Membranes/membrane proteins

Question 1

biomembranes

Answer 1

• lipids, sterols, and protein components

- due to amphipathicity (both philic/phobic) spontaneously form bilayer

Question 2

Properties of biomembranes

Answer 2

• fluidity
• closed compartments
• seme-permeable
• asymmetry

Question 3

Fluidity

Answer 3

• rapids lateral diffusion
• slow transverse (flip-flop) b/w leaflets
• composition dependant (fatty acid length, unsaturation, steroids, proteins, temperature)

Question 4

measuring fluidity

Answer 4

FRAP
- label with fluor, bleach with laser, fluorescent recovery
- intensity will reach about 50% of initial
slower diffusion in PM’s that contain proteins, restrict movement
- 50% of the fluorescence returned to bleached area = 50% of receptor molecules were mobile, 50% immobile

Question 5

Closed Compartments

Answer 5

• cytoplasmic vs exoplasmic faces
• PM: cytosolic face= internal
• Vesicle membrane: cytosolic face = external

Question 6

Semi-permeability

Answer 6

• small, uncharged, non-polar molecules pass through freely

- large, hydrophilic, charged cant pass

Question 7

Asymmetry

Answer 7

• phospholipid composition differs b/w leaflets
• glycans only found at exoplasmic face, make glycocalyx
• proteins are either embedded in bilayer in a fixed manner or associated with only one side

Question 8

Membrane proteins

Answer 8

• integral
• lipid-anchored
• peripheral
  ARE ALL SYMMETRIC

Question 9

Integral

Answer 9

• 3 domains
• Cyto: philic. often charged with Arg/Lys to interact with polar head groups
• Transmembrane: phobic. Secondary/tertiary. generally Ahelix or Bbarrel
  Exoplasmic: philic. Most domains are glycosylated

Question 10

Lipid-Anchored

Answer 10

• has lateral mobility in membrane
• GPI anchor: exoplasmic. Requires sugar residue
• Fatty Acyl: Cytoplasmic. Attaches to N-term glycine by acylation
• Prenyl: Cytoplasmic. Attaches to cysteine residue at/near C-term by prenylation

Question 11

Peripheral

Answer 11

• non-covalent interactions. Ionic, H-bonds, protein-protein, vander waals
• cytoskeleton filaments can assiciate with bilayer through peripheral proteins (adaptors) as can ECM components
• dystrophin

Question 12

Integral protein classification

Answer 12

• tail-anchored

- Type 1,2,3,4

Question 13

Tail-anchored

Answer 13

• no extra-cellular domain
  1. Get3-ATP binds hydrophobic C-term tail of protein
  2. Complex docks at Get1/Get2 receptor
  3. ATP hydrolysis–> tail embeds in ER membrane
  4. Get-3 binds ATP, dissociates from ER

Question 14

Type 1

Answer 14

• cleaved N-term signal sequence targets to ER
• Stop transfer (STA) sequence becomes embedded in ER membrane
• protein synthesis continues and C-term remains in cytosol

Question 15

Type 2

Answer 15

• Signal-anchor internal sequence (SA) directs insertion of nascent polypeptide
• orientation determined by postive amino acids, kept in cytosol
• N TERM KEPT IN CYTOSOL

Question 16

Type 3

Answer 16

• Signal-anchor internal sequence (SA) directs insertion of nascent polypeptide
• orientation determined by postive amino acids, kept in cytosol
• C TERM IN CYTOSOL
• SA fxns like STA in Type 1
• type 3 CC

Question 17

Type 4

Answer 17

• orientation of initial helix determined by positive aa’s next to SA
• alternating STA and SA
• odd/even number of transmembrane domains
• 4A: N and C term in cytosol
• 4B: C term cytosol

Question 18

Hydropathy profile

Answer 18

• can predict whether protein of interest has/doesnt have topogenic signalling sequences
• positive index: hydrophobic, NP.
• negative index: hydrophilic, P

Question 19

topgenic sequences examples

Answer 19

• direct specific insertion of transmembrane proteins

- Cleaved N-term, STA, SA, hydrophobic C term