Flashcards in Intracellular Compartments And Protein Sorting II Deck (37):
What do N terminal and internal signal sequences form?
Am phiphilic alpha helix.
What is the mechanism of mitochondrial signal sequences?
Positively charged residues cluster on one end and hydrophobic on the other end of the helix.
Specific receptor proteins recognize this configuration rather than the precise sequence.
Multi-subunit protein complexes called protein translocators mediate translocation.
What is the translocase of the outer membrane required for?
The import of all nuclear encoded proteins.
TOM inserts the encoded proteins in the outer membrane.
Where is the translocase of the inner membrane (TIM) present?
In both the outer and inner membrane
What are the two TIM complexes?
TIM22 and TIM23
What is the function of TIM 23?
It transports soluble proteins into the matrix and helps insert membrane proteins into the inner membrane.
What is the function of TIM22?
It mediates the insertion of a specific subclass of proteins.
What are the two components of the TOM and TIM complexes?
Receptors for mitochondrial precursor proteins
What is the function of the SAM (sorting and assembly machinery)?
It translocates and inserts/folds beta barrel lproteins in the outer membrane.
What is the function of the OXA complex?
It mediates insertion of proteins synthesized in the mitochondria.
What is protein unfolding maintained through?
Interactions with chaperone proteins (cytosolic Hsp 70 family)
What complex binds signal sequences of incoming proteins?
Import receptors of the TOM complex
Once import receptors of TOM complex bind signal sequences of the incoming protein, what happens next?
Chaperone proteins are stripped off and unfoldeed protein is fed signal sequence first into the translocation channel.
Where does TOM transport a protein?
Across the outer membrane to the inner membrane space.
What complex do proteins bind to as it moves through the channel into the matrix?
What is required for Hsp 70 to be dissociated with a protein?
ATP hydrolysis outside of the mitochondria.
After passing through TOM, protein interacts with __
What is the transport of a protein through TIM dependent on?
A membrane potential.
What is the function of mitochondrial Hsp 70?
It is bound to TIM 23 on the matrix side.
This pulls the protein into the matrix space.
Hsp 70 then releases the protein in an ATP dependent step.
Hsp 60 helps in folding of imported protein using ATP.
What are the steps of import of a protein into the outer membrane?
It passes through the TOM complex and enters the inter-membrane space.
The protein binds to chaperone proteins.
They then bind to SAM complex in the outer membrane, and SAM inserts and folds them in the outer membrane.
What is the ER organized into?
A network of branching tubules and sacs.
The ER membrane is continuous with the nuclear membrane.
What are the roles of ER?
The ER plays a central role in protein and lipid synthesis.
What is the ER signal sequence?
A sequence of 8 or more non-polar amino acids.
The ER signal is guided to the ER membrane by 2 components.
The ER sigal sequence is guided to the ER membrane by 2 components. What ar they?
Signal recognition particle (SRP)
What is the SRP made up of?
6 different polypeptides bound to a single small RNA molecule.
SPR cycles between the ER membrane and cytosol and binds to the ER signal sequence.
What does SRP wrap around?
A large ribosomal subunit.
One end binds to an ER signal sequence of emerging protein and the other end to elongation factor binding site.
What does SRP wraped around a large ribosomal subunit do?
It blocks protein synthesis transiently giving enough time for a protein to enter the ER membrane.
What is the function of the SRP-ribosome complex binding to an SRP receptor present in the ER membrane?
The interaction brings the assembly to a translocator.
SRP and the receptor are released and the protein is translocated across the ER membrane.
Where is a translocator present?
In the ER membrane.
It has a water-filled port.
What is the core of the translocator in the ER membrane made up of?
It has 3 subunits with the largest subunit surrounding the central pore. The pore is gated by a short helix, which opens and closes the pore as needed.
How is a protein translocated across the ER?
SRP binds to ER signal sequence to bring it into the membrane.
The signal sequence triggers the pening of translocator pore and SRP is released.
The signal sequence interacts with a specific site within the pore, thereby opening it (start-transfer signal)
Dual recognition ensures specificity.
Signal peptidase cleaves off a signal which escapes from a lateral opening in the pore.
What is required for the integration of membrane proteins?
Integration of membrane proteins require that some portions of protein pass through the membrane and others do not.
The integration of membrane proteins require that some portions of the protein pass through the membrane and others do not. What ensures this?
Additional hydrophobic region in the polypeptide stops the transfer process before the entire polypeptide is translocated.
This is the stop transfer signal.
What is the function of the stop transfer signal?
It anchors the protein in the membarne after a start-transfer signal has been cleaved and released.
What is the function of lateral gating?
It helps to remove the cleaved start-transfer peptide and to integrate stop-transfer signals into the bilayer.
What determines the topology of multiple TM proteins?
Several combinations of start-transfer and stop-transfer signals.