Membranes

1
Stephen Fish, Ph.D. Marshall University J. C. E.
School of Medicine Fish_at_Marshall.edu
2
Note to instructors I use these PowerPoint
slides in cell biology lectures that I give to
first year medical students. Copy the slides, or
just the illustrations into your own teaching
media. We all know that teaching science often
requires compromises and simplification for
specific student populations, or the requirements
of a specific course. Please feel free to offer
suggestions for improvements, corrections, or
additional illustrations. I would be pleased to
hear from anyone who finds my work useful, and am
always willing to make it better. Also, the
images have been compressed to screen resolution
to keep PowerPoint file size down, and I can
provide them at any resolution. Stephen E.
Fish, Ph.D.
3
Membrane Proteins 1
4
Membrane proteins, the fluid mosaic model

• Proteins afloat in a lipid sea (so profound)

5
Classification of types

• Peripheral membrane proteins
• Integral membrane proteins
• Single leaflet
• Single pass transmembrane
• Multiple pass transmembrane

6
Peripheral membrane proteins

• Non-covalent bonds with proteins or lipids (ionic
  interactions or hydrogen bonds)
• Glycocalyx glycoproteins outer leaflet example
• Spectrin accessory proteins inner leaflet
  example

Glycoprotein
Spectrin
Accessory
7
Single leaflet integral proteins

• The protein itself is not amphipathic
• Covalent bond with lipid
• Outer- GPI anchor (glycosylphosphatidylinositol)
• Inner- fatty acid (myristic, palmatic) or
  hydrocarbon chain (prenyl group)

8
Single pass transmembrane proteins
Polar domain

• Amphipathic with
• Hydrophobic a-helix in membrane
• Polar domains on both sides of membrane

a-helix
Polar domain
9
The hydrophobic a-helix
10
The hydrophobic a-helix
11
Multipass transmembrane proteins

• Multiple hydrophobic (a-helix), hydrophilic
  domains
• They form complex membrane machines
• One example is the band-3 ion transporter
  (although it would have 14 segments)

12
Folded multipass transmembrane proteins

• a-helices fit together in complex ways
• Polar regions also folded
• Expose hydrophilic side chains to aqueous
  environment
• Hide hydrophobic side chains away from water on
  the inside

13
Multipass proteins can make aqueous channels in
membranes

• Side chains are arranged so that hydrophobic ones
  contact lipid
• Hydrophilic ones surround the central opening
• These sorts of channels are fussy about what is
  allowed to pass through the membrane

14
Porins Another type of multipass protein that
forms aqueous pores

• Multiple transmembrane segments are not a-helices
  
• Arranged in a barrel shape
• Large aqueous channel called a pore
• Porins are in the outer mitochondrial membrane
• Allow any molecules smaller than 6 kDa to pass
  through

15
Porin is a ß-barrel made froma rolled up ß-sheet

• ß sheets are made of similar sequences of amino
  acids bound together side by side
• They can be bound in parallel, or antiparallel,
  with their ends connected by loops of the protein

16
Making an antiparallel ß-sheet
17
Making a parallel ß-sheet
18
ß-sheets are pleated arranged with hydrophilic
hydrophobic side chains on opposite sides
19
To form a barrel, the sheet rolls up
20
With hydrophobic side chains outside,
hydrophilic inside, the barrel locks in the
membrane forms an aqueous pore
21
Individual proteins can bind to assemble large
multisubunit protein machines

• Separate proteins diffuse in the membrane
• They can assemble to perform important functions

22
Multisubunit membrane protein machines perform
important tasks
Bind together
23
Sherman says
I dont like hops, but the boss does
24
Visualization of membrane proteins by freeze
fracture EM

• Flash freeze fresh specimen in a block of ice
• Strike the block with a knife
• The specimen breaks along lines of least
  resistance
• Breaks near membrane usually separate leaflets
• Proteins stick in one leaflet or the other

25
When leaflets separate
26
Next, some of the exposed ice is removed the
surface is coated with metal
27
The exposed surface is madeinto a model
28
Use scanning EM (SEM) to make a photo

• Electrons are attenuated relative to the
  thickness of the metal coat
• A pseudo 3D image is captured

29
Freeze fracture EM