
- Type II (uses TAT or SEC)
- Type V (SEC)
List the translocation linked secretion pathways (two steps)
- allow for formation of cell surface structure (cell wall, cell membrane, capsule)
- nutrient acquisition
- pathogenic mechanisms (infection of host cells)
- competition with other bacteria
Why do bacteria export/secrete protiens?
- Type I pilus
- Toxins/adhesins
- P-type pilus
- Curli
- OMPs/adhesins
Structures involved in virulence
- siderophore receptors
- TonB-dependent iron-uptake receptors
Nutrient receptors
- many exported/secreted proteins are hydrophilic
- for proper localization and regulation of release
Why do bacteria need mechanism of protein secretion
Proteins that are translocated across the cytoplasmic membrane
Exported proteins
Proteins translocated across the cytoplasmic membrane and subsequently transported across the entire cell envelope
Secreted proteins
cytoplasmic membrane, cell wall, *capsule, environment
List the potential locations where proteins can end up in gram positive bacteria
cytoplasmic membrane, periplasm, cell wall, *outer membrane, capsule, environment
List potential locations where proteins can be brought in gram negative and cutoff for secretion
- protein folded in the cytoplasm (stays intracellularly)
- intrinsically disordered proteins (rare)
- unfolded proteins are exported/secreted
- folded proteins are exported/secreted
List the four pathways proteins can take after translation
- targeting and sorting in the cytoplasm
- translocation through a channel/energy in the membrane
- maturation and release in the periplasm (and potential folding)
List the steps of translocation/what happens a each location
- universal, most well studied mode of translocation
- proteins are in an unfolded state
- essential for viability (cannot be deleted from cell)
- all proteins have a signal sequence
SEC-dependent protein translocation
- Co-translational
- Post-translational (chaperone dependent and independent)
What are the two modes of SEC mediated translocation
- integral membrane protein in SEC pathway
- generates a gated pore (the channel)
- made up of many alpha helices
- essential
SecYEG
- involved in the sorting and recognition of the signal peptide in the SEC pathway (binds preprotein)
- provides the energy for translocation (ATPase) (along with Ftsy)
- can act as a chaperone to stabilize the unfolded protein in the cytoplasm
- essential
SecA
- protein being secreted in SEC pathway
- has signal sequence
- unfolded
Preprotein
- GTPase provides the energy
- ribosome localizes to the membrane
- sorting and targeting (1) –> translocation (2) –> maturation (3)
- signal peptidase cleaves protein so folding can occur
Co-translational Sec Pathway
- maintain proteins in an unfolded state
- bind the signal peptide in the cytosol or ribosome
- bind SecA
Function of chaperones
- plug in place when these two proteins are not interacting
- plug opens when they interact
SecYEG-SecA complex
- non-essential for SEC translocation but improves efficiency
- may help release the signal sequence from the SecYEG channel
- may help formation of SecYEG
SecDF-YajC
negatively charged glutamic acid group interacts with positive charge, pulling the protein through (uses ATP)
PMF electrophoresis
Lysine is negatively charged at the base and becomes positively charged with the addition of a hydrogen (cannot go back through the channel) – uses ATP
PMF proton ratchet
- less common translocation system
- translocates folded proteins
- requires a signal sequence
- not essential but can affect proper cell function
- ATP independent (only driven by PMF)
Twin Arginine Translocation (TAT) System
- forms pore structure through which proteins move
- interacts with the signal peptide and folded protein
- serves as the initial docking site of the signal peptide
Functions of TAT A, TAT B, and TAT C
Only certain cofactors are found inside the cell
Why do proteins fold inside the cell?
- Signal Peptidase I cleaves signal peptide
- Signal Peptidase II cleaves signal peptide from pre-lipoproteins
What happens after translocation?
- charged n region with Arg/Lys, binds to heads of phospholipids
- hydrophobic h region that associates with acyl chains of phospholipids
- c region (BXZ) where b and z are small amino acids
Standard signal peptide components
- n region
- h region
- c region (cleavage domain) “BXZ” where B is large hydrophobic and cysteine is +1
Lipoprotein signal peptide components
- 1st is diacylation
- 2nd is acylation
steps for the addition of groups to cysteine
- twin arginine RR
- longer and less hydrophobic than Sec signal sequence
TAT signal peptide components
- donates for the diacylation from phosphatidylglycerol
Lgt
- removes signal peptide for lipoproteins
Lsp
- donates for the final acylation in lipoproteins
Lnt
