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Cell Biology > Membrane Proteins > Flashcards

Membrane Proteins Flashcards

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1
Q

what is the process of eukaryotic protein synthesis?

A

ribosomes bind to mRNA and direct synthesis of polypeptides

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2
Q

what does the newly formed polypeptide contain?

A
  • information or a post code that directs the polypeptide to a particular site in the cell
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3
Q

what is a signal peptide?

A
  • has a signal
  • co-translational ER targeting: peptide targets ribosome and protein to the ER and is continued to be translated
  • get the formation of membrane proteins and luminal/secretory proteins
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4
Q

what proteins are produced when there is no signal?

A
  • forms a cytosolic protein

- can be targeted to organelles

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5
Q

what do the majority of type II membrane proteins have?

A
  • an internal sequence
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6
Q

what is protein targeting?

A
  • pieces of information that enable the cellular transport machinery to correctly position a protein
  • information contained in the polypeptide chain or in the folded protein
  • continuous stretch of amino acid residues in the chain that enables targeting called signalling or targeting peptides
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7
Q

what is a targeting signal?

A
  • could be at the N terminal and just linear (primary amino acid sequence)
  • formed by 3D folding of the protein
  • would be important for targeting specialised organelles
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8
Q

what could be indicative of targeting the nucles?

A

a lot of positive charge

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9
Q

what could be indicative of targeting the ER?

A

a lot of hydrophobic amino acids

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10
Q

what signal would proteins destined to be secreted or integrated in the ER?

A

have N terminus signal sequence 6-12 hydrophobic amino acids

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11
Q

what is the Signal Recognition Particle (SRP)?

A

recognises the signal sequence of Nascent polypeptide

- cystolic ribonucleoproteins of proteins bound to RNA scaffold

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12
Q

how does the signal allow entrance to the ER?

A
  • SRP docks with SRP receptor on cytosolic face of ER
  • the interaction triggers GTP hydrolysis and releases SRP
  • GDP and Pi release
  • on SRP release nascent polypeptide enters translocon membrane channel
  • as it grows nascent polypeptide is fed through translocon in the ER (translocates through a hydrophillic pore)
  • signal sequence is removed inside ER by signal peptidase enzyme
  • protein is released into the ER lumen
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13
Q

what happens in co-translational import?

A

proteins to be tarrgeted to the ER initially have an N-terminal peptide, the ER signal sequence, translated by a cytosolic ribosome

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14
Q

what is the ER signal sequence bound by?

A
  • an SRP
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15
Q

how does SRP cause translational arrest?

A
  • polypeptide has an ER signal which binds SRP
  • arrests translation until it starts entering the sec61
  • translation arrest gives the ribosome enough time to bind to the ER membrane
  • means the polypeptide isnt released into the cytoplasm
  • polypeptide does not fold before reaching the translocon wouldnt be able to go through the channel
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16
Q

what is the Sec61 complex?

A
  • the translocon

- four subunits that form a channel

17
Q

how is the Sec61 complex involved in translocation?

A
  • polypeptide chain enters the ER through the translocon
18
Q

what is post translational insertion?

A
  • requires the sec 62 and the larger Sec63(71 and 72) complex
  • the SRP is not involved
  • protein folds up in the cytoplasm
  • after translation its inserted into the ER
  • requires molecular chaperones
  • deposits BIP (a chaperone) molecules onto the translocating chain as it emerges from the ER lumen
19
Q

how is the ER signal peptide cleaved?

A
  • cleaved by signal peptidase
  • after protein translation, Sec61 opens and releases the signal peptide into the membrane where its rapidly degraded
  • production of a luminal protein can be secreted or targeted to the lumen of an organelle
20
Q

what is the signal pepride?

A

can be a stop/start transfer sequences

21
Q

how do stop/start sequences work?

A
  • high quantity of hydrophobic amino acids are recognised by the cell
22
Q

how do the proteins respond to the stop/start sequences?

A
  • protein gets moved laterally and is released from the Sec61 (proteins can have internal signal peptides that can cause the protein to stop transfer through Sec61)
  • signal peptide cleavage
  • this is then the transmembrane protein (crosses the membrane once)
23
Q

what happens if the signal peptide not at the N terminus?

A
  • transported to the ER lumen
  • N terminus in the cytoplasm
  • continue to make the C terminus
  • type II membrane is created (SNARE proteins)
  • multiple stop-transfer sequences results in polytopic membrane protein
24
Q

what does a 19-21 hydrophobic region indicate?

A
  • a transmembrane region

- hydrophobic side chains stick out of a alpha helical coil

25
Q

what happens if it is a transmembrane protein?

A
  • translocation halted
  • signal peptidase cleaved
  • carries on making the rest of the protein in the cytoplasm
26
Q

what happens if you have internal start-transfer sequence?

A
  • its hydrophobic
  • binds but continues to make the protein
  • get a SNARE protein
27
Q

what might membrane proteins have instead of an N terminal transfer sequence?

A

the start transfer can be internal

28
Q

what will the SRP scan for in a polypeptide?

A

for a region of hydrophobicity

29
Q

what will sequences contain?

A
  • a start sequence and then a stop sequence

- order of a start-stop sequence that determines membrane topology

30
Q

which end of the transmembrane protein is the luminal end?

A

the cytosolic of TM domains is more positively charged than the luminal end (positive inside)

31
Q

what do the majority of type II proteins have?

A

an internal sequence

32
Q

what is a polytopic protein?

A

crosses the membrane multiple times

33
Q

what is a type I protein?

A

NH3+ is on the lumenal side and COO- is on the cytosol side

34
Q

what is a type II protein?

A

NH3+ is on the cytosol side and COO- is on the lumenal side

Cell Biology (10 decks)

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