Lecture 3

Question 1

Membranes define what two cell properties?

Answer 1

Membranes define two cell properties:
- They are boundaries
- They serve as permeability barriers

Question 2

Where are proteins made?

Answer 2

Proteins are made in the cytoplasm where ribosomes are.

Question 3

Why don’t we make proteins inside the nucleus?

Answer 3

We don’t make proteins inside the nucleus because they would begin translating the mRNA strand before it was complete! This would likely lead to misfolding, which can be very very bad for the cell.

Question 4

The fluid mosaic model?

Answer 4

The fluid mosaic model envisions the membrane as a fluid bilayer of lipids with a mosaic of associated proteins

Question 5

Protiens in the membrane?

Answer 5

Proteins can be embedded in the membrane. Proteins also move slower int he membrane compared to other components.

Question 6

Impact of different phospholipids?

Answer 6

Different phospholipids make the membrane behave differently.

Question 7

Membranes contain diffenret classes of lipids, like…?

Answer 7

Membranes contain several classes of lipids. Some of these classes include phospholipids, glycolipids, and sterols/steriods (like cholesterol).

Question 8

Membranes vary in composition of..?

Answer 8

Membranes vary in their lipid composition.
- This variation depends on organelles, species present, and such. It also lends to them having different abilities and behaviours.

Question 9

The lipid bilayer is fluid and asymmetric, in what ways can it move?

Answer 9

There are three main ways that phospholipids in bilayers can move:
1. Transverse diffusion (flip-flop, move vertically)
2. Rotation (no impact, spin)
3. Lateral diffusion (slowly move in random directions laterally)

Question 10

What are translocases?

Answer 10

Translocases are proteins that move phospholipids between layers
and maintain the lipid asymmetry
- they actually perform transverse diffusion and other diffusions.

Question 11

Examples of membrane translocases?

Answer 11

Flippase: moves outside phospholipid inside. (flips them)
Floppase: Moves inside phospholipids outside (flops them)
Scramblase: Does both of these at the same time.

Question 12

What is FRAP?

Answer 12

FRAP is fluorescence recovery after photobleaching

Question 13

What do we use FRAP for?

Answer 13

It is a method to study the mobility of molecules in living cells.

Question 14

How does FRAP look (step-wise)?

Answer 14

1. Unlabeled cell surface
2. Cell surface molecules labeled with fluorescent dye
3. Laser beam bleaches an area of the cell surface (patch of no cells)
4. Fluorescent-labeled molecules diffuse into bleached area
5. Bleached area disappeared as lipids move laterally

Question 15

FRAP used to…? Describe what it looks like photographically and graphically.

Answer 15

FRAP is used to test the movement of membrane proteins.
- We see in a photo that a bleached area recovers very quickly (by 40 seconds around half are recovered).
- the graph has a steep decline, but quickly moves back to where it was before (more than 50% recovered after 120 seconds)

Question 16

Proteins at the membrane…?

Answer 16

Proteins at the membrane must have hydrophobic amino acids.
- The stretch of hydrophobic amino acids are the hydrophobic region of membrane proteins.
- Some of them are fully across the membrane, others are only partially associated.

Question 17

Graphical representation of the hydrophobic region?

Answer 17

2 segments below the line and one above. This means that the membrane is crossed at least three times by this hydrophobic region of this protein.

Question 18

Types of proteins in the membrane?

Answer 18

Membranes contain integral, peripheral, and lipid-associated proteins.

Question 19

Integral proteins? q

Answer 19

Integral proteins have a hydrophobic region.
- Monotopic (only on inside)
- Singlepass
- Multipass
- Multi-subunit

Question 20

Peripheral proteins?

Answer 20

Peripheral proteins associate through another protein in the membrane.

Question 21

Lipid-anchored proteins?

Answer 21

Lipid-anchored proteins bind fatty acids which gets inserted into the membrane.
- Fatty acid/isoprenyl anchor on inside
- GPI anchor on outside.

Question 22

Membrane proteins detect..?

Answer 22

Membrane proteins detct and transmit electrical and chemical signals. This isn like insulin!

Question 23

Insulin receptor?

Answer 23

Insulin receptors are dimers, cell will know when insulin is present due to. protein going inside the membrane.
- This is due to a chemical signal.

Question 24

Membrane proteins mediate?

Answer 24

Membrane proteins mediate cell adhesion and cell-cell communication.
Ex. Cadherin to Catherine binding keeps cells together. They are attached via the B-catenin on the cytoplasmic side (peripheral association) and bind together in the intercellular space. They only have one transmembrane pass.

Question 25

Transmembrane proteins moving ions?

Answer 25

Transmembrane proteins can also move ions across cell membranes. Ex. Calcium pump: The calcium pump undergoes a cylce of changes in the process of pumping. Open when trying to move the Ca ions, and this conformational change allows Ca to move across the membrane.

Question 26

Most integral proteins are ________?

Answer 26

Most integral proteins are transmembrane proteins (cross the membrane).

Question 27

Example of transmembrane protein? Glucose receptors....

Answer 27

Glucose binds on the outisde of the cell, is carried throuhg the channel following conformational change, and is released on the intracellular side of the membrane (inside cell).

Question 28

Example of transmembrane protein? Acetylcholine receptirs?

Answer 28

Acetylcholine binding causes a conformational change in the receptor, opening the pore and allowing ions through.

Question 29

Integral monotopic proteins are located on ______?

Answer 29

Integral monotopic proteins are only on one side of the membrane. Very few monotonic proteins are present in our cells

Question 30

Integral monotopic proteins criteria?

Answer 30

1. embed into a single face of the membrane. 2. Irreversible 3. reactions involving hydrophobic or amphiphilic substrates not readily soluble in water.

Question 31

How do monotopic proteins differ in their association to the membrane (%)?

Answer 31

They can go from 7.6% embedded (COX-2), 1.6% embedded (SHC), to 1% embedded (GlpD) based on their function.

Question 32

Peripheral membrane proteins can form..?

Answer 32

Peripheral membrane proteins form weak and reversible associations to the membrane, typically through binding to integral membrane proteins.

Question 33

Examples of peripheral membrane proteins binding to integral membrane proteins?

Answer 33

Examples ● Enzymes that metabolize membrane lipids ● Regulatory subunits of transmembrane proteins

Question 34

Phospholipase A2 and pancreatic PLA2?

Answer 34

Phospholipase A2 breaks membrane lipids. Pancreatic and cytosolic phospholipase A2 both digest lipids (one is just on the outside and the other on the inside).

Question 35

Lipid anchored proteins are...?

Answer 35

Lipid-anchored membrane proteins are covalently bound to fatty acids or isoprenyl groups - Isoprenyl is very hydrophobic and attaches strongly to the membrane, making it a good candidate for this.

Question 36

How are hydrophobic phenyl groups anchored?

Answer 36

post-translational modification that resulted in attachment of hydrophobic prenyl groups that anchor the small GTPAse proteins to intracellular membranes.

Question 37

Trehalase - lipid anchored membrane protein?

Answer 37

Trehalase is bound to glycosylphosphatidylinositol, which keeps it in the membrane.

Question 38

What is glycosylated?

Answer 38

Proteins and lipids in the outside of the membrane are glycosylated - they have sugars attached to them via glycpsylation.

Question 39

Types of glycoslyation?

Answer 39

There is N-linked (carb attached to the N of the amino group) like asparagine There is O-linked (carb attached to the O of OH group) like in serine or threonine.

Question 40

Glycodlyation process?

Answer 40

Glycosylation is the process by which a carbohydrate is covalently attached to a target macromolecule