MVU7 MEMBRANES -2

Question 1

how are phospholipids synthesised?

Answer 1

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

Question 2

what needs to happen for the distribution of phospholipids to equal in the ER membrane?

Answer 2

scramblase protein randomly flips the phospholipids
ATP independent function, makes a balanced membrane

Question 3

how is assymmetry maintained in the plasma membrane?

Answer 3

flippase maintains membrane asymmetry
ATP dependent
directional
lipid specific

Question 4

how are lipids transported in the cell?

Answer 4

by vesicles between organelles of the secretory pathway
by carrier proteins through the cytosol
through contact sites between organelles (ER and mitochondria)

Question 5

what does the primary structure of a protein determine?

Answer 5

structure, function and localisation
membrane proteins have contact with lipids

Question 6

what does the localisation of membrane proteins require?

Answer 6

protein based targeting mechanisms

Question 7

how are integral membrane proteins integrated to interior of lipid bilayer?

Answer 7

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

Question 8

how are lipid anchored proteins attached?

Answer 8

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

Question 9

how are peripheral membrane proteins attached?

Answer 9

attached by non covalent interactions (protein protein interactions)
strong interactions with integral membrane proteins
with lipid head group, weak interactions

Question 10

what is usually the function of transmembrane protein?

Answer 10

function in both cellular components
cell surface receptors, transporters

Question 11

what is usually the function of lipid anchored proteins?

Answer 11

function on one side of the membrane
intracellular signalling

Question 12

what is structure of transmembrane helices?

Answer 12

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

Question 13

what are characteristics of TM helices?

Answer 13

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

Question 14

why do TM helices have some polar AAs

Answer 14

can be in contact with the outside, or transport molecules

Question 15

what are the characteristics of beta barrels?

Answer 15

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

Question 16

when is TM protein orientation determined?

Answer 16

during its insertion into the membrane

Question 17

how are lumenal/extracellular domains modified in secretory pathway?

Answer 17

differently from cytosolic domains
disulfide bonds between cysteine
oligosaccharides (glycosylation)
stabilise protein structure

Question 18

what are protein modifications only found on cytosolic domains?

Answer 18

phosphorylation
ubiquitination
acetylation
methylation

Question 19

how do protein channels work?

Answer 19

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)

Question 20

how do protein pumps/transporters work?

Answer 20

cause movement against the gradient
require energy, ATP
regulated by turning ATPase on and off

Question 21

what is an example of a protein channel?

Answer 21

voltage gated rat brain K+ homotetramer
neuron signaling
homologs: heart rhythm

Question 22

how do channels pore select for ions?

Answer 22

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

Question 23

how does the multidrug resistance transporter work?

Answer 23

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

Question 24

how do lipid anchored proteins work?

Answer 24

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

Question 25

what is the AA rule for acylation?

Answer 25

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

Question 26

what is the code for prenylation?

Answer 26

has to have a specific motif:
Cys-A-A-X-COO-
A= alkyl side chain
X= any
thioester linkage between cysteine and prenyl group

Question 27

which lipid modifications are reversible and which are not?

Answer 27

they are not reversible except thioester bond to cysteine

Question 28

what are GPI anchored proteins?

Answer 28

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