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Medical Cell Biology & Genetics > Protein Targetting > Flashcards

Protein Targetting Flashcards

1
Q

How do most proteins achieve their final conformation?

A

Themselves

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2
Q

State 2 additional post-modifications of proteins after translation, and what they entail.

A
  • proteolytic cleavage - breaking peptide bonds to remove part of the protein
  • chemical modification - addition of functional groups to amino acid residues
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3
Q

What is the peroxisome targeting sequence? What does addition of this mean?

A

SKL (serine - lysine - leucine) - addition of this will target a protein to the peroxisome

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4
Q

What is the peroxisome targeting sequence? Which end of a protein is this sequence added?

A

SKL (serine - lysine - leucine) - this sequence is added to the C-terminus end of a protein (towards the end)

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5
Q

Once tagged, how does a protein transverse the peroxisome membrane?

A

Through a transport channel comprised of 13 PEX proteins - this transfer requires ATP hydrolysis

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6
Q

What must the protein-targeting sequence bind in order to move the protein through the PEX transport channel in the peroxisome membrane?

A

A PEX5-cargo complex

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7
Q

Failure to target proteins to peroxisomes can lead to abnormal peroxisomes - what biological affects may this have? List 1 disease associated with abnormal peroxisomes.

A

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

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8
Q

What is constitutive secretion?

A

Secretion that is continual (occurs all the time)

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9
Q

Describe the conformation and composition of the sequence that is added to molecules that require secretion from the cell.

A

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

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10
Q

What is required to target a protein to the ER for secretion? How is this molecule composed?

A

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

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11
Q

What organelle does the signal recognition particle direct a protein with a target sequence and an associated ribosome towards?

A

The ER

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12
Q

Does binding of the signal recognition peptide instigate or inhibit translation of the RNA molecule associated to a ribosome?

A

The signal recognition particle prevents translation, while it directs the free ribosome to the surface of the ER

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13
Q

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?

A

It contains a stop-transfer anchor sequence - an alpha-helical membrane-spanning sequence

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14
Q

List 7 functions of the endoplasmic reticulum.

A
  • proteolytic cleavage
  • insertion of proteins into membranes
  • glycosylation
  • formation of disulphide (s-s) bonds
  • proper folding of proteins
  • assembly of multi-subunit proteins
  • hydroxylation of selected lysine and proline residues
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15
Q

What is N-linked glycosylation?

A

The addition of sugars (a glycan) to the amide nitrogen of an asparagine residue of a protein

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16
Q

Give 3 reasons why glycosylation is important.

A
  • correct folding of proteins
  • give a protein stability
  • facilitates interactions with other molecules
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17
Q

Between what 2 residues does a disulphide bond form?

A

Between 2 cysteine residues

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18
Q

What is the role of protein disulphide isomerase?

A

Protein disulphide isomerase ensures that the correct disulphide bonds are formed - it can remove wrong ones and correct them

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19
Q

What is O-linked glycosylation? Where specifically does this occur?

A

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

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20
Q

Describe the processing of preproinsulin.

A

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

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21
Q

In which molecules is O-linked glycosylation an important characteristic?

A

Glycoaminoglycans (GAG’s)

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22
Q

What is the opposite of constitutive secretion? Give 3 examples.

A

Regulated secretion:

  • endocrine secretion
  • exocrine secretion
  • paracrine secretion
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23
Q

What cell type secretes collagen?

A

Fibroblasts

24
Q

What is the basic unit of collagen? Describe its structure.

A

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

25
Q

What is special about the properties of glycine that it features so prominently in procollagen/collagen?

A

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

26
Q

What other amino acids are associated with the structure of procollagen/collagen? How are these important?

A

Proline and/or hydroxyproline - these allow the formation of hydrogen bonds between the alpha subunits, which give procollagen structural stability

27
Q

What molecule brings procollagen to the ER? Where is the signal peptide then cleaved?

A

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

28
Q

Where does the hydroxylation of certain glycine and proline residues occur on a procollagen polypeptide?

A

In the lumen of the RER

29
Q

What type of glycosylation does procollagen undergo in the ER?

A

Procollagen undergoes N-linked glycosylation in the RER

30
Q

What is scurvy the result of?

A

Scurvy is the result of unstable connections between the alpha subunits the comprise a collagen helix - this is due to poor prolyl hydroxylase activity, which catalyses increased hydrogen bonding between the 3 alpha subunits

31
Q

What is required by prolyl hydroxylase in order to function properly?

A

Iron ions and vitamin C

32
Q

How does prolyl hydroxylase function?

A

Prolyl hydroxylase catalyses hydrogen bonding between the triple helix, meaning that collagen is more stable

33
Q

What happens with a lack of prolyl hydroxylase?

A

Proline isn’t hydroxylated, so less hydrogen bonds form between the alpha subunits, meaning the helical structure of collagen is much weaker

34
Q

Where and how are procollagen subunits first linked? What then forms to further increase their stability?

