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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
larger molecules are imported into the nucleus via
active transport
26
what are nucleoporins ?
proteins that make up the nuclear pore complexes
27
for nuclear import
the protein needs to cross through the outer membrane and inner membrane of the nucleus
28
nuclear localisation signals are recognised by
nuclear import receptors ( soluble cytosolic proteins) which are of family of proteins called the karyopherin family.
29
what is the name of the nuclear import receptor
importin.. binds to the signal sequence and directs the protein through the nuclear pore complex.
30
importin receptors binds to..
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
for nuclear pore export.. signal sequence usually has
two leucine amino acids. (double leucine signal)
32
for export out of the nucleus.. the signals are recognised by
nuclear export receptors which are again part of the karyopherin family. The nuclear export receptor is called exportin.
33
Nucleoproteins have FG repeats that serve as binding sites
for the importin receptors and exportin receptors
34
proteins that are transported into or out of the nucleus are..
are folded
35
T antigen
is a signal sequence for nuclear import.
36
nucleoplasmin
is a signal patch for nuclear import.
37
describe the form of Ran in the nucleus.
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
high concentration of Ran-GTP in the
nucleus
39
what does Ran-GAP do ? Ran-GTPase activating protein
stimulates the activity of Ran-GTPase resulting in the hydrolysis of the GTP bound to Ran
40
where is Ran-GAP found
in the cytosol
41
where is Ran-GDP more concentrated ?
in the cytosol
42
what is the function of Ran-GTPase
drives directional transport through nuclear pores
43
describe the process of proteins entering the nucleus
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
major difference between nuclear import and nuclear export
Ran-GTP and the protein bind simultaneously to exportin. Ran-GTP does not displace the protein the receptor.
45
summary of protein import into the nucleus
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
proteins are kept........until they are transported into the mitochondria
unfolded
47
a typical signal sequence involved in mitochondrial import of proteins
is an amphipathic alpha helix
48
describe an amphipathic alpha helix
residues are positively charge on one side of the helix, uncharged residues on the other side
49
the signals for mitochondrial import are usually at..
the end of the protein
50
where does translocation across of the mitochondrial membrane occur ?
at special sites where the inner and outer membrane are close together
51
the mitochondrial genome only encodes ...
13 proteins and 24 RNAs..99% of the proteins it requires are coded for by nuclear DNA
52
Protein import into mitochondria :
Receptor : receptor protein in TOM complex (membrane bound) | Channel: TOM and TIM complexes
53
Describe protein import into mitochondria
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
what keeps the polypeptide to be translocated unfolded ?
binding of the cytosolic Hsp70, which is an ATPase protein. Hsp70 used the energy released by ATP hydrolysis to keep the protein unfolded.
55
Translocation across the inner membrane requires the..
the electrochemical gradient
56
The signal peptide is cleaved in the mitochondrial matrix by
a protease
57
what drives further translocation into mitochondria ?
Mitochondrial Hsp70 binds to further drive translocation. Hsp70 used the energy released by ATP hydrolysis to pull the polypeptide into the matrix.
58
what helps the translocated mitochondrial protein fold correctly
Hsp60, another ATPase
59
how are membrane proteins inserted into the outer mitochondrial membranes
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
what stops an inner membrane protein from entering into the matrix ?
stop-transfer sequence (hydrophobic sequence)
61
OXA complex is involved in..
inserting proteins into the inner mitochondrial membrane
62
what do chaperons do
keep the proteins unfolded
63
what is the TIM22 complex involved in ?
inserting proteins into the inner membrane
64
what is protein cleavage involved in ?
inserting proteins in the inter membrane space
65
protein import into mitochondria summary points
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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