Protein Processing And Targeting Flashcards
Define regulatory secretion
Proteins packaged into vesicles and only released when they are required and a signal binds
Define constitutive secretion
Continuous pathway where proteins are packaged into vesicles and constantly released via exocytosis.
Provide an overview of the secretory pathway in mammalian cells
1) protein synthesis iniated on free ribosomes
2) n-terminal sequence produced
3) signal sequence recognised by the srp
4) GTP bound srp directs the ribosome synthesising the secretory protein to srp receptors on the cytotoxic face of the er
5) srp dissociates
6) protein synthesis continues and the newly formed polypeptide is fed into the er via a pore in the membrane (peptide translocation complex)
7) signal is removed via a signal peptidase
8) ribosome dissociates and is regulated
List protein modification that occurs in the er
Signal cleavage by signal peptidase as no longer needed
Disulphides bind formation by protein disulphides isomerase
N-linked glucosylation - addition of sugar to dolichol phosphate molecule then transfer sugar onto amino acid on protein
List protein modification that occurs in the Golgi
O linked glycosolation - add sugar to aa of protein using glycosyl transferase
Trimming and modification of n- linked sugar (oligosaccharide)
Further proteolytic processing
Describe the proteolytic processing in the formation of Important secreted proteins
Removal of signal peptidase changes the protein from prepro to pro, pro part removed in Golgi
Outline the formation of mature insulin
Initially inactive prepro insulin is synthesised
Signal sequence is removed and 3 disulphides bonds formed (pro)
Proteases recognise pairs of basic aa residues and cut it into 3 peptides
Mature insulin composed a and b chains held together by disulphides bonds, c peptide is released
Describe the structure of collagen
Basic unit is troop collagen, synthesised as preprocollagen
Gly end every 3rd position
Left handed triple helix of a chains
Mostly hydroxyl rolling and proline residues
Describe the synthesis of collagen
Clevage of the signal peptide Hydroxylation kf prolin/lysine residues Addition of n linked oligosaccharides Disulphides bond formation Pro collagen (triple helix) O linked glycosylation Golgi then exocytosis Removal of n and c peptides Lateral aggregation to form fibrils
Explain nuclear targeting
1) a fully folded protein with a nuclear localisation sequence is bound by importin a and b in the cytosol
2) the resulting complex binds to the nuclear pore and translocate into the nucleus in an energy dependant mechanism
3) once inside the nucleus the nuclear protein is released and the importin bind to a small protein known as ran
4) importin are exported from thE nucleus and can be recycled to transport more nuclear proteins
5) protein alone in nucleus
Explain mitochondrial targeting
1) have an n-terminal sequence that indicates they are destined for mitochondria
2) signal sequence recognised by proteins in the outer membrane,TOM proteins form a channel
3) transported into mitochondrial in an unfolded state with chaperones
4) ones for matrix recognised by TIM proteins
5) n-terminal sequence removed by mitochondrial processing peptidase
6) ATP dependent folding with help from chaperones
Explain lysosomal targeting
1) lysosomal enzyme passes through the Golgi apparatus
2) phosphate group added to hydroxyl group of carbon 6 manose sugar using n-acetyl glucosamine transferase and phosphodiesterase
3) binds to m6p receptor
4) transported from Golgi to lysosomes
5) removal of phosphate
6) dissociation of receptors at acid ph
Explain the consequences of problems with lysosomal targeting
I cell disease
Genetic defect with n-acetyl glucosamine phosphotransferase means m6p signal not added
Proteins are mistargeted for secretion, end up in blood and urine
Explain how mis targeted proteins are dealt with
Proteins meant to stay in the er have KDEL sequence near c terminus which interacts with the KDEL receptors on the Golgi and are packaged into vesicles and sent back to the er
The neutral ph of the ER leads to dissoxiation of the receptor and protein, protein goes back to ER
