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Molecules to Cells - Biochemistry > Lecture 2. Protein Transport Mechanisms in Cells > Flashcards

Lecture 2. Protein Transport Mechanisms in Cells Flashcards

1
Q

What is the lipid bilayer?

A

Lipids contain a hydrophobic head and hydrophyllic tail. Lipids come together to form a bilayer to bury the hydrophobic tail in aqueous environments

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2
Q

What do internal membranes do ?

A

They create enclosed compartments that segregate different metabolic processes

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3
Q

What is the nucleus surrounded by ?

A

A double membrane called the nuclear envelope

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4
Q

Where are proteins synthesised ?

A

On ribosomes

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5
Q

Where can ribosomes be found ?

A
  1. Free in the cytosol

2. On the surface of rough endoplasmic reticulum

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6
Q

What do cells need to do constantly to function ?

A

Deliver proteins to their appropriate organelles within the cell

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7
Q

What do signal sequences do ?

A

Direct proteins to the correct compartment

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8
Q

What are signal sequences composed of ?

A

A sequence of amino acids

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9
Q

What is a sorting signal ?

A

A signal sequence of a protein

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10
Q

What are the three protein transport mechanisms ?

A
  1. Transport through nuclear pores
  2. Transport across membranes
  3. Transport by vesicles
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11
Q

What is on the nuclear envelope ?

A

Nuclear pores

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12
Q

What are nuclear pores ?

A

The exit and entry for all molecules entering and leaving the nucleus

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13
Q

What must larger molecules have to move out through nuclear pores ?

A

The correct signal sequence

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14
Q

What must proteins in the cytosol have to gain entry to the nucleus through the nuclear pores ?

A

Nuclear localisation sequence

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15
Q

What way are proteins transported through the nuclear pores ?

A

In their fully folded form

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16
Q

What are nuclear pores made up of ?

A

Multiple copies of a set of about 30 proteins called nucleoporins

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17
Q

What projects out from both sides of a nuclear pore ?

A

Protein fibrils

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18
Q

In the cytosol, what are nuclear localisation sequence containing proteins bound by ?

A

Nuclear transport receptors

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19
Q

What do nuclear transport receptors do ?

A

Interact with fibrils and guide itself and the protein to the pore and from there to the nucleus

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20
Q

What happens once the protein is delivered to the nucleus?

A

The nuclear transport receptor returns to the cytosol via the nuclear pore

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21
Q

What provides the energy required for nuclear transport ?

A

GTP hydrolysis

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22
Q

What happens once the nuclear transport receptor directs the protein to the nucleus ?

A

A Ran protein binds to GTP, which then binds to the nuclear transport receptor causing it to release the protein

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23
Q

What happens after the protein is released in to the nucleus ?

A

The nuclear transport receptor remains bound to Ran-GTP and gets transported back to the cytosol via the nuclear pore.

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24
Q

What happens nuclear transport receptor and its bound Ran GTP once back in the cytosol ?

A

The Ran GTP is hydrolysed to Ran GDP, causing the nuclear transporter receptor to be released

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25
Q

What do proteins destined for the mitochondria have ?

A

A signal sequence

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26
Q

In transport across membrane, where is the signal sequence recognised and bound ?

A

The transport outer membrane complex

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27
Q

In transport across membranes, where does the bound receptor go ?

A

It diffuses along the membrane to a contact site on the inner membrane (translocase of the inner membrane)

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28
Q

In transport across membrane, where does the protein move to and from ?

A

Translocates across the outer and inner membrane

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29
Q

In transport across the membrane, what is the role of chaperone proteins ?

A

Assist in the translocation and refolding once inside the organelle

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30
Q

When is the signal sequence/peptide removed ?

A

Once the protein reaches its destination

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31
Q

What are mitochondria ?

A

Organelles that have their own DNA and can make their own proteins

32
Q

What do precursor proteins destined for mitochondria have ?

A

A short segment of amino acids, the signal sequence that targets the proteins to this organelle

33
Q

What does the signal sequence of a precursor protein have an affinity for ?

A

A receptor on the surface of a mitochondria

34
Q

How/where does the precursor protein at the contact site of the two mitochondria membranes move?

A

Snakes in an unfolded state through two sequential protein translocators

35
Q

What do the chaperone proteins prevent ?

A

The protein chain backsliding through the translocator tunnel ?

36
Q

What does the enzyme signal peptidase do ?

A

Cleaves the signal sequence from the precursor protein

37
Q

What is the entry point of proteins destined for the golgi, endosomes, lysosomes, cell surface and the endoplasmic reticulum ?

A

The endoplasmic reticulum

38
Q

Do proteins re-enter the cytosol once inside the endoplasmic reticulum ?

