MVU7 MEMBRANES -2 Flashcards
how are phospholipids synthesised?
made on the cytosolic side of the ER membrane
fatty acids (acyls) are brought and inserted into the membrane by fatty acid binding protein (protects hydrophobic tail)
CoA is added to the fatty acid (intermediate)
a phosphate is added, removed, and then glycerol phosphate, the head group, is added
what needs to happen for the distribution of phospholipids to equal in the ER membrane?
scramblase protein randomly flips the phospholipids
ATP independent function, makes a balanced membrane
how is assymmetry maintained in the plasma membrane?
flippase maintains membrane asymmetry
ATP dependent
directional
lipid specific
how are lipids transported in the cell?
by vesicles between organelles of the secretory pathway
by carrier proteins through the cytosol
through contact sites between organelles (ER and mitochondria)
what does the primary structure of a protein determine?
structure, function and localisation
membrane proteins have contact with lipids
what does the localisation of membrane proteins require?
protein based targeting mechanisms
how are integral membrane proteins integrated to interior of lipid bilayer?
anchored by hydrophobic interactions
1 or more transmembrane alpha helices
transmembrane beta barrel
less common amphipathic alpha helices where it is attached to protein on one side and the other side is polar and facing the outside
how are lipid anchored proteins attached?
covalently linked to one or more lipids or fatty acid groups
type of post translational modification
strength of anchor depends on number and type of lipid
how are peripheral membrane proteins attached?
attached by non covalent interactions (protein protein interactions)
strong interactions with integral membrane proteins
with lipid head group, weak interactions
what is usually the function of transmembrane protein?
function in both cellular components
cell surface receptors, transporters
what is usually the function of lipid anchored proteins?
function on one side of the membrane
intracellular signalling
what is structure of transmembrane helices?
most common form of attachment
like all alpha helices, amino acid side chains point outwards
side chains are hydrophobic to interact with lipids
internal hydrogen bonds maintain structure of helix
what are characteristics of TM helices?
proteins can have one or more helical transmembrane domains
TM helices are longer and more hydrophobic than interior of soluble protein
usually 18-24 amino acids long
can be predicted from hydrophobicity of primary sequence
length of TM helices matches width of membrane
longer TM helices can partition into thicker membranes or insert at an angle in thinner membranes
proteins with single TM helix can rotate easily
multiple TM helices can fold together into functional structures embedded in the membrane
why do TM helices have some polar AAs
can be in contact with the outside, or transport molecules
what are the characteristics of beta barrels?
beta strands form a TM barrel: beta sheet wrapped into a cylinder
number of TM strands can vary
side chains point to lipid bilayer and inside hydrogen bonds hold strands together
when is TM protein orientation determined?
during its insertion into the membrane
how are lumenal/extracellular domains modified in secretory pathway?
differently from cytosolic domains
disulfide bonds between cysteine
oligosaccharides (glycosylation)
stabilise protein structure
what are protein modifications only found on cytosolic domains?
phosphorylation
ubiquitination
acetylation
methylation
how do protein channels work?
allow movement along gradient
do not require energy
regulated by opening and closing
allow regulated flow of ions across membranes
TM helices can form water filled channel
opening is controlled by cytosolic domains or subunits
some TM helices must be polar on one side
selective for ion (depends on how much the channel can open)
how do protein pumps/transporters work?
cause movement against the gradient
require energy, ATP
regulated by turning ATPase on and off
what is an example of a protein channel?
voltage gated rat brain K+ homotetramer
neuron signaling
homologs: heart rhythm
how do channels pore select for ions?
carbonyls from peptide backbone line the pore
ions with right size can exchange water for carbonyls
wrong size cannot bind carbonyl completely and is rejected
how does the multidrug resistance transporter work?
cancer cells use it to resist chemo, pump out toxins
ABC transporter with 2 symmetrical ATPase domains
1. no nucleotide: inward open high affinity for substrate
2. ATP bound: closed
3. ATP hydrolysis: outward open, low affinity for substrate
how do lipid anchored proteins work?
cytosolic proteins covalently linked to acyl (fatty acid) or prenyl chains
single lipid chain provides transient interaction with membrane
two or more lipid chains needed for strong membrane anchor
specific enzymes attach lipid to N terminus or cysteine side chains
what is the AA rule for acylation?
methionine is removed
the N terminus AA has to be cysteine or glycine
amine linkage formed between N terminal and myristic
or cysteine can be anywhere in the protein
thioester bond formed between cysteine and palmitic group
what is the code for prenylation?
has to have a specific motif:
Cys-A-A-X-COO-
A= alkyl side chain
X= any
thioester linkage between cysteine and prenyl group
which lipid modifications are reversible and which are not?
they are not reversible except thioester bond to cysteine
what are GPI anchored proteins?
some proteins have special TM helix removed and become covalently linked to glycosul-phosphatidyl inositol (GPI) anchor
strong membrane attachment
only on lumenal/extracellular side, attached at ER, function at exterior of PM
gives more rotational freedom than proteins with TM helices