Lecture 12- Organelles 2

Question 1

Functions of the ER

Answer 1

Protein synthesis
Protein modification
Protein quality control
Lipid synthesis
Synthesis of steroid hormones
Detoxification of lipid soluble drugs
Ca 2+ storage

Question 2

Protein synthesis in the ER is responsible for producing:

Answer 2

Proteins destined for the lumens or membranes of ER, Golgi, lysosomes, or endosomes

Proteins destined for the plasma membrane

Proteins destined for secretion to the cell exterior

Proteins DO NOT return to the cytosol–go to other locations in vesicles

Question 3

Import of proteins into the ER

Answer 3

COTRANSLATIONALLY (ie while the protein is being synthesized)

This is the ONLY place where co-translational transport occurs

All protein synthesis begins in the cytosol–an ER targeting signal sequence will direct the ribosome to the ER membrane where synthesis will continue

Question 4

ER signal sequences are directed to the ER membrane by what? And how?

Answer 4

SRP and SRP receptor.

Signal recognition particle in the cytosol binds to the ER signal sequence and an SRP receptor or docking protein embedded in the ER membrane

Steps:

• SRP binds to ER signal sequence thereby slowing protein synthesis
• Ribosome-SRP complex binds to the ER membrane with the SRP binding to the SRP receptor and the ribosome binding to the translocation channel
• Binding of SRP to its receptor causes the SRP to release the signal sequence thereby allowing protein synthesis to resume with the polypeptide being threaded through the translocation channel.
• SRP recycled back to the cytosol

Question 5

Import of soluble proteins into the ER lumen

Answer 5

• Signals almost always at the N-terminus
• One the ribosome-SRP complex has bound to the ER membrane the N terminal signal sequence opens the translocation channel and remains bound to it.
• Rest of the protein threaded through the channel as a large loop
• Signal sequence is cleaved off by signal peptidase on the luminal side of the ER membrane, releasing the newly synthesized protein into the ER lumen

Question 6

Import of membrane proteins into the ER membrane

Answer 6

• ER signal sequence may be located at the N-terminal or internally
• Some parts of the polypeptide chain are transported across the membrane
• Membrane spanning domains are released laterally from the translocation channel to become embedded in the ER membrane
• Will only be inserted in one particular topology

Question 7

Signal peptidase

Answer 7

Cleave ER signal sequences (the N-terminal ones)

Located in the ER lumen

Question 8

N-linked Glycosylation

Answer 8

• Most proteins imported into the ER lumen or membrane are converted to glycoproteins by the covalent additions of sugars
• A preformed oligosaccharide of 14 sugars is covalently attached to asparagine residues during translocation (cotranslationally)
• Oligosaccharyl transferase in the ER lumen transfers the oligosaccharide block from dolichol to the polypeptide as it emerges from the ribosome
• Attached to NH2 residue of Asn

Question 9

Hydroxylation of collagen

Answer 9

Collagen molecules are hydroxylated on prolines and lysines to allow interchain hydrogen bonds to help stabilize triple stranded helix of collagen molecules

Question 10

Protein folding and disulfide bond formation

Answer 10

Chaperone proteins help to ensure correct folding

Disulfide bonds between cysteine side chains are formed in the ER by protein disulfide isomerase (ER lumen)

Question 11

Assembly of multisubunit proteins

Answer 11

Assembled with partner polypeptides to form the mature protein within the ER

Question 12

Retention of ER resident proteins

Answer 12

ER retention signals indicate which proteins will reside in the ER

KDEL at C terminus

Question 13

Unfolded protein response

Answer 13

Triggered when the quality control signal becomes overwhelmed and misfolded proteins accumulate in the ER.

• Signals ER to expand in size and increase its chaperones
• If load is still too large, will instruct apoptosis

Question 14

Membrane lipid synthesis in the ER

Answer 14

Enzymes in the cytosolic half of the ER bilayer synthesize new phospholipids from free fatty acids and insert them exclusively into the cytosolic half of the bilayer

lipids delivered from ER to golgi, lysosomes, endosomes, and plasma membrane via vesicles

Transfer to peroxisomes and mitochondria requires cytosolic lipid carrier proteins

Question 15

Scramblases

Answer 15

Move random phospholipids from one half of the bilayer to the other to redistribute evenly

Question 16

Flippases

Answer 16

Located in the Golgi and plasma membrane
Move specific phospholipids from one side of the bilayer to the other to create an asymmetric distribution of phospholipids that is maintained during vesicular transport

Question 17

Functions of smooth ER

Answer 17

Synthesis of steroid hormones

Detoxification of lipid soluble drugs

Question 18

Two major pathways along which transport vesicles travel

Answer 18

Secretory pathway (outward)–proteins synthesized in the ER are delivered to the cell surface or lysosomes via the Golgi

Endocytic pathway (inward)–extracellular molecules are taken up at the plasma membrane and delivered to lysosomes, via endosomes, for degradation

Question 19

Protein coat of vesicles

Answer 19

• Drives vesicle budding

- Discarded prior to fusion with target membrane

Question 20

Clathrin coated vesicles

Answer 20

• Bud from golgi apparatus in the outward secretory pathway and from the plasma membrane in the inward endocytic pathway
• Assembles on cytosolic trans surface of golgi
• Basketlike network of hexagons and pentagons
• Introduce curvature into the membrane

Question 21

Adaptins

Answer 21

• Second major coat protein in clathrin-coated vesicles
• Bind the clathrin coat to the vesicle membrane
• Help select cargo molecules for transport via its interaction with both clathrin and various transmembrane cargo receptors
• Vary according to the nature of the cargo they are binding

Question 22

Dynamin

Answer 22

• Small monomeric GTP binding protein
• Assembles as a ring around the neck of each bud
• Hydrolysis of bound GTP causes the ring to constrict, pinching off the vesicle from the membrane
• After pinching the clathrin coat is quickly removed

Question 23

Rabs

Answer 23

Large subfamily of monomeric GTPases that serve as the molecular markers identifying each membrane type

Recognized by tethering proteins on target membranes

Question 24

Tethering proteins

Answer 24

Capture vesicles via their interaction with Rabs

Question 25

SNARES

Answer 25

Interact to dock vesicles
Play a central role in catalyzing the fusion events
Wrap around each other and pull the two membranes close enough to fuse
SNARE disassembled after fusion and recycled

t-snares–on target membranes
v-snares–on vesicles