ER, traficking, Co-translational import

Question 1

Functions of the smooth ER

Answer 1

1. Lipid synthesis
2. Production of steroid hormones like glucocorticoids and estrogens - endocrine cells
3. Detoxification - liver cells contain enzymes that modify foreign compounds
4. Sequestration (storage) of Ca2+

Question 2

Intracellular handling in cells

Answer 2

-Cells invest a significant amount of their energy to control changes in Ca2+ concentration

Question 3

Why is Ca2+ so avidly excluded from the cytosol

Answer 3

-The principal reason is that Ca 2+ does not bind water very well

-Ca 2+ will precipiatet phosphates and make proteins insoluable

-As a result, calcium in the cytosol is either bound by a range of Ca2+ binding proteins, forced out through pumps and transporters, or sequested into specific organelles (often referred to as Ca 2+ or ‘sinks or pools’

Question 4

Functions of the rough ER

Answer 4

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

Question 5

Glycosylation of proteins

Answer 5

-Addition of carbohydrate chains to specific proteins

Question 6

Protein folding

Answer 6

-Quality control
-Involve the activity of molecular chaperones
-Specific type of protein that assist in the folding process

Question 7

Protein synthesis, modification and transport

Answer 7

-Proteins targeted ER

-Proteins targeted to other endomembrane compartments (soluble and transmembrane)

-Proteins targeted to plasma membrane (secreted and transmembrane)

Question 8

Searching for the start codon

Answer 8

-Initiation factors recruit the small ribosomal subunit and tRNA Met and scan the mRNA for an AUG codon

Question 9

Beginning of elongation

Answer 9

-When the complex reaches an AUG, the large ribosomal subunit joins and the initiation factors are released

-A tRNA which is complementary to the next codon binds to the A site

Question 10

Elongation

Answer 10

• A reaction transfers the Met to the amino acid on the tRNA in the A site, forming a peptide bond

-The ribosome moves down one codon, which puts the amino acid carrying the polypeptide into the P site and the now -uncharged tRNA into the E site where is ejected

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

Question 11

RER and protein translation

Answer 11

-All protein translation begins on free ribosomes (ie not associated with the ER)

-Translation is completed in 1 of 2 ways Free of ER bound ribosomes

Question 12

Free ribosomal translation

Answer 12

• Cytosolic proteins

-Peripheral membrane proteins

-Proteins targeted to nucleus, mitochondria, peroxisomes, chloroplasts

Question 13

ER bound ribosomal translation

Answer 13

-Secreted proteins

-Integral membrane proteins

-Soluble proteins associated with inside (lumen) of endomembrane system

-(ie proteins that function within the ER, Golgi, lysosomes)