Flashcards in Protein Targetting Deck (57)
How do most proteins achieve their final conformation?
State 2 additional post-modifications of proteins after translation, and what they entail.
- proteolytic cleavage - breaking peptide bonds to remove part of the protein
- chemical modification - addition of functional groups to amino acid residues
What is the peroxisome targeting sequence? What does addition of this mean?
SKL (serine - lysine - leucine) - addition of this will target a protein to the peroxisome
What is the peroxisome targeting sequence? Which end of a protein is this sequence added?
SKL (serine - lysine - leucine) - this sequence is added to the C-terminus end of a protein (towards the end)
Once tagged, how does a protein transverse the peroxisome membrane?
Through a transport channel comprised of 13 PEX proteins - this transfer requires ATP hydrolysis
What must the protein-targeting sequence bind in order to move the protein through the PEX transport channel in the peroxisome membrane?
A PEX5-cargo complex
Failure to target proteins to peroxisomes can lead to abnormal peroxisomes - what biological affects may this have? List 1 disease associated with abnormal peroxisomes.
Peroxisomes break down fatty acids - if this function is disturbed fatty acids may accumulate which can impair neuronal function - long bone shortening may occur as a result of abnormal protein targeting to peroxisomes - Zellweger syndrome is associated with abnormal protein targeting to peroxisomes
What is constitutive secretion?
Secretion that is continual (occurs all the time)
Describe the conformation and composition of the sequence that is added to molecules that require secretion from the cell.
The sequence is added to the N-terminus of the protein, and is around 5-30 amino acids in length - the central part of this signalling sequence is composed mainly of hydrophobic residues, which form an alpha helix in solution
What is required to target a protein to the ER for secretion? How is this molecule composed?
The signal recognition particle recognises the targeting sequence and moves the targeted molecule to the ER - it is composed of 6 subunits and a small stretch of RNA
What organelle does the signal recognition particle direct a protein with a target sequence and an associated ribosome towards?
Does binding of the signal recognition peptide instigate or inhibit translation of the RNA molecule associated to a ribosome?
The signal recognition particle prevents translation, while it directs the free ribosome to the surface of the ER
How does a type I membrane protein, designed to reside in the membrane of the ER, stop itself from being completely excluded into the lumen?
It contains a stop-transfer anchor sequence - an alpha-helical membrane-spanning sequence
List 7 functions of the endoplasmic reticulum.
- proteolytic cleavage
- insertion of proteins into membranes
- formation of disulphide (s-s) bonds
- proper folding of proteins
- assembly of multi-subunit proteins
- hydroxylation of selected lysine and proline residues
What is N-linked glycosylation?
The addition of sugars (a glycan) to the amide nitrogen of an asparagine residue of a protein
Give 3 reasons why glycosylation is important.
- correct folding of proteins
- give a protein stability
- facilitates interactions with other molecules
Between what 2 residues does a disulphide bond form?
Between 2 cysteine residues
What is the role of protein disulphide isomerase?
Protein disulphide isomerase ensures that the correct disulphide bonds are formed - it can remove wrong ones and correct them
What is O-linked glycosylation? Where specifically does this occur?
O-linked glycosylation is the attachment of a sugar molecule to the O of an OH- (hydroxyl) group of a serine or threonine residue - this occurs specifically in the Golgi apparatus
Describe the processing of preproinsulin.
The N-terminal signal sequence is cleaved as the protein enters the ER lumen - a pair of disulphide bonds form between the a & c chain within the ER lumen, causing the b chain to bend - the b chain is then cleaved in the Golgi apparatus, to form insulin
In which molecules is O-linked glycosylation an important characteristic?
What is the opposite of constitutive secretion? Give 3 examples.
- endocrine secretion
- exocrine secretion
- paracrine secretion
What cell type secretes collagen?
What is the basic unit of collagen? Describe its structure.
The basic unit of collagen is procollagen - it is a 300nm rod-shaped protein, consisting of 3 alpha chains, which are each around 1000 amino acids long - a glycine residue is present every 3rd amino acid along each alpha chain
What is special about the properties of glycine that it features so prominently in procollagen/collagen?
Glycine is the amino acid with a side chain small enough to fit into the middle of the helix form by the 3 alpha subunits
What other amino acids are associated with the structure of procollagen/collagen? How are these important?
Proline and/or hydroxyproline - these allow the formation of hydrogen bonds between the alpha subunits, which give procollagen structural stability
What molecule brings procollagen to the ER? Where is the signal peptide then cleaved?
The signal recognition particle binds the signal sequence of pre procollagen and brings it (and its associated ribosome) to the ER - the signal sequence is then cleaved off within the lumen of the ER
Where does the hydroxylation of certain glycine and proline residues occur on a procollagen polypeptide?
In the lumen of the RER
What type of glycosylation does procollagen undergo in the ER?
Procollagen undergoes N-linked glycosylation in the RER