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Biochem > Intracellular Protein Trafficking lecture 1 > Flashcards

Intracellular Protein Trafficking lecture 1 Flashcards

1
Q

Cell =

A

Cytoplasm and nucleus

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2
Q

Cytoplasm =

A

Cytosol + organelles

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3
Q

from the cytosol polypeptides are transported to..

A 
nucleus
mitochondria 
endoplasmic reticulum (ER)
plastids eg. cholorplasts 
peroxisomes
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4
Q

how does a protein know where to go in a cell ?

A

sorting signals guide the proteins to their destination.

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5
Q

what are the two types of sorting signals

A

signal sequence and signal patch. These are recognised by specific receptors.

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6
Q

what is a signal sequence ?

A

exposed, short single stretch of amino acids often at the end of the polypeptide chain

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7
Q

what is a signal patch ?

A

amino acids contributing to the signal are separate until after the protein folds

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8
Q

nuclear import sequences are

A

lysine and arginine-rich which are positively charged

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9
Q

sequences that direct proteins to mitochondria (mitochondrial import)

A

amphipathic alpha helix

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10
Q

hydrophobic amino acids direct proteins to..

A

ER import

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11
Q

signal sequences are always cleaved after targeting ? true of false

A

false they are cleaved or uncleared after targeting

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12
Q

which terminus is synthesised first ?

A

N terminus followed by C terminus

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13
Q

small proteins cross the lipid bilayer through,

A

by diffusing through the pores in the membrane

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14
Q

large proteins cross the lipid bilayer..

A

through pores via an active transport mechanism

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15
Q

what do receptors in cell membranes recognise ?

A

signal sequence in the protein .. the signal binds to the receptors directing the protein through the channel in the membrane

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16
Q

The receptors and channels themselves are made of..

A

proteins

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17
Q

the receptor can either be..

A

membrane bound or soluble

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18
Q

signal sequences that direct proteins into nucleus

A

are made up of positively charged amino acids.. don’t have to be at the extreme end of the polypeptide

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19
Q

signal patches only work for nuclear import ?

A

true, protein folds into 3D structure.. two areas of positively charged amino acids come together to form a single patch

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20
Q

signals that direct proteins to the nucleus are known as ..

A

nuclear localisation signals (NLS)

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21
Q

proteins are transported into the nuclei through..

A

nuclear pores

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22
Q

mesh-like structure in the nucleus which known as the.

A

nuclear lamina

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23
Q

nuclear pore complex is made of..

A

lots and lots of proteins 50 - 100 different proteins.. each nuclear pore complex have a molecular weight of 60-80 million daltons

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24
Q

small molecules (<5000) can..

A

freely diffuse through nuclear pores

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25
Q

larger molecules are imported into the nucleus via

A

active transport

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26
Q

what are nucleoporins ?

A

proteins that make up the nuclear pore complexes

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27
Q

for nuclear import

A

the protein needs to cross through the outer membrane and inner membrane of the nucleus

28
Q

nuclear localisation signals are recognised by

A

nuclear import receptors ( soluble cytosolic proteins) which are of family of proteins called the karyopherin family.

29
Q

what is the name of the nuclear import receptor

A

importin.. binds to the signal sequence and directs the protein through the nuclear pore complex.

30
Q

importin receptors binds to..

A

phenyl alanine glycine repeats on nucleoporins which allow the protein and its receptor to be taken through the nuclear pore and into the nucleus.

31
Q

for nuclear pore export.. signal sequence usually has

A

two leucine amino acids. (double leucine signal)

32
Q

for export out of the nucleus.. the signals are recognised by

A

nuclear export receptors which are again part of the karyopherin family. The nuclear export receptor is called exportin.

33
Q

Nucleoproteins have FG repeats that serve as binding sites

A

for the importin receptors and exportin receptors

34
Q

proteins that are transported into or out of the nucleus are..

A

are folded

35
Q

T antigen

A

is a signal sequence for nuclear import.

36
Q

nucleoplasmin

A

is a signal patch for nuclear import.

37
Q

describe the form of Ran in the nucleus.

