Proteins - Post translational Modification Flashcards
Where are proteins made on free ribosomes in the cytosol targeted for?
Cytosol
Organelles - e.g. nucleus, mitochondria, peroxisomes
Where are proteins made on RER bound ribosomes targeted for?
For ER, Secretion/Lysosomes/Plasma membrane
Protein targeting: Which out of ER, Lysosome, Mitochondria, Nucleus, Peroxisome
1) Doesn’t require direct energy
2) Has a C terminal signal
3) Doesn’t require receptor
4) Is kept unfolded on entry to organelle
1) ER - indirect by H+ pump to keep lysosome pH5
2) ER - KDEL is C terminal, Peroxisome SKL is C terminal.
3) Nucleus - importin through pores
4) Mitochondria - chaperones keep unfolded
What is constitutive vs regulated secretion? What 3 cell types tend to do regulated secretion?
Constitutive constant - collagen, albumin
Regulated - at specific times e.g. insulin
Endocrine/exocrine/neurocrine
Where are signal sequences on secretory proteins e.g. collagen/albumin, how many amino acids, are they hydrophillic/phobic, and what do they form to help get across membrane?
N terminus
5-30 amino acids
Central region hydrophobic
Can form alpha helix
What does the ‘pre’ part denote in preproalbumin etc
The signal sequence
What are the 5 basic stages to synthesis of secretory proteins and their translocation across the ER membrane?
1) Signal - N terminus
2) SRP - binds signal and ribosome - halts translation
3) SRPreceptor translocon binds all of above
4) Peptide enters - signal sequence cleaved by signal peptidase
5) Translation continues - fully folded proteins
What are the basic steps of preproinsulin processing in ER/Golgi? (4)
1) In ER- signal peptidases cleaves signal sequence
2) Folding of proinsulin with 3 x cysteine-cysteine disulphide bonds
3) Endopeptidases cleave B chain
4) To golgi - mature insulin 2 peptides joined with disulphide bonds packages in zymogen granules for secretion
What are the differences in membrane protein biosynthesis compared to secreted after the signal sequence has been cleaved by endopeptidases (3)?
1) Membrane proteins have a stop transfer sequence which halts protein in membrane
2) Moves out of translocon = transmembrane domain
3) Translation continues and more transmembrane domains are made if needed
What 7 functions of ER in protein procesing?
Proteolytic cleavage Proper folding Insertion into membrane Assembly of multi subunits Glycosylation Hydroxylation of Lys Pro Dilsuphide bonds
3 reasons glycosylation of proteins is so important - what disease can occur if there is a deficiency in N-Linked glycosylation
1) Correct protein folding
2) Protein stability
3) Facilitates interactions with other molecules
Congenital disorders of glyosylation - very severe
Where is the sugar added to in N-Linked glycosylation?
Asparagine side chain
Which enzyme plays a role in formation of disulphide bonds in ER?
Protein Disulphide Isomerase
What do chaperone proteins in ER do if proteins are misfolded? Give examples of chaperones that do this
Can actively fold/restrict folding to allow for misfiled proteins to be refolded correctly.
E.g. BiP - a Heat Shock Protein 70, Calreticulin, Calnexin
What if misfolding cannot be corrected (2)
1) Back to cytosol for degradation
2) May accumulate in ER - toxic
