Lecture 5: Protein Export Flashcards
Localization Signal
Portion of the polypeptide usually in the primary sequence that acts as a signal for where the protein needs to go. Often on N-terminus
Secretion Signal
One of the localization signals whose cleavage can reduce improper folding which may negatively impact normal cell processes
-Characteristic features include partially conserved sequence of amino acids and location in nascent polypeptides
-Recognized by a specific secretion system
-May not be present in the mature protein
Most proteins in the outer membrane
Beta barrels
Most proteins in the inner membrane
Alpha helix transmembrane domain
Sec/Tat dependent secretion system
-In gram positive: Movement of protein from inner membrane to the outer membrane
-In gram negative: Into the periplasm
Type 2 secretion system
Gets protein out of the cell past the outer membrane completely
Type 5 secretion system
Gets protein INTO the outer membrane
Common characteristics of all protein secretion systems
-All involve multi protein complexes to cross on or more membranes
-Structurally unique from one another
-Each has a unique mailing label in the nascent polypeptide
-Secrete different proteins
-Have some kind of energy source to drive membrane translocation
-Many translocation systems use cytoplasmic chaperones to restrict folding of the peptide until secreted
Sec-dependent secretion systems (3 characteristics)
-Dominant protein secretion mechanism that crosses cytoplasmic/inner membrane
-Present in ALL bacteria
-Contains a N-terminal secretion signal
SecYEG/BA characteristics
-In both gram positive and negative
-Transport of unfolded proteins
SecYEG/BA Mechanism
-SecB recognizes a secretion signal and acts as a chaperone to transport and prevent unfolding towards YEG
-SecA uses ATPase activity (ATP hydrolysis) that guides the unfolded protein through SecYEG via translocation
-Secretion signal is cleaved (Usually from N-term)
SecSRP chracteristics
-Both gram positive & gram negative
-Cotranslational Transport
-Uses signal recognition particle (SRP) to for secretion signal recognition
-N-term is synthesized first by ribosome in order for it to be recognized by SecYEG
SecSRP Mechanism
-SRP binds to co-translated protein from ribosome while still attached
-FtsY uses GTPase activity to dock the ribosome-SRP complex to SecYEG
-SecYEG supports translocation INTO the membrane to become an intermembrane protein
Twin Arginine Translocation (TAT) secretion system pathway characteristics
-Found in gram positive & gram negative bacteria
-Transports (FOLDED) proteins
-Uses proton motive force as energy
TAT secretion system pathway
-TatB and TatC bind N-terminal peptide with twin arginines
-TatB and TatC recruit this peptide to TatA that acts as a membrane channel for translocation
Tat secretion system in gram negative bacteria
Proteins will either remain within the cytoplasm or be exported by Type II secretion systems
Type II Secretion System
Sec/TAT-dependent secretion to the periplasm
MOSTLY GRAM NEGATIVE
-Secretion signal I removed
-Secretes folded proteins
-N-term secretion signal II directs the periplasmic polypeptide to the GSP
-GSP system uses cytoplasmic ATP to export protein
Three domains of Type V Secretion System Autotransporter Secretion
-N-terminal secretion signal (Cleaved in periplasm)
-B-barrel domain (Pore in the outer membrane)
-Passenger domain (Remain attached or cleaved)
Two partner secretion system of type 5 secretion system
-One partner carries B-barrel domain
-Other partner is secreted
-Transports some large virulence factors (Usually its partner)
Steps of autotransporter secretion in Type V secretion systems
-Secretion across inner membrane by Sec
-Signal I is removed in periplasm
-B-domain forms pore helped by Bam complex
-Passenger domain passes out of cell through B-domain pore
-Passenger domain forms the active protein
Type I secretion system
-Translocator composed of 3 proteins
-Single step to cross both membranes (Mainly in gram neg)
-ATP driven, resembles transporters for small molecules
-Secretion signal within C-term
3 proteins of the Type I translocator
-Cytoplasmic ATPase
-Periplasmic tube
-Outer membrane port
Steps of Type I Secretion system
-Polypeptide translated by cytoplasmic ribosomes
-Protein not correctly folded
-C-term secretion signal docks with ABC subunit via ATPase activity
-Polypeptide exported across inner membrane, periplasm, and outer membrane in a single step
-ATP hydrolysis
-Secreted protein refolds
Examples of proteins who use T1SS
-RTS (repeats-in-toxin) toxin
-Cell surface protein
-Proteases
-Lipases
-Bacteriocins
-Heme-acquisition proteins
-Nodulation-related proteins
Type III secretion system
Type III Secretion System Characteristics
-Polypeptide synthesis: N-term signal made & binds to multi-cargo chaperone
-Complex docks with cytoplasm face of secretion system
-ATP used to load protein into central lumen (protein remains unfolded)
-Protein released at the tip of the protein like a syringe
Similarities between Type III Secretion systems and flagellum
Type 3 secretion systems and flagellar assembly machinery look alike, have similar component and functions in a similar manner. Only differ since flagella use proton gradient while T3SS uses ATP hydrolysis
Type IV Secretion System Characteristics
-Composed of different proteins than other systems
-Secretes proteins and/or DNA
-ATP dependent
-Carried by many pathogenic bacteria (Conjugation, DNA Uptake, Effector translocation)
-Both gram negative and gram positive cells
Type IV Secretion System Process
One step process
-Proteins synthesized in cytoplasm
-Signal can vary
-Proteins loaded into lumen of a translocated protein
-Pump-like secretion
-May not require pilus but they may still be attached
Type VI Secretion System effectors
-Peptidoglycan-degrading enzymes
-Membrane targeting enzymes
-Nucleases
Type VI Secretion System Process
-Effectors assembled into apparatus
-Released upon “Firing”
-Signal unknown
-Recycling of apparatus component
Differences in secretion systems in gram positive bacteria
-Sec/TAT
-Additional proteins for Sec
-Injectosome similar to T3SS
-Mechanism unclear
Type VII Secretion System in mycobacteria
-Cross mycomembrane heavily modified by lipids
-Sec/TAT-independent
