Lecture 19

Question 1

What are the 2 ways of translation?

Answer 1

1) translation on FREE ribosome: cytosolic proteins, peripheral membrane proteins, proteins targeted to nucleus mitochondria, peroxisomes, chloroplasts
2) translation with ER BOUND ribosomes: secreated proteins, integral membrane proteins, soluble proteins associated w/ inside of endomembrane system

Question 2

Rough ER functions

Answer 2

1. synthesis of membrane phospholipids
2. glycosylation of proteins: addition of carbohydrate chains to specific proteins
3. protein folding- quality control: involve activity of molecular chaperones, specific protein assisting in the folding process
4. protein synthesis, modification, and transport: proteins targeted to ER, targeted to other endomembranes compartments, targeted to plasma membrane

Question 3

What is the smooth endoplasmic reticulum?

Answer 3

• lacks ribosomes and is primary site of lipid synthesis

Question 4

Why do proteins move through channels to get into the ER?

Answer 4

• cotranslational import

Question 5

How do ribosomes synthesize polypeptides from mRNA?

Answer 5

1) searching for START codon: AUG
- initiation factors recruit small ribosomal subunit and tRNA and scan mRNA for AUG codon
2) beginning of elongation
- when complex reaches AUG, large ribosomal subunit joins, initiation factors are released and tRNA complementary to the next codon binds to A site
3) elongation
- reaction transfers met to amino acid on tRNA in A site, forming a peptide bond
- ribosome moves down 1 codon, putting amino acid carrying polypeptide into P site and now-uncharged tRNA into E sire where its ejected
- new tRNA complementary to next codon binds to A site
4) termination

Question 6

What is cotranslational import?

Answer 6

1. after translation of SIGNAL SEQUENCE, signal recognition particle binds to signal sequence and stops translation process
2. SRP binds SRP receptors to target whole translation complex to ER
3. SRP released and ribosome binds to translocon: once done, protein synthesis resumes
4. polypeptide enters ER as its translated: in the end, signal peptide is cleaved off and chaperone folds protein

Question 7

Rough ER functions

Answer 7

1. synthesis of membrane phospholipids
2. glycosylation of proteins: addition of carbohydrate chains to specific proteins
3. protein folding- quality control: involve activity of molecular chaperones, specific protein assisting in folding process
4. protein synthesis, modification and transport: proteins targeted to ER
5. protein translation beings on free ribosomes

Question 8

What is the rough endoplasmic reticulum?

Answer 8

• associated w/ ribosomes

- many proteins involved destined fro secretion are synthesized by ribosomes associated w/ rough ER

Question 9

How does protein targeting on mitochondria work?

Answer 9

• diff strategies exist for diff proteins synthesized on free ribosomes targeted to mitochondria/chloroplasts
• for mitochondria, TOM complex is equivalent of SRP complex and transcolon

Question 10

How are ribosomes targeted to ER membranes and what is it?

Answer 10

Signal sequence

• located in amino-terminus
• contains several consecutive hydrophobic amino acids
• directs synthesis to ER compartment

Question 11

Smooth ER functions

Answer 11

1. lipid synthesis
2. production steroid hormones
3. detoxification- liver cells contain enzymes that modify foreign compounds
4. sequestration of Ca^2+

Question 12

What is the ER?

Answer 12

a compartment of flattened sacs and tubules