Intracellular Compartments and Protein Sorting 2 Flashcards
Where are mitochondrial proteins synthesized and then imported?
on ribosomes and imported into mitochondria
Where are most mitochondrial proteins encoded?
In the nuclear DNA
What do proteins need to cross to get in mitochondria?
Outer and/or inner mitochondrial membranes
Protein movement is called what?
Translocation
What directs proteins to correct compartments in mitochondria?
Signal sequences
What are mitochondrial signal sequences?
N terminal and internal
What do the mitochondrial signal sequences form?
amphiphilic alpha helix
________ charged residues cluster on one end and _______ on the other end of the helix
Positively
Hydrophobic
What recognizes the positively charged and hydrophobic configuration of mitochondrial signal sequences rather than precise sequence?
Specific receptor proteins
What are multi-subunit protein complexes called that mediate translocation?
Protein translocators
What is TOM
Translocase of the outer membrane
Where is TOM present?
In the outer membrane of the mitochondria
What is TOM required for?
For import of all nucleus encoded proteins and inserts them in outer membrane
What is TIM?
Translocase of the inner membrane
Where is TIM present?
inner membrane
What are the two TIM complexes?
TIM 22 and TIM 23
What does TIM 23 do?
Transports soluble protein into matrix and helps insert membrane proteins in inner membrane
What does TIM 22 do?
Mediates the insertion of a specific subclass of proteins (e.g. ATP, ADP, and Pi transporter)
TOM and TIM complexes have ___ components
2
What are the components of TOM and TIM responsible for?
Receptors for mitochondrial precursor proteins
Translocation channels
What is SAM complex?
Sorting and assembly machinery: translocates and inserts/folds beta barrel proteins in the outer membrane
What does the OXA complex do?
Mediates insertion of proteins synthesized in mitochondria
Precursor proteins entering mitochondria are (folded/unfolded)
Unfolded
Unfolding of mitochondrial proteins is maintained through what interactions?
With chaperone proteins: cytosolic Hsp 70 family
What binds signal sequence of incoming mitochondrial protein?
Import receptors of TOM complex
What are stripped off so that the unfolded protein is fed through TOM?
Chaperone proteins
What enters first into translocation channel?
Signal sequence
What does TOM transport?
Protein across the outer membrane to the inner membrane space
What to proteins bind to in order to move through to the matrix?
TIM complex
What are signal sequences of mitochondrial proteins cleaved off by?
Peptidase
Bound Hsp ___ dissociated at expense of _________ outside mitochondria
70
ATP hydrolysis
After passing through TOM, what do proteins interact with?
TIM
What does transport through TIM depend on?
Membrane potential
What drives translocation of positively charged sequence through TIM
Energy in electrochemical gradient drives translocation by electrophoresis
What is bound to TIM 23 on the matrix side?
Mitochondrial Hsp 70
What pulls protein into matrix space?
Mitochondrial Hsp 70
What releases the protein in the matrix in an ATP dependent step?
Hsp 70
What helps folding of imported protein using ATP?
Hsp 60
How are proteins imported into outer membrane?
Pass through TOM complex and enter intermembrane space
Bind to chaperone proteins
Then bind to SAM complex in outer membrane
SAM inserts and folds them in outer membrane
What is an example of proteins imported into outer membrane?
Porins (beta barrel proteins)
What is the ER organized into?
A network of branching tubules and sacs
What is the ER membrane continuous with?
The nuclear membrane
What is the internal space of the ER called?
ER lumen
What does the ER play a central role in ?
Proteins and lipid synthesis
What is the site of production of all TM proteins?
ER
destined to other organelles and ER
What do ER signal sequences vary in?
IN amino acid sequence
What do ER signal sequences have at its center?
8 or more non-polar amino acids
What are ER signal sequences guided to ER membrane by?
2 components:
Signal recognition particle (SRP)
SRP receptor
What is the SRP made up of?
6 different polypeptides bound to a single small RNA molecule
What do SRP cycle between?
ER membrane and cytosol
What do SRP bind to?
ER signal sequence
What is the shape of SRP?
Rod shaped with large hydrophobic pocket lined by methionines
What can the pocket of SRP accommodate?
Hydrophobic signal sequences of different size, shape and sequence
What does SRP wrap around and bind to?
Large ribosomal subunit
One end binds to ER signal sequence of emerging protein and other end to elongation factor binding site
What does the SRP block?
Protein synthesis transiently giving enough time for protein to enter ER membrane
What binds to SRP receptor present in ER membrane?
SRP-ribosome complex
What does the interaction of SRP -ribosome complex and SRP receptor bring about?
The assembly to a translocator
What is released and then the protein translocates across the ER membrane?
SRP and receptor
Where is the translocator of the ER present?
In the ER membrane
What type of pore does the translocator in the ER membrane have?
Water filled pore
What is the core of the translocator made of?
Sec61 complex
Has 3 subunits with largest subunit surrounding the central lpore
What is the pore of the translocator gated by?
Short helix which opens and closes pore as needed
What triggers the opening of the translocator pore and SRP released?
Signal sequences that was brought to the ER membrane by SRP
What does the signal sequence interact with, and thereby opening the pore?
Specific site within the pore called the start-transfer signal
What ensures specificity of protein being translocated into ER?
Dual recognition
What does the start-transfer signal also interact with?
Lipid core of ER membrane
What cleaves off the ER signal?
Signal peptidase
Where does the cleaved ER signal escape from?
Lateral opening in the pore
What is the integration of trans-membrane proteins similar to?
The initial process is similar to soluble proteins
What sequence initiates translocation of trans-membrane proteins?
N terminal sequence
What does the integration of membrane proteins require?
Require that some portions of the proteins pass though membrane and some others do not
What additional region in polypeptide stops transfer process before entire polypeptide is translocated?
Hydrophobic region called stop transfer signal
What does the stop transfer signal anchor?
Protein in membrane after start-transer signal has been cleaved and released
What helps to remove the cleaved start-transfer peptide and to integrate stop-transfer signal into bilayer?
Lateral gating
What determines topology of the transmembrane proteins?
In multiple TM proteins several combinations of start-transfer and stop-transfer determines topology
