Transcript Slide 1

Chapter 5 of Yeagle Structure of Biological Membranes
•Non-lamellar phases
•Spontaneous curvature
•Actual curvature
What is the physical basis of non-lamellar phase structures? Can we
understand the competing forces that stabilize a lamellar versus a non-lamellar
phase?
In cells, primarily lamellar structures are
found, yet lipids extracted from cells will
form non-lamellar phases in vitro.
Bilayer thickness
Surface charge
Dielectric constant
Lipid composition
•POLYMORPHISM
•MESOMORPHISM
General Motivation:
(1) by studying structural
polymorphism/mesomorphism, one
gaines an understanding of the
forces that are “locked-up” in
biomembranes that can affect the
organization and function of
membrane proteins
(2) Generally extended to
surfactant/detergent chemistry
This chapter is interested in phase changes that change the CURVATURE of
the lipid-water interface.
These phases occur at temperatures above the gel-liquid transition, but below
the transition temperature to an isotropic liquid.
Why do we care about
CURVATURE?
•Cell division
•Endocytosis
•Membrane fusion
•Structure
•organelles
X-ray diffraction and NMR
EPR, UV-VIS, IR, Calorimetry, Neutron diffraction
GOAL: INTUITIVE UNDERSTANDING OF THE FORCESTHAT DRIVE THE
FORMATION OF CURVATURE ALTERING PHASE TRANSITIONS
TERMINOLOGY:
•NON-BILAYER PHASE: really means non-lamellar, but still a bilayer
of phospholipids
•INVERTED/WATER-IN-OIL PHASE: HII phase, possess a net
concave curvature when viewed from the water domain.
•NONINVERTED/OIL-IN-WATER PHASE: HI phase, possesses a net
convex curvature when viewed from the water domain
OIL-WATER SURFACTANT MICELLES
What happens when you have detergents and a small amount of oil in water?
What happens when the oil is the majority constituent?
THERMOTROPISM and LYOTROPISM
Phenomenological Approach: like Hooke’s Law – the force required to stretch an
elastic object is linearly proportional to displacement from equilibrium position.
This disregards the molecular forces at play
For lipid bilayers: the fundamental unit of all lipid mesomorphs is the lipid
monolayer, and that this monolayer may be endowed with a spontaneous
tendency to curl.
Co = spontaneous curvature
Ro = radius of spontaneous curvature
Co = 1/Ro
Rigidity of object
DE = 0 when R = R0
Parabolic dependence
Perform X-ray crystallography and obtain d-spacing and reflections
By adding 16%(w)
tetradecane, it is
possible to minimize the
unfavorable packing of
the acylchains, thus
lowering the phase
transition temperature to
the hex phase
As you raise the
Temperature, the tube
radius becomes smaller
Inverted Hex phase is a cylinder
Bending membranes using:
1) Lipids themselves
2) Molecular motors
3) Protein binding
Bending by proteins:
Scaffolding Mechanism The protein coats that cover budding
membrane surfaces function as “scaffolds” to curve the
membrane. Constraint is that protein must be curved and rigid for
the membrane to “follow”. There must also be a tight binding
examples: dynamin and BAR-domain proteins
Local Spontaneous Curvature Mechanism. Spontaneous
curvature is generated by the penetration of a protein into the
membrane.
Example is the ENTH domain of epsin. It is involved in clathrin
mediated endocytosis. The ENTH domain binds to PIPs (PI-4,5
biphosphate)
Amphiphysin is another protein example. Both have amphipathic
helices that insert into the bilayer which may cause a local
curvature strain.
Sensing by proteins:
A new concept in cell trafficing and membrane curvature is that proteins can
sense curvature.
Proteins have been discovered that have binding affinities that are dependent
upon the radius of membrane curvature.