Topic 8: Protein secretion Flashcards
what is protein targeting
to correct location to work
those destined for cytosol, or posttranslational important into organelles - synthesised on free ribsosomes
those destined for secretion or into membrnae or lysosomes - synthesised on ribosomes on RER
what are the two different types of secretion
- constitutive
2. regulated: - endocrine cells, exocrine cells, neurocrine cells -> only when need them
what organelles are needed for protein targeting for secretion
lots of secretory granules
lots of RER
what is required for protein sorting
a signal (only for certain protein)
receptor (recognise and direct to correct membrane)
a translocation machinery (to get across membrane)
energy to transfer the protein
what do all secretory proteins possess
a secretory sequence: mainly hydrophobic region always at end terminus of protein 5-30 amino acids in length able to form alpha helix
what is preproalbumin
pre - defines the signal sequence which will be removed when processing
pro - bit of protein not incorporated into final protein
how does the protein get into the endoplasmic reticulum
translation:
signal sequence recognised by signal recognition particle, binding to it, stops any further translation
the ribosome complex and signal recognition aprticle bind to ER through signal recognition particle receptor
the translocon allows newly synthesised protein to travel through lumen of ER. exhange of GTP -> GDP driving opening of the channel to allow translation to restart. signal sequence is chooped off by peptidase, then proteins folds
what is the function of the ER
insertion of proteins into memrbane specific proteolytic cleavage glycolysation formation of disulfide bond proper folding of proteins assembly of multisubunit proteins hydroxylation of selected Lys and Pro residues
what is the function of golgi
movement of proteins to the cis-golgi through budding
further protein modification
release through trans-golgi to membrane
what are the key features of collagen
basic unit is tropocollagen 300nm rod shaped protein secondary structure 3 polypeptides glycine in every 3rd position right handed triple helix mostly proline or hydroxyproline in X and some Y (afterglycine) positions H bonds between chains stabilise structure
how is collagen synthesised and modified in the ER
- synthesis and entry of chain into ER
- cleavage of signal peptide (prepro alpha chain -> pro alpha chains via signal peptidase)
- hydroxylation of selected proline and lysine residues
- addition of N linked oligosaccharides
- addition of galactose to hydroxylysine residues
why is prolyl hydroxylase
important in reactions in the ER
requires vit C and fe2+ for activity
allows increased H bonding to stablise triple helix, without - scurvy, due to weak tropocollagen triple helices
what else happens in ER in terms of forming triple helix
chain alignment by formation of disulfide bonds
formation of triple helical procollagen from C to N terminus
how does synthesis and modification of collagen in golgi work
completion of O-linked oligosaccharide chains by addition of glucose
transport vesicles leave cell
exocytosis
removal of N and C terminal propeptide to form tropocollagen molecules (catalysed by procollagen peptidases)
why are N and C terminus so important
prevent procollagen molecules coming together so prevent formation of larger molecules
