Membranes - Biological Funciton

Question 1

Evidence for proteins in the membrane

Answer 1

Functional - membrane allows for :
– Facilitated diffusion
– Ion gradients
– Specificity of cell responses

Biochemical - can see proteins using :
– Membrane fractionation + gel electrophoresis
– Freeze fracture

Question 2

Mobility of proteins in bilayer

Answer 2

Three modes of motion permitted:

• Conformational change (shape changes)
• Rotational
• Lateral
• No flip flop (as thermodynamically its a challenge

Question 3

restriction of membrane protein mobility

Answer 3

Restraints on mobility:
lipid mediated effects - proteins tend to separate out into the fluid phase or cholesterol poor regions
membrane protein associations - can get interactions with proteins from another cell
association with extra-membranous proteins (peripheral proteins), e.g. cytoskeleton

Question 4

Membrane proteins

Answer 4

Peripheral -
Bound to surface
Electrostatic and hydrogen bond interactions
Removed by changes in pH or in ionic strength

Integral -
Interact extensively with hydrophobic domains of the lipid bilayer
Cannot be removed by manipulation of pH and ionic strength
Are removed by agents that compete for non-polar interactions
e.g. detergents and organic solvents

There proteins produce the lipid mosaic model - refers to patchwork of membrane proteins that are present

Question 5

Membrane protein topology

Answer 5

Usual structure of a transmembrane protein = NH2 end present in the cytosol and the COOH end in the ECM, with a transmembrane helix present in the lipid bilayer

Can also get oligosaccharides binding to the ECM part of the protein, as well as disulphide bonds forming with sulphydryl groups binding as well to the cytosolic park of the protein

Question 6

Erythrocytes cytoskeleton

Answer 6

Different types of proteins present to give the RBC its concave shape

2 integral proteins - Band 3 and glycoprotein A
The rest are peripheral proteins or protiens that bind to the integral proteins
E.g. ankyrin binds to Band 3 in the cytosol and allows spectrin to bind to it as well allowing spectrin to connect to the PM
Band 4.1 bind to glycoprotein A and to spectrin - which also helps connect spectrin to the PM

Question 7

Haemolytic anaemias

Answer 7

E.g. Hereditary Spherocytosis
Spectrin depleted by 40-50%
Erythrocytes round up
Less resistant to lysis
Cleared by spleen

Hereditary Elliptocytosis
Defect in spectrin molecule
Unable to form heterotetramers
Fragile elliptoid cells

Question 8

Secreted and membrane protein biosynthesis

Answer 8

Signal recognition particle binds to mRNA with ribosome present, takes it to docking protein present in the PM
During this signal sequence is made via ribosomes and synthesis stops there
Ribosome binds to signal sequence receptor and continues to secrete protein
Signal peptidase cleaves off the signal sequence before the protein is secreted out of the cell

In the membrane the ribosome stops secreting before its injected into the ER, so PM reforms around the protein making it a membrane protein

Question 9

Secretory protein synthesis

Answer 9

N-terminal signal sequence folds into the membrane positioning positively charged resides on the cytoplasmic side, signal peptidase cleavage, nascent polypeptide extruded into ER lumen during synthesis

N-terminal signal sequence, signal peptidase cleavage, nascent polypeptide extruded into ER lumen during synthesis until hydrophobic stop transfer sequence synthesised, ribosome detaches from ER and completes protein synthesis in the cytoplasm)