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Cell Biology and Biochemistry > 8 Protein Sorting and Intracellular Traffic > Flashcards

8 Protein Sorting and Intracellular Traffic Flashcards

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1
Q

Summarise the ER

A
  • ER is continuous with the outer membrane of the nuclear envelope
  • It consists of a vast network of tubules which extend through the cytoplasm
  • Rough ER has ribosomes for protein synthesis
  • Smooth ER is for protein synthesis
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2
Q

Give all the purposes of the Smooth ER

A

1 - synthesis of lipids such as phospholipids and cholesterol which form cell membranes
2 - synthesis of steroid hormones like testosterone
3 - storage and release of Ca2+ (sarcoplasmic reticulum in muscles)
4 - detoxification (present in the liver)

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3
Q

What is the role of specifically the Rough ER

A
  • The site of protein synthesis and processing of transmembrane proteins destined for:
    ER, Golgi, Plasma membrane, Lysosomes, Endosomes
    and also for the cell exterior
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4
Q

where are proteins synthesised if not on the RER

A
  • On cytosolic ribosomes, which are free in the cytoplasm
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5
Q

what is co-translational translocation

A

The process by which proteins are synthesised on the ribosome at the RER

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6
Q

Describe the process of co-translational translocation

A
  • A specific signal sequence on a protein destined for the ER is recognised by a signal recognition particle (SRP)
  • The SRP also binds to the ribosome pausing synthesis of the polypeptide
  • The SRP takes the ribosome and polypeptide to an SRP receptor on a membrane which is close to a protein translocator
  • The SRP releases the ribosome and the protein translocator transports the polypeptide into the ER, lumen
  • The SRP and SRP receptor dissociate and are recycled
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7
Q

Give a brief summary of co-translation translocation

A
  • SRP recognises SSS on polypeptide and binds to it and the ribosome
  • SRP takes the complex to an SRP receptor on ER membrane
  • Polypeptide is transported through protein translocator into ER lumen
  • SRP and ribosome are recycled
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8
Q

Where does the SRP bind

A

onto the Specific signal sequence on the polypeptide chains end terminus

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9
Q

proteins collect in the ER lumen and on the ER membrane why?

A
  • proteins destined to be released out of the cell via exocytosis are fully transported into the ER lumen first
  • Proteins destined to be a membrane protein will just get embedded within the ER membrane
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10
Q

once a protein has been translated and inside the ER how is it modified?

A

1) N-linked Glycosylation, where carbs groups are added

2) The protein is folded into its 3D shape via bond formation, often glycosylation helps this

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11
Q

what is N-linked glycosylation

A
  • When a precursor oligosaccharide composed of 14 sugars is added to the protein via the N terminus (amine group) of an asparagine amino acid
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12
Q

When do these oligosaccharides bind to the protein

A
  • The ER lumen is packed with oligosaccharides and these bind to the protein N terminus in the ER while the back end is still being translated outside of the ER by a ribosome
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13
Q

describe the structure of a regular oligosaccharide

A
  • 14 sugars
  • 3 glucose at 1 end
  • 2 N-acetyl glucosamine at the other end
  • 9 Mannose sugars in the middle which branch out
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14
Q

once the protein has folded correctly in the ER lumen what change occurs to the oligosaccharide

A
  • 3 glucose and 1 mannose sugar is cleaved from the oligosaccharide
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15
Q

What is the importance of glycosylation

A
  • the Oligosaccharide ensures the protein folds correctly

- this prevents build up of toxic proteins

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16
Q

describe the mechanism by which proteins are checked and altered to ensure they fold correctly

A
  • When the protein is first folded 3 glucose and 1 mannose are cleaved from the oligosaccharide
  • If not folded correctly glucosyl transferase enzymes add a glucose back onto the oligosaccharide
  • a Chapperone protein then binds to this glucose structure and holds down the protein until it is refolded
  • another enzyme called glucosidase then cleaves off the glucose
  • if the protein is still folded incorrectly glycosyl transferase adds another glucose to repeat
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17
Q

What occurs when the unfolded protein response is triggered in a cell due to accumulation of too many misfolded proteins

A
  • Protein synthesis is inhibited
  • misfolded proteins are degraded
  • More chaperone proteins are transcripted to bind onto the misfolded proteins until they fold correctly
  • Apoptosis as a last resort
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18
Q

How does the pulse chase experiment discover the secretory pathway in cells

A
  • A cell is fed radioactive labelled amino acids for 3 minutes
  • The cell is then fed regular amino acids
  • proteins which have been synthesised with the radioactive amino acids can be traced
  • so the pathways of the proteins can be viewed and worked out
  • this discovered the secretory pathway
19
Q

what are the 3 main pathways involved in intracellular membrane traffic

A

1) Secretory pathway
2) Endocytic pathway
3) Retrieval pathway

20
Q

Things are moved in the cell using vesicles, what types of vesicles are there?

