Proteins in Membranes

Question 1

Why are proteins synthesised in the ER, transported to the Golgi before being packaged and sent to the membrane?

Answer 1

All of those areas have an oxidative environment with Na+, Ca++. The proteins are glycosylated for protection and there are S-S bonds. However the cytoplasm is reductive with Mg++ and K+, minimally glycosylated proteins and no S-S bonds.

Question 2

Explain integral membrane proteins

Answer 2

Totally embedded TMD with a lipid attachment to the inner or outer leaftlet

Question 3

Explain peripheral membrane proteins

Answer 3

Non covalently (reversibly bound to -ve charged membrane patches and to adaptor proteins or protein kinases - to cytoplasmic domains of receptors

Question 4

Explain features of a TMD

Answer 4

Alpha helix
3.6 AA per turn
7 turns
Approx 25 AA
Hydrophobic AA

Question 5

How can topology be useful?

Answer 5

Topology = shape and orientation in membrane of a protein.

Can use known intracellular motifs - they should be inside

Question 6

What is a GPI anchor?

Answer 6

Glycosylphosphatidylinositol anchor - attached to the membrane surface

Question 7

Explin rafts/cholestrol/Lo phases etc

Answer 7

Lipids in fluid membranes are disordered - the chains fill up the space around proteins (numerous proteins can fit).
Cholestrol orders these - cause FA chains to straighten up = ordered phase where only certain proteins can fit.

Where you have saturated lipids and cholesterol = raft = Lo phase. 95% of proteins cant enter these - those that can are then protected from inactivation by the bulk of enzymes in the fluid phase.

Raft proteins need:
Large TMD - 26aa - or a conformation that allows the alpha helix to be stretched
Palmitoylation

Question 8

Explain how Gproteins and lipid rafts interact

Answer 8

Trimeric G protein - alpha subunit is anchored and upon activation, alpha and the beta gamma subunits separate. The alpha subunit goes into the raft where it is protected from inactivation and therefore can act for longer.

Question 9

Explain the Beta adrenergic receptor

Answer 9

Asscociated with GP
7TMD
Agonist or antagonist binding
Areas involved in densentisiation

Question 10

Explain the nicotinic ach receptor

Answer 10

5 subunits, 4 TMD in each, dynamic pore (4th P loop = voltage sensor)

Question 11

Trk receptor?

Answer 11

Binds NGF, receptors dimerises on ligand binding - intracellular portion has tyrosine kinase activity,