Membrane Structure 1

Membrane StructureChapter 11

Questions in this chapter you should be able to answer:Chapter 11- #s1 - 19

Membrane Structure 2

Membranes are described as a “2-dimensional liquid”. Why?

Laser Tweezers

Membrane Fluidity

Jmol membrane model

Membrane Structure 3

How can we measure membrane fluidity?

FRAP: ‘Fluorescence Recovery After Photobleaching’

GFP: Green fluorescent protein

Cell fusion alsoshows fluidity

See Figure 11-30

P 378

FRAP

Membrane Structure 4

What factors determine how fluid a membrane is?

Properties of P-lipids: Chain length Saturation

Properties of membranes: Cholesterol content Cytoskeleton association

Membrane Structure 5

What are the principal membrane lipids?

PhospholipidsGlycolipids

Other membrane lipids(not phospholipids)

Cholesterol

CerebrosidesSphingolipidsCeramide

Membrane Structure 6

How are P-lipids distributed in lipid bilayers?

P-lipids are synthesized on the ER membrane….

How do they get to other side of membrane?

How is asymmetry achieved?

Scramblase vs Flippase

Membrane Structure 7

What are the principal functions of membrane proteins?

How are membrane proteins connected to the membrane?

Membrane associated (peripheral)Covalent vs Noncovalent

Transmembrane (integral)Single-pass Multi-pass

Membrane Structure 8

Why do transmembrane proteins occur as alpha helices and beta-pleated sheets??

Jmol Transmembrane proteins

Membrane Structure 9

How can membrane proteins be purified and studied?-- detergents ‘mimic’ P-lipid structure around proteins

Question 11-5Why is red part hydrophilic and blue part hydrophobic?

Membrane Structure 10

What do we know about the structure of bacteriorhodopsin?

Function?

Structure?

Mechanism?

Bacteriorhodopsin

Membrane Structure 11

How is the cell membrane structurally reinforced?

-- Cell cytoskeleton-- also influences fluidity

Membrane Structure 12

Why are carbohydrates particularly abundant on the cell surface?

Functions:

Surface protectant

Cell recognition

Cell adhesion-- extracellular matrix

Membrane Structure 13

How can protein movement in cell membrane be restricted?

Consider challenge of intestinal epithelium…

Fig 12-16 p 395

Membrane Structure 14

You have isolated two mutants of a normally pear-shaped microorganism that have lost their distinctive shape and are now round. One of the mutants has a defect in a protein you call A and the other has a defect in a protein you call B. You grind up mutant and normal cells separately and separate the plasma membranes from the cytoplasm by centrifugation. You then wash the membrane fraction with a low concentration of urea ( which disrupts their ability to interact with other proteins) and centrifuge the mixture. The membranes and their constituent proteins form a pellet while the proteins liberated by the urea wash remain in the supernatant. When you check each of the fractions for the presence of A or B, you obtain the results given below.

First cell extractAfter urea wash and

centrifugation

Membrane Cytosol Membrane Supernatant

Normal cells A and B no A or B B A

Mutant A B A B no A or B

Mutant B B A B no A or B

Answer the following statements about your results?(a) Which is an integral and which is a peripheral membrane-associated protein. The results for which cell-type shows this?(b) How does the mutation to protein-A alter its properties?(c) How does the mutation to protein-B alter its properties?(d) Which result(s) most indicate an interaction between A and B? Explain.