11.2 Membrane Dynamics Flashcards
liquid-ordered (Lo) state
gel-like state in which all types of motion of individual molecules are strongly constrained
VDW- much stabilization energy
tightly packed
liquid-disordered (Ld) state
state in which individual hydrocarbon chains are in constant motion (lateral and rotational)
VDW not as stabilizing
more space to move
L0 to Ld transition can be caused by
heat which produces thermal motion of side chains
fatty acid composition affects
membrane fluidity
at physiological temperatures, long chain saturated fatty acids:
tend to pack into an Lo phase (more VDW)
at physiological temperatures, kinks in unsaturated fatty acids:
interfere with packing, favoring the Ld state (less VDW)
shorter-chain fatty acyl groups favor the
Ld state
less SA, less VDW
based on the amount of VDW, for packing:
increasing VDW favors ordered
decreasing VDW favors disordered
sterols have paradoxical effects on bilayer fluidity
they interact with phospholipids containing unsaturated fatty acyl chains, compacting them and constraining their motion (ordered)
they associate with sphingolipids and phospholipids having long, saturated fatty acyl chains, making the bilayer fluid (disordered)
transbilayer movement of lipids requires
catalysis
transbilayer (“flip flop”)
movement has a large, positive free-energy change
membrane proteins facilitate the
translocation of individual lipid molecules
flippases
catalyze the translocation of the amino-phospholipids phosphatidylethanolamine (PE) and phosphatidylserine (PS) from the extracellular to the cytoplasmic leaflet of the plasma membrane
consume ~1 ATP per molecule of phospholipid translocated
related to the P-type ATPases (active transporters)
floppases
move plasma membrane phospholipids and sterols from the cytoplasmic leaflet to the extracellular leaflet
are ATP-dependent
members of the ABC transporter family
each specializes in movement of specific lipids
scramblases
move any membrane phospholipid across the bilayer down its concentration gradient
not dependent on ATP; some require Calcium
lead to controlled randomization of the head-group composition on the two faces of the bilayer – toward equilibrium
uncatalyzed transbilayer (“flip flop”) diffusion
is very slow
uncatalyzed lateral diffusion is
very fast
catalyzed transbilayer translocations
flippase- outside in
floppase- inside out
scramblase- eiether or
phosphatidylinositol transfer proteins
move phosphatidylinositol lipids across lipid bilayers
-believed to have roles in lipid signaling and membrane trafficking
lipids and proteins diffuse ____ in the bilayer
laterally !
individual lipid molecules undergo
Brownian movement
FRAP
fluorescence recovery after photobleaching
rate is a measure of the rate of lateral diffusion of the lipids
hop diffusion of individual lipid molecules
single particle tracking confirms lipid molecules diffuse laterally within small regions
movement from one region to another (“hop diffusion”) is rarer
some membrane proteins are free to diffuse, whereas others are not
membrane proteins are limited in movement by:
-associating to form large aggregates (“patches”)
-anchoring to internal structures
slower, bigger
sphingolipids and cholesterol cluster together in
membrane rafts
microdomains (rafts)
clusters of cholesterol and sphingolipids that make the bilayer slightly thicker and more ordered than neighboring, phospholipid-rich regions
-can be up to 50% of the cell surface
protein rafts
proteins must have hydrophobic helical sections long enough to segregate into the thicker bilayer regions of rafts
lipid rafts are enriched in
1) proteins that have two long-chain saturated fatty acids covalently attached through Cys residues
2) GPI-anchored proteins
caveolae “Little caves”
specialized rafts
have caveolin protein
caveolin
integral protein that binds to the cytoplasmic leaflet of the plasma membrane
-forms dimers
-associates with cholesterol-rich membrane regions
-forces the bilayer to curve inward to form caveolae
protein that induces curvature, can increase lipid particles there, increases SA without changing the size of the cell
membrane curvature and fusion are central to many biological processes
budding of vesicles from golgi complex
exocytosis
endocytosis
fusion of endosome and lysosome
viral infection
fusion of sperm and egg
fusion of small vacuoles (plants)
separation of two plasma membranes at cell division
cardiolipin
can create or recognize membrane curvature
-located in mitochondrial and bacterial membranes
protein-induced curvature of membranes
BAR domains
BAR domains
domains consisting of coiled coils that form long, thin, curved dimers with a positively charged concave surface (interact w membrane)
BAR domain proteins assemble into crescent-shaped scaffolds and favor membrane curvature
septins
family of GTP-binding proteins that polymerize at curved regions of the plasma membrane
-participate in cell division, exocytosis, phagocytosis, and apoptosis
-have an amphipathic helix that is important in vesicle `trafficking and neurotransmitter release
*increase curvature of PM
fusion proteins
mediate specific fusion of two membranes by bringing about specific recognition and a transient local distortion of the bilayer structure
mediating fusion and not curvature*
*to get fusion, you need membranes close enough together
steps of fusion proteins
- 2 membranes recognize each other
- bring membranes into close contact- remove water
- bilayer becomes locally disrupted (hemi fusion)
- bilayer fuses to form single continuous bilayer hydrophobic areas interact
SNARE proteins
snap receptors- family of proteins
v-SNAREs
t-SNAREs
*very big in neurotransmitters
v-SNAREs
SNAREs in the cytoplasmic face of the intracellular vesicle
t-SNAREs
SNAREs in the target membrane with which the vesicle fuses
integrins
surface adhesion proteins that mediate a cell’s interaction with the extracellular matrix and with other cells
-carry signals in both directions across the plasma membrane
-heterodimeric proteins composed of two unlike subunits, alpha and beta
cadherins
involved in surface adhesion
-undergo homophilic interacts with identical cadherins in an adjacent cell
selectins
have extracellular domains that bind specific polysaccharides on the surface of an adjacent cell
-require calcium
-essential part of the blood-clotting process
