BIO130: Week 7 - Membrane Proteins

What are integral membrane proteins?

proteins DIRECTLY attached to bilayer:

inserted OR attached

What are the 3 types of integral membrane proteins?

1. Transmembrane (amphipathic): fully through

2. Mono-layer associated (amphipathic): semi-through

3. Lipid-linked: not through

What are the 3 types of transmembrane proteins?

1. Single-pass: one a-helix

2. Multipass: 2+ a-helices

3. B-barrel: rolled sheet

What are membrane-spanning alpha helices?

a-helix shape used to pass through the membrane

How long are membrane-spanning alpha helices?

about 20 hydrophobic amino acids

How do single-pass proteins work?

hydrophobic side chains of a-helix interact w/ lipid tails

How do multipass proteins work?

protein channel w/ aqueous core surrounded by amphipathic a-helices

- aq. transfer!

How do B-barrel proteins work?

protein channel w/ hydrophilic core surrounded by hydrophobic beta sheets

- aq. transfer!

What are the 2 ways transmembrane structures can be identified?

1. X-ray crystallography: 3D structure

2. Hydrophobicity plots

What are hydrophobicity plots?

scans 20-30 hydrophobic amino acids & checks their hydrophobicity

- to find single/multipass proteins

How do monolayer-associated proteins work?

amphipathic a-helix = some part inside bilayer

- only in CYTOSOL leaflet

- curves membrane = vesicle budding

What are the 3 components of lipid-linked membrane proteins?

1. Protein

2. GPI anchor/Lipid anchor

3. Lipid

How do lipid-linked membrane proteins work?

lipid is inside bilayer while lipid/GPI anchors are outside

- GPI: in ER lumen

- Lipid anchor: in cytosol

How are integral membrane proteins extracted?

detergents = bilayer destruction

What are peripheral membrane proteins?

proteins NOT directly attached to bilayer:

bound to other proteins OR lipids

- through non-covalent interactions

What is the 1 types of peripheral membrane proteins?

1. Protein-attached

How are peripheral membrane proteins extracted?

gentle buffer = bilayer safe!

- hydrophobic and hydrophilic sides match with bilayer to denature them

What are the 3 steps to create a liposome?

1. Purification: addition of detergent

2. Addition of phospholipids (mixed w detergent)

3. Removal of detergent

What process is used to study protein movement?

FRAP: Fluorescence Recovery After Photobleaching

How does FRAP work?

transmembrane proteins are marked green by GFP (Green Fluorescent Protein) then a patch of the bilayer is bleached white

- movement = recovered green

- no movement = white spot remains

What 2 things can restrict lateral diffusion/movement?

1. Binding to other structures

2. Barriers

What is the rate of fluorescence recovery?

time it takes for green proteins to take over bleached area