Protein Transport and Secretion

Question 1

proteins entering the er enter the ________ pathway

Answer 1

secretory

Question 2

peroxisome v endosome v lysosome

Answer 2

peroxisome = break down fa's with oxygen
endosome = formed from csm. transport of materials from golgi --> lysosome or vacuole. processing of internalized proteins.
lysosome = digest viruses/bacteria. waste removal for cells.

Question 3

function of the ER

Answer 3

post translational modification of proteins

Question 4

protein destination is dependent on

Answer 4

location of the ribosome during translation:

Question 5

by default ribosomes are

Answer 5

free, unless targetedti the endoplasmic reticulum

Question 6

free ribosomes synthesize protein types:

Answer 6

Cytosolic proteins
Mitochondrial proteins
Nuclear proteins
Peroxisomal proteins

Question 7

How do ribosomes get to the ER?

Answer 7

Most proteins that are destined for the ER are directed by an N-terminal ER signal sequence that binds the Signal recognition particle (SRP)

Approx 5-30 amino acids long, mainly charged residues

Question 8

Explain the mechanism of SRP in facilitating ribosomes to the ER

Answer 8

• Translation begins
• SRP via signal sequence binds to ribsome . transaltion is paused.
• srp bound ribosom binds to SRP receptor on the RER membrane
• protein enters RER lumen via protein tranlocator on the RER membrane
• SRP recycled, translation proceeds with protein translocated into ER lumen

Question 9

Smooth ER is site of ………..

Answer 9

site of lipid and lipid membrane biosynthesis

Question 10

Function of ER

Answer 10

Assembly and folding of all transmembrane proteins.

Processing of proteins that are secreted by the cell or onwards to other organelles via the Golgi apparatus.

Site of post-translational modification of proteins e.g. N-linked glycosylation of precursor oligosaccharides

Disulphide bond formation

Error checking / Quality control:- Biosynthetic arrest of misfolded proteins

Question 11

After processing in the ER proteins are delivered to the _____________.

Answer 11

Golgi apparatus.

Question 12

Golgi apparatus structure

Answer 12

Stack of disc like membranes (Cisternae)
Cis or entry face.
Trans or exit face.

Question 13

Functions of the Golgi apparatus

Answer 13

Proteins are sorted for delivery to other parts of the cell

Site of N-linked (amino acid aspargine) oligosaccharide modification - eg like adding an address to the protein

O-linked (amino acid pyrolisine) glycosylation

Oligosaccharide addition/modification acts as a recognition motif directing proteins to their appropriate destination in the cell.

Question 14

Glycosylation is the process by which ………

Answer 14

……… a carbohydrate is covalently attached to a target macromolecule, typically proteins and lipids. This modification serves various functions eg acting as an address. In other cases, proteins are not stable unless they contain oligosaccharides linked at the amide nitrogen of certain asparagine residues.

Question 15

How are proteins transported from the ER to the Golgi other organelles and into and out of the cell?

Answer 15

Process of vesicular transport

**Vesicular transport is also involved in the movement of proteins into and out of the cell - endocytosis and exocytosis.

Question 16

Basic process of vesicular transport

Answer 16

Budding of organelle membrane. vesicle containing cargo protein. fusion of vesicle to target and release of cargo.

Question 17

The process of vesicular transport is governed by

Answer 17

coated vesicle proteins

Question 18

Three well characterised coated vesicles proteins are:

Answer 18

Clathrin: plasma membrane into endosomal and golgi compartments

COP-I: from golgi

COP-II: from ER to golgi

Question 19

How is a clathrin vesicle formed?

Answer 19

1) coat of clathrin is assembled on the membrane. cargo proteins are selected from their cargo receptors.
2) bud is formed including clathrin, the receptors, the cargo proteins, and the membrane part itself.
3) vesicle is formed, from above constituents.
4) uncoating of clathrin, and naked vesicle moves to its location in the cytosol.

Question 20

How can vesicle transport be highly selective?

Answer 20

• Rab proteins on cytosolic surface of vesicles directs specificity
• v-SNARE and t-SNARE mediate membrane fusion.

3 STEPS:
tethering
docking
fusion.

Question 21

process of selectivity of vesicles by RAB

Answer 21

1. Cargo vesicle = car
2. protein = you
3. RAB = tour guide
4. t-SNARE on target membrane = your hotel
5. v-SNARE = your hotel booking.
6. RAB effector = receptionist at hotel, friend of Rab

so, you travel with v-SNARE and RAB in your cargo vesicle.
rab meets rab effector in the hotel, and you give v-SNARE to t-SNARE. Now you can enter the hotel i.e. target membrane.

Question 22

Cytoskeleton - how does it help with cellular vesicular transport?

Answer 22

Microtubules: Components of the cytoskeleton also aid vesicular transport.

form long rope-like filaments - Important in maintaining cell structure.

Act as tracks guiding membrane vesicles to their correct destination. They have motor proteins on them

Aid ER to golgi transport.