Hettema lecture 6

Question 1

Describe anterograde/retrograde transport.

locations and coat proteins involved

Answer 1

Anterograde - RER -> cis Golgi (forward), COPII

Retrograde - cis Golgi -> RER (backwards, COPI, proteins that escape the ER are transported back to the Golgi

Question 2

How does a missorted protein in cis-Golgi return to the RER?

Answer 2

The missorted protein contains a KDEL peptide sequence which binds to the KDEL receptor. Vesicle forms -> RETROGRADE TRANSPORT

Question 3

Describe the 2 models of transport through the Golgi complex.

Answer 3

STATIONARY CISTERNAE MODEL
- each Golgi part is stationary and vesicles transport the protein between stacks
- glycosylation occurs at each stack
CISTERNAL MATURATION MODEL
- each vesicular stack matures into the next one
- no vesicular transport
- only retrograde transport
- late Golgi stage fragments into vesicles

Question 4

Give the 5 locations where proteins can end up after being trafficked by the trans Golgi.

Answer 4

• lysosomes
• endosomes
• retrograde transport (vesicles)
• constitutive delivery of proteins to plasma membrane (happens all the time)
• periodic delivery to PM

Question 5

Describe the process of transporting a protein to lysosomes.

Answer 5

glcNAc phosphortransferase transfers a phosphorylated glcNAc molecule to a carbon 6 atom of a mannose sugar on lysosome
phosphodiesterase cleaves glcNAc molecule leaving a phosphate group attached to lysosome

Question 6

Describe the transport of the lysosome enzyme to the actual lysosome following the attachment of glcNAc.

Answer 6

• Man-6-pho added to lysosomal enzyme
• binds to receptor
• Clathrin coat formation and budding
• uncoating
• fusion w/ late endosome
• release from receptor
• dephosphorylation of glcNAc of lysosome
• fusion with lysosome
• recycling of coat proteins and receptor (to PM)
• phosphorylated lysosomal proteins can also be endocytosed through the plasma membrane

Question 7

Give the overall route taken to direct a lysosomal enzyme to the lysosome.

Answer 7

RER -> cis/medial/trans Golgi -> Late endoscopes -> Lysosomes

Question 8

On a molecular level, what occurs in I-cell disease?

Answer 8

• formation and attachment of Man-6-P to lysosomal enzymes is disrupted
• are not directed to lysosomes
• lysosomes contain high proportions of glycolipids and glycoaminoglycans because no enzymes are present to break them down
• lysosomes missing around 8 different acid hydrolases
• high proportions of lysosomal enzymes in blood and urine

Question 9

Describe the 5 different ways on endocytosis.

Answer 9

• macro pinocytosis
• clathrin dependent
• caveolin dependent
• Cathrin/caveolin independent

Question 10

What does receptor-mediated endocytosis depend on?

Answer 10

Clathrin being used as the coat protein

Question 11

What 4 things does the AP (adaptor protein) complex bind?

Answer 11

• receptors for soluble cargo proteins
• clathrin
• membrane
• factors for vesicle formation and release

Question 12

What role does dynamin play in vesicle formation?

Answer 12

Allows the vesicle to be pinched off via GTP hydrolysis

Question 13

Describe the mode of action of pH dependent release of an LDL particle.

Answer 13

outiside the cell, pH = 7 therefore LDL binds to receptor

in endosome, pH = 5 therefore LDL dissociates from receptor and receptor is recycled back to PM

Question 14

What is the cause of familial hypercholesterolaemia?

Answer 14

• mutations in LDL receptor
• leads to high concentrations of LDL in serum
• leads to fat deposition in certain cell types (-> Xanthomas (accumulation in skin and tendons) and atheromas)