A

1- Clathrin coated vesicles
2- COP1 coated vesicles
3- COP2 coated vesicles

21
Q

Where do clathrin coated vesicles transport their content

A
  • from the plasma membrane

- and between the Golgi and endosomes

22
Q

where do COP1 and COP2 coated vesicles transport their content from

A
  • COP1 - from the Golgi

- COP2 - from the ER

23
Q

Describe how Rab is used to ensure specificity, meaning that the right vesicle finds the right membrane to fuse with

A
  • Rab-GDP is inacitve
  • Inactive Rab-GDP binds to GEF (guanin exchange factor) on a vesicle membrane,
  • GEF exchanges a GDP for GTP, by adding a gaunine to the GDP. this produces Rab-GTP
  • Rab-GTP is active and a rab effector protein on the vesicle binds to it if its the right type of Rab securing it to the vesicle
24
Q

what is GEF

A

Guanine exchange factor, this causes inactive Rab-GDP to gain a guanine becomming active Rab-GTP
- basically GEF activates Rab

25
Q

What is a Rab effector protein

A
  • a protein on a vesicle which binds and secures a specific type of Rab to the vesicle before Rab drives it to the target membrane
26
Q

What type of proteins are needed for membrane fusion

A
  • Snare proteins
27
Q

What types of snare proteins are there and where are they located

A
  • V-snares are located on the vesicle

- T-snare are located on the target membrane

28
Q

How do snare proteins work

A
  • They are very specific but when the right V and T snare proteins come into proximity, they wrap around each other to form a stable trans-snare complex
29
Q

What are the 3 stages in the process of membrane fusion process

A
  • Tethering
  • Docking
  • Fusion
30
Q
  • Describe the process of membrane fusion
A
  • Tethering: Rab-GTP on the vesicle recongises and tethers with the Rab-effector protein on the target organelle membrane
  • Docking: The vesicle is pulled towards the organelle membrane, the T-snare and V-snare wrap around each other forming a trans-snare complex
  • Fusion: Once the vesicle and organelle membrane are within 1.5nm proximity the membranes can fuse releasing the vesicular cargo
  • Finally GDI binds to Rab-GTP, causing Rab to hydrolyse GTP into GDP becoming inactive Rab-GDP and releasing from the vesicle membrane
31
Q

how many proteins are involved in membrane fusion and list their purpose

A
  • Rab-GTP to recognise and tether with the right organelle membrane
  • Rab effector protein, is on the target membrane and tethers specifically with Rab-GTP on a vesicle
  • V-snares on the vesicle membrane ready to form a complex with T-snares
  • T-snares on the target membrane which form a trans-snare complex with specific V-snares
  • GDI which causes Rab-GTP to hydrolyse the GTP into GDP recycling the Rab-GDP
32
Q

what is a vesicular tubular cluster

A
  • when multiple vesicles with the same snare proteins fuse forming a large one
33
Q

How are proteins synthesised in the ER lumen takes to the Golgi

A
  • COP2 coated vesicles bud off from the ER containing the proteins
  • These vesicles can fuse together if they have the same snare proteins forming a vesicular tubular cluster
  • Motor proteins then pull these large sacs of proteins along microtubules to the cis Golgi network
34
Q

What happens to any proteins taken by accident by COP2 coated vesicles or when the golgi wants to send proteins back to the ER ?

A
  • These proteins bud off from the vesicular tubular clusters or the golgi in COP1 coated vesicles back to the ER
  • this is the retrieval transport pathway
35
Q

Describe the Golgi

A
  • stacks of flattened membranes called cisternae
  • 2 sides, Cis-face which receives vesicles from ER and trans-face where cargo is packaged up and sent away
  • Sorting office and site of more modification of proteins
36
Q

What type of modifications occur in the Golgi network

A
  • both O linked and N linked glycosylation

- in each cisternae a different type of modification occurs

37
Q

What are the 2 pathways by which substances from the trans golgi network leave the cell

A
  • Constitutive secretory Pathway

- Regulated secretory Pathway

38
Q

Describe the constitutive secretory pathway

A
  • Vesicles bud off from the trans-golgi network and directly go and fuse with the cell membrane releasing their content via exocytosis
39
Q

Describe the regulated secretory pathway

A
  • Vesicles bud off from the trans-golgi network
  • the vesicles wait at the cell periphery for stimulus such as a hormone binding to the cell membrane
  • once stimulated the vesicle fuses with the cell membrane releasing its content via exocytosis
40
Q

Describe lysosomes

A
  • They are vesicles containing lots of digestive enzymes/ acid hydrolases
  • pH in lysosome is 4-5
41
Q

How are the proteins and enzymes for lysosomes delivered from the golgi

A
  • in clathrin coated vesicles
  • The proteins have a mannose-6-phosphate tag to ensure they are directed the right way as these are very destructive enzymes
42
Q

Describe the cycle of a late endosome to a lysosome

A
  • Late endosomes contain acid hydrolases and waste material from the Golgi
  • The late endosome firstly fuses with a lysosome or an endolysosome, to create a larg endolysosome
  • Once the large endolysosome has digested all the material it becomes a lysosome
  • this fuses with another late endosome to repeat
43
Q

Describe how acid hydrolases get from the RER to the late endosome

A
  • acid hydrolase proteins travel from the RER to the golgi as mannose-6 proteins
  • The golgi modifies them adding a phosphate group becoming mannose-6-phosphate
  • mannose-6-phosphate is sent to a late endosome via a clathrin coated vesicle

Cell Biology and Biochemistry (9 decks)

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