Protein Targeting Flashcards
Which proteins are synthesised on free ribosomes and which on rER ribosomes?
- Free ribosomes: proteins remaining in cytosol or that are post-translationally imported into organelles.
- rER ribosomes: proteins destined for secretion, PM or lysosomes.
What are the 4 requirements for protein sorting?
- Signal (intrinsic to protein, except for cytosolic proteins)
- Receptor: recognises signal and directs it to correct membrane.
- Translocation machinery
- Energy
What are the requirements for peroxisomal protein targeting?
- Peroxisome targeting sequence (PTS). Usually present on C-terminus of protein.
- PTS receptor Pex5.
- Translocon made up of 13 Pex proteins.
- ATP hydrolysis for recycling of PTS receptor.
Describe the process of peroxisomal targeting.
- PTS receptor (Pex5) binds to PTS in cytosol.
- PTS receptor integrates into translocon to open it - dissociates from PTS.
- Folded protein enters PO matrix.
- PTS receptor returns to cytosol via ATP hydrolysis.
What is the result of faulty PO targeting?
Peroxisome biogenesis disorders. E.g.
- ZELLWEGER SYNDROME: mutation in any of 12 PEX genes (inc. Pex5)… no PO protein targeting… VLFCA accumulation… neurone development impairment.
- RHIZOMELIC CHONDRODYSPLASIA PUNCTATE: mutation of Pex7 leads to skeletal abnormalities.
What are the 2 types of cellular secretion of proteins and give examples of each.
- Constitutive secretion
- collagen secretion from fibroblasts
- albumin secretion from hepatocytes
- Regulated secretion
- endocrine cells secreting hormones
- exocrine cells secreting digestive juices
- neurocrine cells secreting NTs
Describe the process of protein targeting to the ER.
Involves co-translational protein translocation.
- N-terminal signal sequence on protein being translated recognised by Signal Recognition Particle (SRP).
- SRP binds to signal sequence and to ribosome.
- SRP binds to SRP receptor in ER membrane. GTP for GDP exchange on SRP and SRP R opens translocon. SRP dissociates.
- Polypeptide is translated through the translocon.
- Signal peptidase cleaves signal sequence as polypeptide enters ER lumen.
- Polypeptide is released into lumen.
What is type I ER membrane protein synthesis? What does this require?
- Insertion of polypeptide destined for PM or internal membrane of secretory pathway into ER membrane.
- Stop-transfer anchor sequence.
Describe the requirements for targeting of proteins retained in the ER.
- KDEL or KKXX signal sequence at C-terminus.
- KDEL receptor.
- Protein remains folded during translocation and signal sequence is retained.
- No energy requires except from GTP hydrolysis required for vesicle budding.
Describe the components of protein targeting to lysosomes. What does the signal sequence involve?
- Signal sequence = N-linked glycosylation of asparagine residues - adds on M6P.
- Recognised by M6P receptor on membrane of Golgi trans face.
- M6P is cleaved by acidic pH of lysosomes.
- Protein is translocated folded.
- Energy is required for vesicle movement and hydrogen pump (ATP hydrolysis).
Describe the components of protein targeting to mitochondria.
- Signal sequence is bi-partite: matrix-specific sequence (cleaved in matrix) and region-specific targeting sequence. Located in N-terminus.
- Signal sequence cleaved by signal peptidase.
- Protein is translocated unfolded.
- Many specialised proteins involved: SRP, SRP receptor, chaperone proteins (MSF) or cytosolic Hsc70, TOMs, TIMs.
- Requires ATP hydrolysis.
Describe the components of protein targeting to the nucleus.
- Signal sequence = internal Nuclear Localisation Signal (monopartite or bipartite).
- NLS is retained so that protein can re-enter nucleus after mitotic envelope degradation.
- Protein is translocated when folded. Involves alpha and beta importin receptors, nuclear pore, RanGTP, RanGAP, etc.
- GTP hydrolysis is required for recycling of importin (RanGTP to RanGDP).
