1. Membrane Structure and Function
MCBG
08/29/2019
Edward Campbell

2. Biological membranes are primarily composed of lipid and protein
Plasma membrane, golgi, nuclear membrane, mitochondria
All these membranes have a common structure
Myelinated neuron ~25% protein, mitochondrial membrane ~75% protein
Membrane Composition:
~50% lipid, 50% protein
This is an average, with the protein content ranging from 25% to 75%

3. Membrane molecules Phospholipids
Length and saturation of fatty acid tails dictates membrane fluidity
3 carbon backbone provides structural stability of phospholipids
The lipids of the PM are all amphiphilic, having a hydrophobic and hydrophilic side
Phosphoglycerides
A cis-double bond induces a “kink” that prevents
phosphoglycerides from packing too tightly

4. Cholesterol Cholesterol present in large amounts in plasma membrane
Can reach 1:1 cholesterol: phospholipid ratio
Cholesterol incorporation into a lipid bilayer:
increases the rigidity of the membrane
Decreases the permeability of the bilayer to water soluble molecules

5. Hydrophobic molecules self-aggregate in water
Phospholipids self-aggregate without external biochemical energy
Self-aggregation is energetically favorable because organizing water around individual hydrophobic molecules is energetically expensive

6. Important implications of the behavior of hydrophobic molecules
Somewhat interesting,
biochemically
What makes all life on
Earth possible
Sealed compartment=win!

7. Demonstration of Membrane Biophysical properties
Hydrophobic interactions provide tremendous plasticity to plasma membranes,
Preventing their rupture under most circumstances.

8. Bonus Example: Membrane plasticity as demonstrated by a neutrophil

9. Lateral diffusion of lipid molecules within the plasma membrane
In most cases, lateral diffusion of lipid molecules is quite high
~2μm/sec
Fluidity depends on
Lipid composition
Temperature

10. Membrane Fluidity and Transition Temperature
Membrane Fluidity: The relative mobility of lipids within a membrane,
Each lipid has a characteristic temperature at which it freezes, called the
transition temperature
More fluid membranes have a lower transition temperature
-More double bonds  lower transition temperature (more fluidity)
cis bonds produce kinks that reduce intermolecular packing
-shorter hydrocarbon tails  lower transition temperature (more fluidity)
reduced tendency of hydrocarbon chains to interact

11. Lipid Rafts Q: How are phospholipids like M1 students?
A: Both non-randomly associate:
Different types of phospholipids “clique out”, as van der Waals forces between hydrocarbon chains can induce transient self-associations. Lipid rafts are cholesterol and sphingolipid rich regions of transient self-associations

12. Membrane dynamics in action
Adapted From:

13. Biological Lipid Bilayers exhibit considerable asymmetry
Glycolipids are many places, but always on the monolayer AWAY from the cytosol
Generated in the lumen of the Golgi

14. Cellular surveillance mechanisms exploit membrane asymmetry to respond to insults and maintain cellular homeostasis
Galectins contain a Carbohydrate Recognition Domain (CRD) which allows them to bind sugars present on glycolipids
Galectin 3,8,9 are typically localized to the cytoplasm
(where such sugars are not accessible)
When viruses, bacteria or other pathological agents rupture phagosomes/endosomes/lysosomes, the sugars normally present on the inside of the vesicle become accessible to cytoplasmic Galectin proteins, which in turn recruit autophagic effectors

15. Monitoring vesicle rupture by adenovirus
Adenovirus Galectin3 DAPI
Chris Wiethoff Lab

16. Monitoring vesicle rupture by alpha-synuclein
Alpha-synuclein Galectin3

17. PS is translocated to the exterior membrane in apoptotic/dead cells
PS on the exterior leaflet tells macrophages to “eat me”
Involves inactivation of transporter that normally removes PS from exterior leaflet and activation of a scramblase that randomly flips phospholipids
TCB, 16:189, 2006

18. Lipid Bilayer constituents drive signal transduction
The interior side of the PM contains high amounts of phosphatidylserine, imparting a net negative charge to this membrane leaflet relative to the outer leaflet
Phospholipase C cleaves inositol phopholipids to generate relay
Signal and scaffold for protein kinase C.

19. Physical properties of membranes: Take home points
The amphiphilic properties of phospholipids make them spontaneously form lipid bilayers, a critical component of life on earth
Because of the properties of lipid bilayers, membranes are fluid, deformable and self sealing, allowing compartmentalization of biological material
Cholesterol is a critical component of eukaryotic membranes. Increasing amounts of cholesterol in membranes increases their rigidity
Some phospholipids and cholesterol spontaneously self-associate to form membrane microdomains, such as lipid rafts
The leaflets of membrane bilayers are asymmetric, and this assymetry is a critical aspect of many biological processes

20. Membrane Proteins
Proteins can associate with membranes in numerous ways

21. Numerous ways that membrane proteins can associate with the CYTOPLASMIC leaflet of the PM

22. Example: alpha-synuclein forms an amphipathic alpha-helix which associated with membranes
Brown = Hydrophobic residues
Blue = Polar residues

23. Transmembrane proteins often form alpha-helices through a membrane
Hydropathy plots reveal membrane
spanning regions in protiens
~30% of proteins are likely membrane proteins

24. Beta Barrels form large pores in membranes

25. Many membrane proteins diffuse laterally within the plasma membrane

26. Cell to cell communication is rooted in the biology of membranes
A combination of membrane biophysics and cytoskeletal organization create dynamic membrane behavior critical for many cellular activities

27. Membrane communication and the cytoskeleton

28. The immunological synapse
Monks et al, Nature, 1998
Cell to cell contact induces clustering of cellular components at the point of contact:
Transmembrane proteins
Actin
Lipid Rafts (cholesterol)
PNAS, 100:

29. Viruses exploit these cellular activities to enhance their replication
HIV-1 directionally buds at points of cell to cell contact
Dendritic cells “present” internalized viruses to T-cells for inspection

30. Membrane Proteins: Take Home points
Proteins associate with membrane in numerous ways
Transmembrane proteins often form alpha-helices which traverse the membrane. These helices are readily identified by plotting the hydropathy index of the protein sequence
Membrane proteins, membrane microdomains and cytoskeletal proteins coordinate to generate dynamic cellular structures which are critical in many biological processes