What is translocation?
- protein movement across membranes
How do proteins get into the mitochondria?
- synthesized on ribosomes
- signal sequence helps direct across the membrane into the mitochondria
What are mitochondrial locating specific signals that are associated with proteins for translocation?
- amphiphilic alpha helix, that are at N terminal or in the polypeptide somewhere
How are the mitochondrial sequences recognized?
receptor proteins recognize the configuration and not the sequence
Why are a majority of the mitochondrial signal sequences positively charged?
the cytosol in the inner membrane is negative, therefore a positive charge will be attracted to the inside
What functions can TOM perform?
- import nuclear proteins into the mitochondria
- insert the proteins in the outer membrane
What functions can TIM perform?
- allows passage of proteins through the inner membrane. Forms "continuous" tunnel with TOM
What is the role of TIM23?
- transport soluble proteins into matrix and inserts membrane proteins in the INNER membrane
What is teh role of TIM 22?
- mediates the insertion of specific subclasses of proteins (ATP, ADP)
What are unique regions on the TOM/TIM complex?
- receptor for mitochondrial precursor
- translocation channel
What is the OXA complex responsible for?
mediating insertion of proteins that were synthesized in the mitochondria, into the inner membrane
How are proteins entered into the mitochondria?
- protein is unfolded and bound with Hsp70 (chaperones)
- Binds with TOM and chaperones are removed as protein is threaded in
- once in the matrix, the signal sequence is cleaved by peptidase
Which complexes require ATP and which rely on membrane potential for protein translocation into the mitochondria?
- ATP required for TOM
- Membrane potential for TIM
How do proteins get placed in the outer membrane?
- pass through TOM complex, and bind with chaperones in the intermembrane space
- SAM complex binds and folds the protein into the outer membrane
- porins are good examples
What is the role of Mia 40?
- provides protein translocation into the intermembrane space
- uses sulfide bridges which become reduced releasing the protein into the membrane space
What guides the ER signal sequence to the ER membrane?
- signal recognition particle (SRP)
- SRP receptor
What is the chemical structure of the SRP
rod-shaped structure with hydrophobic pocket lined by methionines, which can accomodate different sequences and sizes of signal
How does the SRP help move the protein across the ER membrane?
- SRP binds with the ribosomal subunit, at the emerging sequence site and elongation factor binding site
- stops transcription
- SRP binds with SRP receptor and release the ribosome onto the protein translocator
- SRP/SRPreceptor unbind from the ribosome/protein translocator units.
- SRP is recycled. translocation occurs as gene is translated
What is the structure of the translocator that is present on the ER membrane?
- Sec61 core complex that is a water filled pore
- consists of 3 subunits
- Signal peptide causes conformational changes in the gate allowing the pore to become open
If a protein has multiple transmembrane domains, what must have been present in the sequence to produce this effect?