A

The 3 procollagen subunits are first linked in the lumen of the ER, via the formation of disulphide bridges at the C-terminal ends of the structures - hydrogen bonds are then formed along the chain increasing the stability of the triple helix

35
Q

What type of glycosylation does procollagen undergo in the Golgi apparatus?

A

Procollagen undergoes O-linked glycosylation in the Golgi apparatus

36
Q

Where is the next destination of procollagen after the Golgi apparatus?

A

Procollagen is endocytosed to the extracellular matrix in a vesicles from the Golgi apparatus

37
Q

What defines the conversion of procollagen to tropocollagen?

A

The cleavage of the N-terminus and C-terminus - this leaves only the triple helix of tropocollagen

38
Q

How do individual tropocollagen molecules combine to form collagen?

A

Tropocollagen fibrils aggregate to form collagen - lysine molecules cross-link to form aldol cross-links

39
Q

Why is procollagen secreted before the final stage of processing occurs?

A

If collagen were constructed within the cell it would destroy it

40
Q

What enzyme is responsible for the aldol cross-link formation between tropocollagen subunits? What is required for this enzyme activity?

A

Lysyl oxidase - this requires copper ions and vitamin B6 for optimal activity

41
Q

What disorder may a deficiency in lysyl oxidase result in?

A

Elhers-Danlos Syndrome

42
Q

Why must oxygen bad transporters to be transported around the body?

A

It is not very soluble in water

43
Q

When transferred into the ER, what cleaves the signal sequence off a protein?

A

A signal peptidase

44
Q

What happens to proteins that enter the default pathway of the secretory pathway?

A

They are packaged and stored in granules until the are stimulated for release

45
Q

How do proteins designed to stay in the ER remain there, and not secreted?

A

A lys-Asp-Glu-Leu amino acid at the C-terminus of a protein ensures it remains within the lumen of the ER

46
Q

What signal is used to direct a molecule to a lysosomes? Describe how and where this signal is added?

A

Mannose-6-phosphate - this is added to the 6th carbon of a mannose sugar on the protein within the Golgi apparatus

47
Q

What 2 enzymes act to add a phosphate to the 6th position of a mannose sugar? Where will this post-translational modification target the protein?

A

N-acetyl glucosamine phosphotransferase & phosphodiesterase - the addition of mannose-6-phosphate targets a protein to a lysosome

48
Q

What receptor transfers a protein tagged with mannose-6-phosphate to a lysosome? How do these dissociate once within the lysosome?

A

M6P receptor transfers a tagged protein to the lysosome - the acidity of the lysosome is enough to remove the receptor from the mannose sugar, releasing the protein

49
Q

What is I-cell disease?

A

This results from a defective phosphotransferase - here the enzyme can no longer transfer phosphate to the 6th hydroxyl group of mannose, and so the protein isn’t targeted to a lysosome - without this signal, the proteins are instead excreted outside the cell - lysosomes cannot function without the majority of these proteins - substances build up within the lysosome (as they cannot be degraded) resulting in the characteristic I-cells

50
Q

Describe the signal that is added to a molecule to target it to the mitochondria.

A

It is an amphipathic (one side is positively charged while the other is hydrophobic) alpha helical sequence added to the N-terminus

51
Q

What keeps a protein unfolded while it is transported to a mitochondria? What else is required?

A

Hsp70 - ATP hydrolysis is also required

52
Q

Describe how a protein targeted to a mitochondria is moved from the cellular cytoplasm to the mitochondrial matrix.

A

Chaperones (such as Hsp70), using ATP hydrolysis, move the protein to the mitochondria in its unfolded form - the signal then binds an import receptor on the mitochondria - the protein is passed through the TOM, and then secondly through the TIM - within the matrix, Hsp pull the protein through, again with the use of ATP - once in the matrix, the signal sequence has served its purpose, and is cleaved

53
Q

Describe the signal that targets a protein to the nucleus.

A

A positively charged amino acid sequence consisting of arginine and lysine residues on the protein

54
Q

How is a targeted protein moved from the cytosol to the nucleus?

A

The protein is bound by importin, which transfers it through the nuclear pore

55
Q

Once in the nucleus, how is a molecule unloaded from importin?

A

A molecule called Ran-GTP configures a conformational change in importin causing it to release the targeted protein

56
Q

Name 2 diseases that are the result of defects in targeting a protein to the nucleus.

A
  • Swyer Syndrome

- Langer Mesomelic Displasia

57
Q

How is a protein targeted for the ER retirement there from the Golgi apparatus? What mechanism allows for this protein to dissociate from its transporter?

A

A protein is coated in a COPI1 coat - this returns it to the ER - the here possesses a PLZ LOOK UP

Medical Cell Biology & Genetics (11 decks)

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