A

in the nucleus Ran is usually in its Ran-GTP form due the activity Ran-GEF(guanine nucleotide exchange factor) which is a nuclear protein bound to chromatin

38
Q

high concentration of Ran-GTP in the

A

nucleus

39
Q

what does Ran-GAP do ? Ran-GTPase activating protein

A

stimulates the activity of Ran-GTPase resulting in the hydrolysis of the GTP bound to Ran

40
Q

where is Ran-GAP found

A

in the cytosol

41
Q

where is Ran-GDP more concentrated ?

A

in the cytosol

42
Q

what is the function of Ran-GTPase

A

drives directional transport through nuclear pores

43
Q

describe the process of proteins entering the nucleus

A
  1. signal sequence on the protein binds to importin
  2. importin receptor complex then interacts with FG repeats on the nucleoporins directing the receptor and its bound protein into the nucleus
  3. In the nucleus, Ran-GTP binds to the receptor complex, causing dissociation of the cargo from the receptor
  4. Importin which is now bound to Ran-GTP
    is transported out of the nucleus via its own export pathway.
  5. In the cytosol, Ran-GTP dissociates from importin as its hydrolysed to Ran-GDP
44
Q

major difference between nuclear import and nuclear export

A

Ran-GTP and the protein bind simultaneously to exportin. Ran-GTP does not displace the protein the receptor.

45
Q

summary of protein import into the nucleus

A

occurs via nuclear pores: gated transport through aqueous pores (large molecules), free diffusion of small molecules
requires signal sequences (NLS),
post-translational, proteins are folded before import
import/export receptors and Ran-GTPase cycle drive directional transport

46
Q

proteins are kept……..until they are transported into the mitochondria

A

unfolded

47
Q

a typical signal sequence involved in mitochondrial import of proteins

A

is an amphipathic alpha helix

48
Q

describe an amphipathic alpha helix

A

residues are positively charge on one side of the helix, uncharged residues on the other side

49
Q

the signals for mitochondrial import are usually at..

A

the end of the protein

50
Q

where does translocation across of the mitochondrial membrane occur ?

A

at special sites where the inner and outer membrane are close together

51
Q

the mitochondrial genome only encodes …

A

13 proteins and 24 RNAs..99% of the proteins it requires are coded for by nuclear DNA

52
Q

Protein import into mitochondria :

A

Receptor : receptor protein in TOM complex (membrane bound)

Channel: TOM and TIM complexes

53
Q

Describe protein import into mitochondria

A
  1. Signal sequence binds to import receptors
  2. Insertion into membrane by TOM complex
  3. Translocation of the polypeptide into matrix through TIM
  4. cleavage of the signal peptide by signal peptidase
  5. pre-cursor protein folds in the matrix space to form a mature mitochondrial protein
54
Q

what keeps the polypeptide to be translocated unfolded ?

A

binding of the cytosolic Hsp70, which is an ATPase protein. Hsp70 used the energy released by ATP hydrolysis to keep the protein unfolded.

55
Q

Translocation across the inner membrane requires the..

A

the electrochemical gradient

56
Q

The signal peptide is cleaved in the mitochondrial matrix by

A

a protease

57
Q

what drives further translocation into mitochondria ?

A

Mitochondrial Hsp70 binds to further drive translocation. Hsp70 used the energy released by ATP hydrolysis to pull the polypeptide into the matrix.

58
Q

what helps the translocated mitochondrial protein fold correctly

A

Hsp60, another ATPase

59
Q

how are membrane proteins inserted into the outer mitochondrial membranes

A
  1. The polypeptide is inserted into the membrane by TOM complex
  2. chaperons bind to the polypeptide keeping it unfolded in the inter membrane space
  3. the pre-cursor protein is then re-inserted into the outer membrane by SAM complex
  4. fully folded protein
60
Q

what stops an inner membrane protein from entering into the matrix ?

A

stop-transfer sequence (hydrophobic sequence)

61
Q

OXA complex is involved in..

A

inserting proteins into the inner mitochondrial membrane

62
Q

what do chaperons do

A

keep the proteins unfolded

63
Q

what is the TIM22 complex involved in ?

A

inserting proteins into the inner membrane

64
Q

what is protein cleavage involved in ?

A

inserting proteins in the inter membrane space

65
Q

protein import into mitochondria summary points

A

VIA TOM and TIM complexes
Requires targeting signals which are amphipathic alpha helices
post-translational - proteins must be to pass through translocation complexes
requires ATP and an electrochemical gradient across the inner membrane

Biochem (7 decks)

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