B: ER, Trafficking, Co-translational Import (19)

Question 1

What is the ER

Answer 1

Endoplasmic reticulum

Question 2

What is the structure of the ER

Answer 2

Mosaic structure with many cell types inside

Question 3

What is found in the rough ER

Answer 3

Associated with ribosomes
Many proteins, including those destined for secretion are synthesized by ribosomes associated with the rough ER

Question 4

What is found in the smooth ER

Answer 4

Lacks ribosomes
Primary site of lipid synthesis

Question 5

Does the RER or SER have ribosomes

Answer 5

Rough ER

Question 5

What are the functions of the smooth ER (4)

Answer 5

1. Lipid synthesis
2. Production steroid hormones
3. Detoxification
4. Sequestration

Question 6

What steroid hormones are produced in the SER

Answer 6

Ex. glucocorticoids, androgens and estrogens - endocrine cells

Question 7

How does detoxification occur in the SER

Answer 7

Liver cells contain enzymes that modify foreign compounds

Question 8

What sequestration occurrs in the SER

Answer 8

Storage of Ca

Question 9

Why does the cell need to put energy into controlling Calcium levels

Answer 9

Ca2+ does not bind water very well, which will precipitate phosphates and make proteins insoluble

Question 10

How does the cell control Ca levels

Answer 10

Calcium in the cytosol is either bond by a range of Ca2+ binding proteins, forced out through pumps and transporters or sequestered into specific organelles

Often referred to as Ca2+ sinks or pools

Question 11

What are the 4 functions of the rough ER

Answer 11

1. Synthesis of membrane phospholipids
2. Glycosylation of proteins
3. Protein folding - quality control
4. Protein synthesis, modification and transport

Question 12

Explain protein folding within the RER

Answer 12

Involve activity of molecular chaperones, specific types of protein that assist in folding process

Question 13

Where are proteins targeted to in the RER (3)

Answer 13

Proteins targeted to ER

Proteins targeted to other endomembrane compartments (soluble and transmembrane)

Proteins targeted to plasma membrane (secreted and transmembrane)

Question 14

What do ribosomes synthesize in the cytoplasm?

Answer 14

In the cytoplasm, ribosomes synthesize polypeptides from mRNA

Question 15

What are the 4 components of protein synthesis

Answer 15

1. Searching for start codon
2. Beginning of elongation
3. Elongation
4. Termination

Question 16

Explain the first step of protein synthesis:
Searching for start codon

Answer 16

Initiation factors recruit the small ribosomal subunit and tRNA and scan the mRNA for a start codon

Question 17

Explain the second step of protein synthesis: Beginning of elongation

Answer 17

Once the complex reaches AUG, the large ribosomal subunit joins

The initiation factor is released

A tRNA complementary to the next codon binds to the A site

Question 18

Explain the third step of protein synthesis:
Elongation

Answer 18

A reaction transfers the Met to the AA on the tRNA in the A site, forming a peptide bond

The ribosome moves down 1 codon, which puts the AA carrying the polypeptide into the P site and the now-uncharged tRNA into the E site where it is ejected

A new tRNA complementary to the next codon binds to the A site

Question 19

What are free ribosomes

Answer 19

Ribosomes not associated with the ER

Question 20

Where does all protein translation begin

Answer 20

All protein translation begins on free ribosomes

Question 21

Where is protein translation completed

Answer 21

Translation is completed with free or ER bound ribosomes

Question 22

What type of proteins are translated with free ribosomes

Answer 22

Cytosolic proteins

Peripheral membrane proteins

Proteins targeted to nucleus, mitochondria, peroxisomes, chloroplasts

Question 23

What type of proteins are translated with ER bound ribosomes

Answer 23

Secreted proteins

Integral membrane proteins

Soluble proteins associated with inside (lumen) of endomembrane system

Question 24

What is a signal sequence

Answer 24

Signal sequence is:

• Located in amino-terminus (N-terminus)
• Contains several consecutive hydrophobic AAs
• Directs synthesis to the ER compartment

Question 25

What does a signal sequence do

Answer 25

Moves ribosomes to the ER to target protien

Question 26

What happens to ribosomes once they are in the ER

Answer 26

Ribosomes form protein

Protein moves through channel into ER for cotranslational import

Question 27

What is cotranslational protein import

Answer 27

The transport of a polypeptide chain occurs while it is being synthesized on a ribosome bound to the channel

Question 28

What are the 4 steps of cotranslational protein import

Answer 28

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

Question 29

How are proteins targeted to the mitochondria

Answer 29

For mitochondria, the TOM complex is the equivalent of SRP complex and translocon

Question 30

What are the 2 endocytic pathways of protein sorting

Answer 30

1. It is retained in the ER lumen, if that is where it functions
2. It is transported from the ER to the golgi complex for further meditation and delivered to distal paths of biosynthetic/secretory pathways.