A

No, they will be transported by vesicles to other organelles, the plasma membrane or secreted from the cell

39
Q

What happens when the endoplasmic reticulum sequence is detected by ribosomes if it is in the cytosol ?

A

It will direct itself to the endoplasmic reticulum and allow protein to be threaded across the endoplasmic reticulum membrane as it is synthesised

40
Q

What happens to soluble proteins in transport across membranes ?

A

They will be directed through the translocation channel on the endoplasmic reticulum and released into the endoplasmic reticulum lumen

41
Q

What allows proteins to be embedded in the endoplasmic reticulum membrane ?

A

Membrane proteins have hydrophobic stop and start transfer sequences. This gives rise to transmembrane proteins

42
Q

How is a polyribosome formed ?

A

As a MRNA molecule is translated into a protein, many ribosomes bind to it

43
Q

What are free ribosomes ?

A

Unattached to any membrane

44
Q

What are membrane bound ribosomes ?

A

Ribosomes that become riveted to the ER membrane and translate proteins that are translocated into the ER

45
Q

Where are membrane bound ribosomes found ?

A

The surface of the ER creating rough endoplasmic reticulum

46
Q

How do water soluble proteins cross from the cytosol and the ER ?

A

They completely cross the ER and are released into the lumen

47
Q

How do transmembrane proteins cross from the cytosol to the ER ?

A

They partially cross the ER and become embedded in the membrane

48
Q

How are proteins directed to the ER ?

A

A signal sequence of small hydrophobic amino acids

49
Q

How does the signal sequence get to the ER ?

A

Guided by a signal recognition particle

50
Q

What does the signal recognition particle bind to ?

A

The ER signal sequence in the new protein

51
Q

When is the signal recognition particle released ?

A

After protein synthesis slows down to allow time for the signal recognition particle binds to the signal recognition particle receptor in the ER membrane

52
Q

What happens when the signal recognition particle is released ?

A

The ribosome is passed to a protein translocation channel in the ER membrane

53
Q

What other things does the signal sequence do ?

A

To open the translocation channel

54
Q

How is the protein chain moved through the membrane ?

A

The signal sequence remains bound to the channel but the the rest of the protein chain is threaded through the membrane as a large loop

55
Q

What happens to the signal sequence/peptide once it is cleaved from the protein chain ?

A

It is released from the translocation channel and enters the membrane where it rapidly degrades

56
Q

How is the inactive channel closed ?

A

A protein serving as a plug the binds from the ER lumen

57
Q

What initiates translocation of a transmembrane protein ?

A

The N-terminal signal sequence

58
Q

How is the translocation process of a transmembrane protein stopped ?

A

a sequence of hydrophobic amino acids called a stop transfer sequence

59
Q

Where is the stop transfer sequence released from ?

A

Released laterally from the translocation channel

60
Q

What happens the stop sequence in the lipid bilayer ?

A

Forms a membrane spanning segment which anchors the protein in the membrane

61
Q

How does transport from the ER to the golgi occur ?

A

Budding and fusion of membrane enclosed vesicles

62
Q

What is the function of transport vesicles ?

A

Mediate the passage of proteins inwards from the plasma membrane to lysosomes and outwards from the ER to the plasma membrane

63
Q

What drives vesicle budding ?

A

The assembly of the protein coat

63
Q

What drives vesicle budding ?

A

The assembly of the protein coat

64
Q

What may the coat be composed of ?

A
  1. Clathrin

2. Coat protein complex

65
Q

Which vesicles usually coated with clathrin proteins

A

Vesicles originating from the plasma membrane and golgi, destined for endosomes or lysosomes

66
Q

Which vesicles are usually made from coat protein complex proteins ?

A

Vesicles shuttling between the ER and golgi

67
Q

What do adaptins do ?

A

Mediate contact between receptors and clathrin molecules that act to shape the vesicle forming at the plasma membrane

68
Q

What are triskelions ?

A

Individual clathrin molecules that are seen as three legged structures

69
Q

How are vesicles transported ?

A

Along cytoskeletal fibres

70
Q

What is the vesicular transport system responsible for ?

A

Protein secretion from cells

71
Q

How are secretory proteins released from the cell ?

A

Exocytosis

72
Q

How is endocytosis mediated ?

A

By plasma membrane vesicles budding into the cell cytosol

73
Q

What causes low density lipoproteins to receptors to release their cargo ?

A

The low internal pH of the endosome

74
Q

What happens to low density lipoprotein receptors once empty ?

A

Recycled to plasma membrane in vesicles that bud off from the endosome

75
Q

What does the lysosome contain ?

A

A hydrolytic enzyme that digests low density lipoproteins

76
Q

Where are secretory proteins synthesised ?

A

Ribosomes on the endoplasmic reticulum

Molecules to Cells - Biochemistry (17 decks)

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