Chapter 7

Question 1

What are the four general principles for protein synthesis?

Answer 1

1. Ribosomes are located in cytosol (they can be free or located on membrane bound organelles) slide 6 picture
2. Synthesis of all proteins start on free ribosomes, but some will finish up translation while the ribosomes become membrane bound slide 7
3. Results in two major branches of protein sorting (cytosolic vs secretory) slide 8
4. Final destinations of protein are associated with which pathway it came from (cytosolic or secretory) slide 8

Question 2

What is the cytosolic pathway?

What are it’s targets?

Answer 2

Proteins are synthesized on free ribosomes in cytoplasm then released directly to the cytoplasm
Then moved to final destination by recognition of target amino acid sequences (signal seqeunces)
Targets of cytosolic pathways include:
Non membrane bound cytosolic proteins
Nucleus
Mitochondria
Peroxisomes
Chloroplasts
Slide 11

Question 3

What is the secretory pathway?

What are it’s targets?

Answer 3

The ER, Golgi apparatus, lysosomes and vesicles form the secretory pathway
Carries out sorting of both free and membrane bound proteins to intracellular destinations and secretes proteins from cell
Targets: nuclear membrane proteins, ER proteins, lysosomes, endosomes, Golgi proteins, secretory vesicles
Slide 14

Question 4

Where is the rough ER located?

What’s the differences between rough ER and smooth ER?

Answer 4

It is continuous with the outer nuclear membrane
Slide 17 pictures

Difference is Rough ER has ribosomes and is protein metabolism and is entry point for most proteins into secretory pathway and smooth ER doesn’t have ribosomes and is lipid metabolism

Question 5

How do the majority of proteins destined for secretory pathway translocate into the ER during synthesis on membrane bound ribosomes?

Answer 5

The signal for ribosome attachment to the RER was demonstrated to be an amino acid sequence (signal sequence) near amino terminus of growing polypeptide chain
Polypeptide structure on slide 20

Question 6

Where is the C terminus, N terminus, and signal sequence located on a secretory pathway?

Answer 6

Carboxyl terminus (end of peptide)
Amino terminus (start of peptide)
Signal sequence

All on slide 21

Question 7

How does a signal sequence target a protein to the endoplasmic reticulum? (5 steps)

Answer 7

1. A signal sequence at beginning (Nterminus) of protein being synthesized
2. A signal recognition particle (SRP) in cytoplasm binds to signal sequence
3. SRP receptor on ER binds to SRP
4. Channel called translocon runs through ER membrane and is attached to SRP receptor on cytosolic side of ER
5. Signal peptidase enzyme at bottom of translocon cleave off signal sequence
   Slides 23-25

Question 8

What happens when a protein is translocated across the ER?

Answer 8

Passage into the ER lumen and ER membranes is done by the action of molecular chaperones that are in the ER
Proteins cross unfolded then once through they fold into their 3D confirmations

Question 9

What are the two major classes of proteins targeted to the RER?

Answer 9

Proteins destined for secretion
Integral membrane proteins- once protein is in membrane, it’s orientation is fixed (this is why it’s important for proteins to get inserted in the correct way in the ER during translation)
Slide 30

VERY FEW PROTEINS ARE EMBEDDED IN THE OUTERMEMBRANE, MOST ARE IN ER

Question 10

What is the transmembrane sequence in the poly peptide being synthesized?

Answer 10

1. It stops the polypeptide from entering the lumen of the ER
2. it changes the confirmation of the translocon channel (Releasing the polypeptide chain to the ER membrane)

Question 11

What is the secretory pathway of proteins from lumen to plasma membrane?

Answer 11

Study picture on slide 32 and 33

Question 12

How are membranes created?

Answer 12

Membranes always arise from pre existing membranes!!
They are not generated spontaneously

New phospholipids are synthesized from the precursor glycerol by enzymes bound to the membrane on the cytoplasmic facing side of the phospholipid bilayer of the ER membrane

Question 13

What are flippases?

Answer 13

Enzymes that facilitate the translocation of newly synthesized phospholipids to the opposite side of the bilayer
Maintains stable membrane making sure the growth of both sides of phospholipid bilayer is even
Slide 35

Question 14

What is the cis face of Golgi and trans face of Golgi?

Answer 14

Cis- closest to ER (where proteins enter)
Trans- furthest from ER (where proteins exit)

Slide 38

Question 15

How are proteins and lipids exported from the endoplasmic reticulum?

Answer 15

Proteins free in the ER and membrane proteins and membrane phospholipids travel along the secretory pathway in transport vesicles which bud from the membrane and fuse with the membrane of another membrane

Vesicles leave via ERGIC and move to Golgi via these vesicles
Picture on slide 40

Question 16

Where does real sorting of protein start?

Answer 16

In the Golgi!!

Proteins can be exported out of ER, sorted in Golgi and sent back into the ER

Question 17

What is protein glycosylation?

Answer 17

Mannose 6 phosphate residues target proteins for lysosomes (all proteins going to lysosomes should have this glycosylation)

It is protein processing within Golgi with synthesis/extensive modification of carbs

Question 18

What are the 2 key steps of vesicular transport?

Answer 18

VT is responsible for molecular traffic between a variety of membrane enclosed compartments
1. Get the correct protein into the correct vesicle
2. Get the vesicle to its correct final destination within the cell
Picture on slide 46

Question 19

What are coated vesicles?

Answer 19

Transport vesicles costed with cytosolic cost proteins that carry secretory proteins from ER to Golgi and from Golgi to other targets
Make sure everything gets where they need to go

Question 20

What are lysosomes?

Answer 20

Membrane enclosed organelles that contain an array of enzymes capable of breaking down all types of biological polymers (proteins, nucleus acids, carbs and lipids)

Question 21

How are lysosomal storage diseases caused?

Answer 21

Deficiency or absence of any one of a number of lysosomal enzymes
Results in failure of lysosome to degrade contents

Question 22

How are vesicles docked and fused?

Answer 22

GTP bound Rab proteins recognize and bind tethering factors on target which makes first contact with vesicle and target membrane
These tethering factors stimulate formation of SNARE complexes between vesicle and membrane which begins fusion

Tethering is like hanging on to boat from dock to make sure it’s the right dock, if it’s the right dock you’ll tie it off with SNARE complexes

Question 23

What is a nuclear pore complex?

Answer 23

Consists of a structure with eight-fold symmetry organized around a large central channel
Composed of 30-50 different nucleoporins
Selective transport
Picture on slide 62

Question 24

What are the two mechanisms of transport through the nuclear pore complex?

Answer 24

1. Passive- molecules less than 20-40 kDa

2. Energy dependant- larger molecules (allows for selectivity of transport)

Question 25

How do proteins processes through the cytosolic pathway move into the nucleus?
What recognizes these signals?

Answer 25

Nuclear localization signals (NLS)
Specific amino acid sequences that are recognized by transport receptors and direct the transport of proteins from the cytoplasm through nuclear pore complex

Nuclear transport receptors recognize nuclear localization signals

Question 26

What are the 3 key points of protein import through the nuclear pore complex?

Answer 26

1. Importin (nuclear transport receptor) binds to NLS in the protien going in nucleus
2. Interaction of complex with Ran/GTP on nuclear side of pore causes release of nuclear protein
3. Ran/GTP/importin recycled

Slide 67

Question 27

What proteins would be exported out of nucleus and by what signal?

Answer 27

Most RNAs are exported from nucleus as RNPs (ribonucleoprotein)
Exported out by nuclear export signals (NES)