Lecture 20 - Protein Trafficking

Question 1

constitutive secretion:

Answer 1

vesicles move directly from the golgi to the plasma membrane

Question 2

regulated secretion:

Answer 2

fusion of vesicles with the plasma membrane depends on a signal

Question 3

exocytosis as an example of constitutive secretion:

Answer 3

some cytokines are secreted from macrophages via the constitutive route - this requires recycling endosomes and Rab as well as VAMP interactions with syntaxin-4 at the plasma membrane

Question 4

insulin exocytosis:

Answer 4

insulin is secreted by beta cells which is triggered by a rise in blood glucose levels

the glucose uptake is taken in by a transporter called GLUT2

Question 5

what do lysosomes consist of?

Answer 5

lysosomes consist of membrane bound sacs of hydrolytic enzymes

Question 6

lysosome pH:

Answer 6

lysosomes have a low pH due to an ATP driven pump that pumps H+ ions into the lumen

the pH is made low this way due to the hydrolytic enzymes being active in acids conditions

Question 7

how are dangerously proteins from the lysosome handled?

Answer 7

• All proteins delivered via ER and Golgi
• Dangerous: hence made as inactive precursors with ‘extra’ peptide
• ‘Extra’ peptide cleaved off in lysosome
• ..and a sorting problem

Question 8

what are sorting/signal patches?

Answer 8

sorting or signal patches are a specific 3D arrangement on the surface of a folded protein similar to the active site on an enzyme

Question 9

importance of mannose-6-phosphate:

Answer 9

lysosomal proteins recognised in cis-golgi by ‘sorting patch’

two stages: add P-GlcNAc to mannose on N-linked sugar then remove GlcNAc - mannose6phosphate

receptor in trans0golgi picks this up and sorts to vesicles, which transfer to lysosomes

Question 10

clathrin geometry:

Answer 10

inside face of the membrane (also on golgi)

‘coated pit’ consisting of flat hexagons and curved pentagons - the 12 pentagons make a sphere

Question 11

how does the vesicle get pinched off:

Answer 11

the vesicle will finally form as it is pinched off via a protein called dynamin which polymerises around the neck - the hydrolysis of GTP drives this conformational change that separates the vesicle from the membrane

Question 12

phagocytosis:

Answer 12

the ingestion of large particles such as microorganisms

Question 13

receptor-mediated endocytosis:

Answer 13

specific cargo taken up by a transmembrane receptor

Question 14

cathrin mediated endocytosis - adaptins:

Answer 14

adaptin complexes bind to ‘signal’ in cytoplasmic tail of receptor and to clathrin - hence selective sorting into clathrin coated vesicle

Question 15

the endosome - key sorting station:

Answer 15

first destination for endocytosis vesicles, sorting station for the endocytosis pathway, some molecules sent to lysosome, some recycled

Question 16

where next from the endosome?

Answer 16

the next destination depends on whether the ligand and receptor within the membrane of the vesicle endocytosed stay associated

If receptor bound for recycling to plasma membrane it will go there; if receptor bound for transfer to other edge of cell then they will transcytose.

If ligand and receptor do not remain bound (due to acidic nature of endosome – then it will go to lysosome for degradation

Question 17

three examples of receptors for clathrin mediated endocytosis:

Answer 17

•LDL receptor
•Transferrin receptor
•EGF receptor

Question 18

lipoprotein particles:

Answer 18

‘packages’ of lipids and cholesterol bound by apoprotein B-100

Question 19

low density lipoprotein (LDL):

Answer 19

‘bad cholesterol’

Question 20

the LDL receptor:

Answer 20

a transmembrane protein, the LDL receptor binds to LDL outside cell and adaptins inside the cell, adaptins bind to clathrin

LDL is internalised in coated vesicles which are delivered to lysosomes

Question 21

what is the aim of the transferin receptor?

Answer 21

aim: to get iron into the cell (Fe3+)

Question 22

The EGF Receptor:

Answer 22

‘Epidermal’ growth factor
•EGF binds
•Receptor becomes dimer
•Tyrosine kinase phosphorylates itself
•…and other things
•Cells proliferate
•Growth factor = ‘on’ switch for cell proliferation
•Must have an ‘off’ switch
•‘Off’ switch = endocytosis of EGF and receptor to lysosome

Question 23

what can result from problems occuring with Mannose-6-phosphate?

Answer 23

the hydrolytic lysosomal enzymes can be secreted into the bloodstream

I-cell disease or mulcolipidosis II

Question 24

cystic fibrosis cause:

Answer 24

problems with airways as the ATP dependant chloride channel protein can be mutated with the common F508 mutation which causes the protein to not be able to retain its shape