Lecture 19: ER, Trafficking, Cotranslational Import

Question 1

Transmembrane protein transport orientation

Answer 1

Longer end of protein faces lumen (glycosylated part of protein with oligosaccharides) and shorter side faces cytosol

Question 2

ER types

Answer 2

• rough ER - associated with ribosomes, protein synthesis
• smooth ER - lacks ribosomes and synthesizes lipids

Question 3

Smooth ER functions

Answer 3

1. Lipid synthesis
2. Production steroid hormones - in endocrine cells
3. Detoxification - in liver cells contain enzymes that modify foreign compounds
4. Sequestration (storage) - in muscle cells

Question 4

Where is the lumen and where is the cytosol in the Rough ER?

Answer 4

• lumen - light parts
• cytosol - dark structures

Question 5

Rough ER function

Answer 5

1. Synthesis of membrane phospholipids
2. Glycosylation of proteins - adding carbohydrate chains
3. Protein folding - molecular chaperones which fold
4. Protein synthesis, modification and transport - proteins targeted to ER, other endomembrane compartments, PM, all proteins synthesized in ER

Question 6

Protein synthesis: overview

Answer 6

mRNA molecule with 5’ cap, ribosomes scan and translation begins, makes protein, terminates at STOP codon

Question 7

All translation in our bodies is completed in 1 of 2 ways:

Answer 7

Free or ER bound ribosomes

Question 8

Free (not associated with ER) ribosomes translation

Answer 8

• cytosol if proteins
• peripheral membrane proteins
• proteins targeted to nucleus, mitochondria, peroxisomes, chloroplasts

Question 9

All translation happens in…

Answer 9

Cytosol

Question 10

When transporting proteins, a signal indicates…

Answer 10

Where the protein will do, signals must be indicated to go to specific compartments, to go to the cytosol, you dont need to have a signal, it will automatically go tot he cytosol if not signal is present

Question 11

ER membrane ribosomes Translation:

Answer 11

• secreted proteins
• integral membrane proteins
  Soluble proteins associated with inside (lumen) of endomembrane system

Question 12

How is the site of translation determined?

Answer 12

• signal sequence is located in: amino terminus
• contains hydrophobic AA
• directs synthesis to ER compartment
• protein. moves through channel into ER for cotranslational import

Question 13

Cotranslational protein import steps:

Answer 13

1. After translation of signal sequence, signal recognition particle (SRP) binds to signal sequence and ribosome and STOPs the translation process
2. SRP binds SRP receptor to target the whole translation complex to ER (SRP + ribosome + mRNA + new polypeptide)
3. SRP is released and ribosome binds to translucent. Once this is done, protein synthesis resumes
4. Polypeptide enter the ER (through the translocon) as it is translated. In the end, the signal peptide is cleaved off and chaperone folds the protein

Question 14

Exocytosis is..

Answer 14

When cellular components are moves to the plasma membrane (exterior) via bio synthetic/secretory pathway

Question 15

Endocytosis is…

Answer 15

Cellular components are moved from the plasma membrane to the organelle within the cell via endocytic pathways

Question 16

Example of exocytosis

Answer 16

Organelle to plasma membrane

Question 17

Exocytosis: two kinds

Answer 17

1. Secretion - all proteins that function outside of PM is trafficked to PM through secretory pathways, produced in ER eventually to PM and released
2. Synaptic cleft -

Question 18

Example of endocytosis

Answer 18

Plasma membrane to organelle

Question 19

Two functions associated with endocytosis

Answer 19

• bring in material that serves as a nutrient source - degraded into its most basic components which is released into the cell
• recycling of material

Question 20

Uptake of LDL particle in liver cells steps: (endocytosis)

Answer 20

1. LDL is particle that contains cholesterol, phospholipid mono layer surrounds it, ApoB protein
2. LDL particle binds to receptor and forms a vesicle on the cytosol and buds off
3. LDL particle is degraded to AA, cholesterol and FA, it is done in the late endosome as the receptor is released from the LDL
4. Receptor is reused back to plasma membrane