Protein Sorting & Secretion Flashcards
What are the requirements for protein sorting?
Signal (intrinsic to protein)
Receptor (recognises signal and directs protein to correct membrane)
Translocation machinery (transports protein across membrane)
Energy (to trasnfer protein to new place)
What is the definition of co-translational insertion?
Process by which protein synthesis is closely coupled with the translocation of the growing protein to the destination within the cell
Outline the steps of mitochondrial matrix transport.
Protein with signal kept unfolded by chaperone proteins
Signal binds to import receptor on outer mitochondrial membrane
Protein fed through pore by translocation machinery (energy req.)
Protein moves through channel in inner mitochondrial membrane (electrochemical gradient helps pull protein through)
Targeting signal cleaved, chaperone proteins released
Outline the steps of nuclear import.
Importin binds to cargo containing nuclear localisation signal in cytoplasm
Importin-cargo moves through nuclear pores in nuclear membrane
Ran-GTP binds to importin to release cargo in nucleus
Importin-Ran-GTP recycled to cytoplasm
Give an example of a disease caused by ineffective mitochondrial matrix transport.
Pyruvate dehydrogenase deficiency
Change in aa of amphipathic helix, therefore reduced uptake into mitochondria
Build up lactic acid (as pyruvate cannot be converted to acetyl CoA
Less ATP produced
Give an example of a disease caused by a loss in nuclear localisation signal.
Swyer syndrome: loss/mutation of nuclear localisation signal in sex-determining region of Y chromosome
XY genotype, outwardly female (no transcription factor for testis-determination produced)
Outline the steps of peroxisome import.
Peroxisomal import receptor binds cargo with peroxisomal targeting signal
Peroxisomal proteins remains folded and receptor integrates into translocon (opening it)
Peroxisome targeting sequence and cargo dissociate from receptor
Receptor recycled to cytosol (energy req.)
Outline the steps of secretory proteins transport.
Signal recognition particle binds to signal sequence.
SRP-signal binds to receptor on membrane
Protein secreted through translocon as it is synthesised
Signal peptidase cleaves signal sequence
Protein folds in cytosol
note: membrane proteins: second hydrophobic aa sequence anchors protein in membrane, preventing further transport into ER lumen
What are the functions of the ER?
Insertion of proteins into membranes
Specific proteolytic cleavage
Glycosylation (protein folding/stability)
Disulfide bond formation
Assembly of multisubunit proteins
Hydroxylation of lysine and proline (collagen formation)
How can proteins be modified in the ER?
N-linked glycosylation: sugars added onto specific asparagine side chains
Peptidyl-prolyl isomerases: accelerate interconversion of cis and trans of proline residues
Protein disulfide isomerases: formation of disulfide bonds in ER lumen
How are ER-resident proteins retained in the ER?
RETROGRADE TRANSPORT:
KDEL rector on soluble resident protein binds to KDEL receptors in the RER
KDEL-protein binds to receptors in acidic Golgi complex
KDEL-protein retrieved and transported back to basic ER
Describe some problems with folding of proteins. What are some mechanisms of preventing protein mis-folding.
Protein trapped in mis-folded conformation due to mutation
May cause protein to be incorrectly associated with other sub-units
ER chaperone proteins retain unfolded proteins in ER and monitor the extent of protein misfolding (autoregulation)
If mis-folding cannot be corrected, the protein will either accumulate in the ER at toxic levels or will be returned to the cytosol for degradation.
Give an example of diseases caused by degradation of proteins and by accumulation of proteins in the ER.
Degradation of proteins:
Alzheimer’s, familial hypercholesterolaemia, diabetes, cystic fibrosis
Accumulation of proteins:
Parkinson’s, Ehlers-Danlos, osteogenesis imperfecta, heart failure (HMG-CoA reductase)
How can proteins be modified in the Golgi complex?
Phosphorylation of oligosaccharides (mannose)
Trimming/addition of sugars
Outline the process by which collagen is synthesised.
Prepro alpha chain has signal sequence removed and selected proline and lysine residues are hydroxylated (prolyl hydroxylase)
Pro alpha chain has addition of N-linked oligosaccharides
Disulfide bonds form between chains
Addition of glucose to oligosaccharide chains
Exocytosis and extracellular cleavage of propetides (procollagen peptidase) to form tropocollagen
Tropocollagen arranged as fibrils and fibres (by lysyl oxidase)
