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Membrane Component Localization, e.g.,, match with, :red_cross:, by -…
Membrane Component Localization
Where
are the components of biomembranes localized? What is the significance of their respective localizations?
Lipids
Between leaflets
Asymmetry
Most PLs in both
Some more in one than the other
PMs from cultured RBCs and MDCK cells
exoplasmic leaflet
PC and SM
more rigid
cytosolic leaflet
PE, PS, PI :no_entry:
more fluid
R + K in segment of single-pass MP
"inside positive" rule
Can influence curvature
How?
Unclear!
Partly reflects where lipids are synthesized
SM
luminal face (Golgi)
PGs
cytoplasmic face (ER)
does not account for
PC
!
flippases
transported to exoplasmic leaflet
Why?
variety of membrane-based functions
PI
important source of 2ndary messengers (
PLC
→ DAG + IP3)
effects on cellular metabolism
PS
flips upon
platelet stimulation :syringe:
apoptosis :skull:
Annexin V staining
recognition by phagocytes
NO flip-flop :red_cross:
Within each leaflet
lipids are not randomly distributed (evenly mixed)
microdomains
Lipid rafts
contain
cholesterol
& SM
more ordered (less fluid)
glycolipids
bound by fluorescently labeled cholera toxin :microscope:
certain PM proteins
involved in
sensing & transmitting extracellular signals into the cytosol :zap:
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lateral mobility & spinning
Excess
Lipid droplets
:cry: triglycerides and cholesterol esters
from
ER
storage function
lipids & proteins (targeted for degradation)
visualized by
lipophilic dye:
Congo red
can increase their formation :meat_on_bone:
feeding cells with oleic acid
lipid
monolayer
formed by
delamination
of ER bilayer
lens formation in hydrophobic core
scission :scissors:
cytosolic leaflet
Proteins
Integral (
™
)
limited number of folded structures
mostly
hydrophobic α helix
continuous segment of
20-25
hydrophobic (uncharged) amino acids
hydrophobic R groups:
Van der Waals
interactions w/PM
hydrophilic :!: amide peptide bond in interior. how?
Each carbonyl (C=O) group forms
H-bond
with amide H atom of the a.a.
four
residues down the protein's C-terminus
polar groups shielded from hydrophobic interior of the membrane :shield:
predicted length (3.75 nm)
= hydrocarbon core of PL bilayer
mostly perpendicular to PM but can be oblique
single α-helical domain is sufficient
e.g.
Glycophorin A :syringe:
23 a.a. helix
Arg and Lys (:heavy_plus_sign:) near cytosolic side
bind negatively charged PL head groups
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Extracellular domain heavily
glycosylated
, with carbohydrate side chains attached to specific Ser, Thr, and Asn (polar) residues :bear: :honey_pot:
some
β Strands
barrels in Porins :oil_drum::oil_drum::oil_drum:
channels (small molecules)
E. coli
,
mitochondria
and
chloroplasts
trimers (identical subunits) :oil_drum:
each subunit:
16
β-strands
sheet
barrel
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single-pass TM protein
dimer
coiled-coil dimer stabilized by
Van der Waals
interactions between adjacent side chains :two_men_holding_hands:
Hydrophobic side chains project
outward
(towards PL fatty acyl chains)
e.g.
Dimeric glycophorin
multipass
Aquaporins :four:
transport water, glycerol, and other hydrophilic molecules
tetramers
(four identical
subunits
)
each subunit has
six
membrane-spanning
α helices
oblique / / / / / /
e.g.
glycerol channel Glpf ½
one long transmembrane helix
with bend in the middle
two α helices that penetrate only
halfway
through the membrane (their N-termini face each other)
aquaporin 0 :eye: :shirt:
annular
phospholipids: form a tight ring :doughnut: (annulus) against protein
Bacteriorhodopsin :sunny:
7 α helices
:triangular_ruler:
retinal covalently attached to
one
helix absorbs light
conformational change
pumping of protons from cytosol → extracellular space
proton gradient
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multimers
:!?: polar/charged a.a. residues in helix
e.g.
T-cell receptor
four dimers
help guide assembly
Topology :mountain:
Lipid-anchored
bound covalently to one or more lipid molecules embedded in
one leaflet
of membrane
polypeptide chain itself
does not
enter the PM
cytosolic lipid anchors
N-terminal Gly
acylation
(FA group)
C-terminal Cys
prenylation
(HC chain)
exoplasmic lipid anchors
glycosylphosphatidylinositols (GPIs)
PIns
sugar residues
PE
protein
Peripheral
bound to the membrane
indirectly
interactions with integral or lipid-anchored membrane proteins
directly
interactions with PL head groups
e.g.
phospholipases :older_man::skin-tone-2::zap::snake: (A2)
at cytosolic face
associate w/cytoskeleton
help determine cell
shape
and
mechanical
properties :frame_with_picture::nut_and_bolt:
at exoplasmic face
associate w/ECM or cell wall (:four_leaf_clover: :microscope:)
interface btwn cell & its environment
:red_cross:
but
do not
directly contact
hydrophobic
core
e.g.,
match with
:red_cross:
by