Intracellular Membrane Traffic

Question 1

Proteins shuttled from cytosol to nucleus and vice versa

Answer 1

Via gated transport

Question 2

Protein Translocation

Answer 2

When proteins move from different membrane bound organelles. EX. Plastids, Mitochondria, Peroxisomes, ER

Question 3

Vesicular Transport

Answer 3

Proteins transported by vesicles ER to Golgi and vice versa. Late endosome, Lysosome, Early Endosome, Secretory Vesicles. Going to cell exterior

Question 4

Transport Vesicles

Answer 4

Bud off from one compartment (donor) and fuse with another (target). Carry material as cargo from donor compartment to membrane of the target compartment.

Question 5

Vesicle Budding

Answer 5

Requires a membrane fusion event triggered from the lumenal side of the membrane.

Question 6

Vesicle Fusion

Answer 6

Requires a fusion event initiated from the donor and target membranes.

Question 7

Different coats used in Vesicular Transport

Answer 7

Select different cargoes and have different transport paths that serve diff functions.

Clathrin, COPI, and COPII

Question 8

Structure of Clathrin Coat

Answer 8

Triskelion Structure made from 3 heavy and 3 light chains. Multiples of 3 pentagons, hexagons, sometimes heptagons resemble deformed soccer balls

Question 9

Assembly/ Disassembly of Clathrin Coat

Answer 9

The assembly of the coat introduces curves in the membrane, leading to uniformly sized coated buds. Adaptor Proteins bound to specific cargo receptors bind to clathrin triskelion’s.

After the clathrin vesicle is formed its coat is lost rapidly. Uncoating regulated by cytoplasmic protiens and pinching off.

Return from plasma membrane endosomes, Golgi

Question 10

Rab Proteins Guide Vesicle Targeting

Answer 10

Rab(GTPases) direct vesicle to specific spots on the correct membrane by interacting with Rab effectors on surface. Establish some connection not really strong.

Ex. Rab proteins can be large multiprotein complexes or long fibrous tethering proteins.

Specific organelles have specific Rab effectors on thier surface that respond to specific Rab proteins.

Question 11

SNARE protiens

Answer 11

Help to dock the Rab directed vesicle to the target membrane and catalyze the fusion of the membranes

Question 12

Ran GTPase also

Answer 12

Imposes directionality on transport through NPC (Nuclear Pore Complexes)

Small GTPase Ran functions like a witch present in ON/ active form (Ran-GTP) or OFF/inactive (Ran- GDP)

Ran GAP activating protein in cytosol (Takes phosphate off ) GDP
Ran GEF located in nucleus binds to chromatin (Puts on to make GTP)

Question 13

Recruitment of Cargo molecules into ER transport vesicles

Answer 13

By binding indirectly or directly wit the COPII coat membrane and soluble cargo proteins become concentrated in the transport vesicle. Some proteins that COPII binds can help act as cargo receptors bring more proteins to package in the vesicle.

Many diff types of proteins.

Question 14

Formation of COPII Vesicle

Answer 14

Membrane bound active Sar-GTP recruits COPII subunits to the membrane.

Question 15

COPI

Answer 15

Mediates the budding of vesicles that return to the ER from Golgi and vesicular tubular clusters.

Usually ER resident proteins. Via KDEL receptor in vesicular tubular clusters recruit budding COPI

Question 16

Compartmentalization of the Golgi

Answer 16

Oligosaccharide processing happens in Golgi compartments. Processing enzymes not restricted to a particular cistern, graded across the stack. Early acting enzymes first in cis, later acting in trans cistern.

Golgi Struc.
cis network
Golgi Stack - Cis cistern, medial cistern, trans cistern
trans network

Question 17

Oligosaccharide Processing Pathway in Golgi

Answer 17

Highly ordered, so that each step shown depends on the previous one

Question 18

Transport through the Golgi

Answer 18

Individual and small proteins transported through vesicle- mediated transport or cisternal maturation

Large proteins through cisternal maturation

Question 19

Three pathways of Protein Sorting in the trans Golgi network

Answer 19

1) Proteins with the mannose 6- phophate (M6P) marker are diverted to lysosomes(via endosomes) in clathrin coated vesicles.

2) Proteins with signals directing them to secratory vesicles are concentrated in vesicles as a part of a regulatory secretory pathway in specialized secretory cells.
3) In unpolarized cells, a constitutive secretory pathway delivers proteins with no special features to the cell surface

Question 20

Transport from Trans Golgi

Answer 20

Cell Exterior Exocytosis

Question 21

Receptor Mediated Endo cytosis of LDL

Answer 21

LDL disassociated from receptor in acidic environment of early endosome LDL in lysosome degraded to give free cholesterol.

LDL receptors returned to plasma membrane clathrin coated vesicle transport.

Question 22

Endosome Pathway

Answer 22

Maturation of early to late endosomes occurs through the formation of multivesicular bodies, contain large amounts of invaginated membrane and internal vesicles.

Multivesicular bodies are transported inward by microtubules by anchor proteins. As they progress downward they increase maturation to late endosomes, when late no longer send vesicles to the plasma membrane.

Question 23

Lysosomes are delivered materials for degradation

Answer 23

Through multiple pathways phagosome, autophagosome, endosome

Question 24

Mannose 6- phosphate receptor

Answer 24

Sorts lysosomal hydrolases in the trans golgi network adds mannose 6-phosphate (M6P) groups to precursors of lysosomal enzymes. Segregate from other types of proteins in the TGN go into clarthin coated vesicle.

The vesicle goes to the endosome low pH receptors disassociate from the receptor, empty receptors are recycled in retromer-coated vesicles to Golgi apparatus. Phosphate removed from the mannose sugars attached to the hydrolases, ensuring that the hydrolases do not return to the Golgi apparatus with the receptor