Protein Sorting II Flashcards
Where are most mitochondria proteins encoded?
in nuclear DNA
What is protein movement called?
translocation
What do signal sequences do?
direct proteins to correct compartment in mitochondria
Mitochondrial signal sequences form an ______ alpha helix
amphiphilic
True or False?
Positively charged residues cluster on one end and hydrophobic residues cluster on the other end of the helix
true
Multi-subunit proteins complexes called ____ ____ mediate translocation
proteins translocators
Do specific receptor proteins recognize a precise sequence?
no, they recognize the configuration of pos and neg residues on opposite ends
What is the mitochondrial signal sequence bound to?
import receptor
Where is the TOM complex located and what is it required for?
present in the outer membrane; required for import of all nucleus encoded proteins - inserts them in outer membrane
Where is the TIM complex located?
present in both outer and inner membrane
What are the two types of TIM complexes and what do they do?
TIM 22 mediates the insertion of a specific subclass of proteins (ex: ATP)
TIM 23 transports soluble proteins into matrix and helps insert membrane proteins in inner membrane
How many components do TOM and TIM complexes have? What are they?
2 components:
receptors for mitochondrial precursor proteins; translocation channels
What is the SAM complex and what does it do?
Sorting and Assembly Machinery complex; translocates and inserts/folds beta barrel proteins in the outer membrane
What does the OXA complex do?
mediates insertion of proteins synthesized in mitochondria
What is the general pathway of protein import into the mitochondrial matrix?
Binding to import receptors; insertion into membrane TOM complex; translocation into matrix by TIM 23 complex; cleavage by signal peptidase
True or False?
Precursor proteins entering mitochondria are folded
false; unfolded
Unfolding of the proteins in maintained through interaction with what?
chaperone proteins (cytosolic Hsp 70 family)
After import receptors of TOM complex bind signal sequence of incoming proteins, what happens to chaperone proteins?
chaperone proteins are stripped off
How is the unfolded protein oriented as it is fed through the translocation channel?
signal sequence first
TOM transports protein across the outer membrane and into what?
inner membrane space
The protein in the inner membrane space binds to the TIM complex and moves through a channel to the ____
matrix
What cleaves off the signal sequence once the protein is inside the matrix?
peptidase
In regards to protein transport needs, bound Hsp 70 is dissociated at the expense of what?
ATP hydrolysis outside mitochondria
What does transport through TIM depend on?
membrane potential
Energy in electrochemical gradient drives translocation of positively charged sequence through TIM by what?
electrophoresis
On what side of TIM 23 is mitochondrial Hsp 70 bound to?
matrix side
After the protein is pulled into the matrix, Hsp 70 releases the proteins in what type of step?
ATP dependent step
Hsp60 helps with what?
folding of imported protein using ATP
In regards to import into the outer membrane, the protein passes through the TOM complex and enters what? What does it bind to when its here?
inter-membrane space; binds to chaperone proteins and then to SAM complex in outer membrane
In regards to import into the outer membrane, Sam ___ and ___ the protein in the outer membrane.
inserts; folds
Is the ER membrane continuous with the nuclear membrane?
yes
Do the ER signal sequences vary in amino acid sequence?
yes; have 8 or more non-polar amino acids at center
What two components guide signal sequences to the ER?
signal recognition partical (SRP); SRP receptor
SRP cycles between ER membrane and cytosol and binds to the ER signal sequence. It is rod shaped with a large _____ pocket lined by methionines.
hydrophobic
In regards to translocation across ER, SRP wraps around large ribosomal subunit. One end binds to ER signal sequence of emerging protein and other end to what? What does this do?
elongation factor binding site; this blocks protein synthesis transiently giving enough time for protein to enter ER membrane
In regards to translocation across ER, after the SRP-ribosome complex binds to SRP receptor present in ER membrane, interaction brings the assembly to a ____. What happens after this?
translocator; SRP and receptor are released and protein is translocated across the ER membrane
Translocator is present in ER membrane and has water filled ___
pore
In regards to the translocator in the ER membrane, what is the core of the translocator made of?
Sec61 complex; has 3 subunits
In regards to the translocator in the ER membrane, what is the pore gated by?
short helix which opens and closes pore as needed
In regards to protein translocation across the ER, describe the sequence of events leading up to the start-transfer signal
SRP binds to ER signal sequence in protein and brings it to the ER membrane. Signal sequence triggers opening of translocator pore and SRP is released. Signal sequence interacts with a specific site within the pore, thereby opening the pore. This is called the start-transfer signal.
In regards to protein translocation across the ER, dual recognition ensures what?
specificity
In regards to protein translocation across the ER, what cleaves off the signal? And where does it go?
Signal peptidases; signal escapes from a lateral opening in the pore
In regards to integration of Trans-membrane proteins, what does the N terminal sequence initiate?
translocation
In regards to integration of Trans-membrane proteins, integration of membrane proteins require what?
that some portions of protein pass through membrane and others do not
In regards to integration of Trans-membrane proteins, an additional hydrophobic region in polypeptide stops transfer process before what?
before entire polypeptide is translocated
note: this is called the stop transfer signal
In regards to integration of Trans-membrane proteins, what does lateral gating help?
helps to remove the cleaved start-transfer peptide and to integrate stop-transfer signal into bilayer
In multiple trans-membrane proteins, several combinations of start-transfer and stop-transfer determine ____ of the protein
topology
