Lecture #5 - Membrane Structure

Front 1

What two types of phospholipid are located on the outerlayer of the plasma membrane?

Back 1

Phosphatidylcholine and Sphingomyelin (cholines)
(mostly)

Front 2

What three types of phospholipid are located on the inner layer of the plasma membrane?

Back 2

Phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol

Front 3

Where are glycolipids located?

Back 3

On the outer layer with the carbohydrate region on the cell surface (these do not flip flop)

Front 4

Does flip-flopping of phospholipids occur?

Back 4

Not in liposomes
Yes in biological systems to achieve an asymmetrical distribution

Front 5

What does this difference elude to?

Back 5

The fact that biological systems have membrane translocators that move these phospholipids. They are called flippases.

Front 6

What is the significance of Phosphatidylserine on the outer layer of the plasma membrane?

Back 6

1) Increased PS on the outer membrane is a signal for aging blood cells and cells dying of apoptosis to be engulfed by macrophages
2) Exposure of PS on outer membrane sequesters blood clotting proteins. The negative charge on the serine binds calcium which then can bind to prothrombin and start the clotting cascade

Front 7

What is the significance of PS on the inner surface?

Back 7

PS and PIP2 are used on the inner cell surface for cell signaling.
It activates PKC via its Ca-serine complex

Front 8

What are the two main classes of membrane protein?

Back 8

Integral and Peripheral

Front 9

Where are integral proteins located?

Back 9

Embedded in the lipid bilayer
Attached to hydrophobic groups
They contain hydrophobic regions that span the bilayer and hydrophilic regions at the ends

Front 10

Do these proteins flip flop like phospholipids?

Back 10

No

Front 11

Glycoprotein carbohydrates attached to the integral protein are located where?

Back 11

Outer surface (extracellular)

Front 12

What is a single pass integral protein?

Back 12

They have a single transmembrane region

Multipass = multiple transmembrane regions

Front 13

What are B-Barrel proteins?
Where are they found?

Back 13

8-22 antiparallel B strands.
Some are porins, some are enzymes, some are receptors.
They are found on the outer mitochondrial membrane and in bacterial membranes

Front 14

What conformation are the hydrophobic sections of integral proteins in? why?

Back 14

They are in the alpha - helix conformation and are forced into this because there is no water to form hydrogen bonds.

Front 15

What is the difference between a homomeric protein and a heteromeric protein?

Back 15

Homomeric = Individual subunits of an integral protein have the same function

Heteromeric = Individual subunits have different functions

Front 16

What are peripheral proteins?

Back 16

Proteins attached to the membrane surface by non-covalent interactions with other proteins or lipid groups.
They interact with the membrane via electrostatic interactions and hydrogen bonding with the hydrophilic domains of integral proteins and lipid polar groups.
They are easily released by salts and or agents that break hydrogen bonds.
They may interact with integral proteins and serve as regulators of membrane bound enzymes.

Front 17

What is a lipid anchor?
What is it's function?

Back 17

A saturated fat or prenyl group covalently bound to the cytosolic portion of the membrane.
Can be involved in cell signaling.

Front 18

What is protein acylation?

Back 18

A mechanism to recruit signaling proteins to the membrane when they are needed to carry out a response to an external signal like a hormone and release them by removal of the acyl group to stop the response.

Front 19

What are GPI proteins?
Where are they located?

Back 19

Glycophosphatidylinositol proteins
They are anchored to the extracellular portion of the bilayer by the two saturated fatty acyl groups.
They are in high concentration in the apical (lumen facing) membrane of polarized cells and in caveolae (small cellular invaginations.

Front 20

What are lipid rafts?
How are they related to caveolae?

Back 20

Lipid rafts are micro-segments of saturated phospholipids and contain cholesterol. They are a little bit thicker and found predominantly in caveolae.
Caveolae are a type of endocytic vesicle in the plasma membrane enriched in glycosphingolipids and spingomyelin and contain the protein caveolin.

Front 21

What is the significance of caveolae?

Back 21

They are a method that pathogens avoid destruction to avoid endosomal phagocytosis and lysosomal destruction.

Front 22

What does the signal peptidase do?

Back 22

Cleaves the start sequence from the translocated protein and releases the secretory protein into the lumen.

Front 23

How are single-pass proteins unique?

Back 23

They contain an internal stop-translocating signal sequence which is released into the ER membrane.

Front 24

How are double pass integral proteins unique?

Back 24

They have both start and stop translocation signal sequences that are internal.

Front 25

How proteins destined for the plasma membrane distinct?

Back 25

They are glycosylated by a lipid molecule called dolichol.

Front 26

Integral proteins are transferred to the golgi apparatus and then the plasma membrane via

Back 26

a budding and fusion process.
The portion of the protein on the ER cytosolic side stay on the cytosolic side of the cell, whereas the lumen side of the protein ends up on the outside of the cell.