Lecture 19 - Vesicular Traffic: the ER & Endoplasmic Reticulum

Question 1

basis of the secretory pathway:

Answer 1

transport vesicles bud from one membrane and fuse with another, carrying membrane components and soluble proteins between compartments of the endomembrane system and the plasma membrane

Question 2

where does cargo bud off the rough endoplasmic reticulum?

Answer 2

cargo buds off from the rER at specific points known as ER-exit sites (ERES)

Question 3

______ _______ ________ are involved in cargo selection and membrane budding and essential for protein export from the ER

Answer 3

COPII coated vesicles

Question 4

COPII vesicles undergo…

Answer 4

… fusion to form vesicular tubular clusters (VTCs)

Question 5

what is Sar1:

Answer 5

SAR1A (mammalian name) or Sar1 (yeast name) is a protein involved in membrane trafficking

is is a monomer in small GTPase found in COPII vesicles and isn a member of the ‘ras superfamily’

Question 6

what does Sar1 specifically do?

Answer 6

Sar1 regulates the assembly and disassembly of COPII coats

GDP-bound Sar1 interacts with the membrane-bound exchange factor Sec12 and exchanges its bound GDP for GTP

Question 7

what is vesicle building driven by?

Answer 7

vesicle building is driven by the assembly of a protein coat

Question 8

what is the first step in COPII vesicle formation?

Answer 8

the first step in COPII vesicle formation is where the ER integral protein membrane protein Sec12 exchanges GDP for GTP bound to Sar1 through its GEF activity

Question 9

second step in COP11 vesicle formation:

Answer 9

membrane assoaciated GTP-bound Sar1 recruits the inner coat Sec23/24 complex where GTP hydrolysis by Sar1 is mediated by the activity of GTPase activating protein GAP, which accelerates GTP hydrolysis

the Sec23/24 complexes assemble along with cargo protein into the pre-budding complex

Question 10

second step in COPII formation:

Answer 10

Outer coat Sec13/31 complexes are recruited to the pre-budding complexes and self- assembled by crosslinking. The polymerization of Sec13/31 by self-assembly drives
membrane curvature to form a spherically shaped vesicle

Question 11

functions of the golgi apparatus trafficking system:

Answer 11

Functions in the trafficking system:
•concentration: less area than ER
•modification: glycosylation
•sorting: plasma membrane, lysosomes,
back to ER

Question 12

appearance of the golgi apparatus:

Answer 12

•electron microscope: ‘stacks’
•light microscope: depends on cell type

Question 13

golgi glycolisation:

Answer 13

O-linked oligosaccharides are linked to the hydroxyl group of serine or threonine via N-acetylgalactosamine (GalNac) or (in collagens) to the hydroxyl group of hydroxylysine via galactose.

Question 14

modification, packaging and sorting with lysosomal proteins:

Answer 14

(eg hydrolytic enzymes) are tagged with mannose-6-phosphate in the Golgi then packaged into specific
vesicles for onward trafficking

Question 15

modification, packaging and sorting with secretory proteins:

Answer 15

(eg insulin) are packaged into secretory vesicles ready for exocytosis where the membrane of the vesicle will fuse with the cell membrane

Question 16

modification packaging and sorting with cell surface proteins:

Answer 16

such as phospholipids and glycoproteins enter the constitutive secretory pathway