Cell Structure and Function Chapter 7

MP (membrane proteins) Functions

• Substrate Binding

• Substrate Transport

• Signal Transduction, etc.

Signal Transduction

• Mechanisms by which signals are transmitted from the outer surface to the interior of a cell

Cell Junctions

• Adhesive Junctions - hold cells together

• Tight junctions - form seals that block the passage of fluids between cells

• Gap Junctions - allow communication between adjacent animal cells (plasmodesmata for plant cells).

Fluid Mosaic Model

Membrane as two fluid layers of lipids with proteins within and on the layers

• Fluid because lipids and proteins easily move laterally in the membrane

• mosaic because there are proteins within the membrane

3 Main Classes of Membrane Lipids

• Phospholipids

• Glycolipids

• Sterols

Sterols

• Most eukaryote membranes have plenty

  • Cholesterol

  • Phytosterols (plants)

• Not Found in Most Bacteria

Fatty Acids

• Components of all membrane lipids (excluding Sterols)

• Hydrocarbon tails bar polar solutes from diffusing 

• Saturated - no double bonds

• Unsaturated - double bonds

Membrane Asymmetry

• Difference between the monolayers regarding the kind of lipids present and the saturation of fatty acids in the phospholipids

• Ex: Most glycolipids in animal cells are out the outer layer of the plasma membrane. 

Lipid Mobility

• Rotation - Phospholipids can rotate about their axis

• Lateral Diffusion - Can move WITHIN the monolayer 

• Rotation and Lateral Diffusion are both rapid and random

• Transverse Diffusion (flip flop) - movement of lipids from one monolayer to another requires their hydrophilic heads to move all the way through the hydrophobic interior of the bilayer.

• Protein Catalyzes for Transverse:

  • Phospholipid Translocators AKA flipasses, floppases, and scramblases

    • Flippase - outer to inner leaflet

    • Floppases - inner to outer leaflet

    • Scramblases - both switching sides at same time

FRAP

Fluorescence Recovery After Photobleaching

Demonstrates Lateral Diffusion

Factors Regulating Membrane Fluidity

Temperature (mainly depends on this)

• Fluidity Increases as temp increases

Fatty Acid Structure

• Saturation:

  • Saturated - less membrane fluidity

  • Unsaturated - more membrane fluidity

• Length of Hydrocarbon Tail:

  • More fluidity when less hydrocarbon length

  • Less fluidity when more hydrocarbon length

Sterols

• Fluidity Buffer:

  • Rigidity prevents membrane fluidity at higher temp

  • Rigidity prevents phospholipids packing close together and reduces the tendency to gel cal cooler temperature

• Sterols decrease permeability of membranes to ions and small polar molecules

  • Fill spaces between the hydrocarbon chains of phospholipids

Liquid Crystal v Gel states

Fatty Acid Saturation and Packing

Most fatty acids vary in chain length and degree of saturation to ensure that membranes are fluid at physiological temperatures

Homeoviscous Adaptation

Compensating for changes in temp by altering the length and degree of saturation of fatty acids in their membranes. 

Freeze Fracturing

• Bilayer is frozen and hit with diamond knife and the resulting fracture follows the plane between the 2 layers of membrane lipid 

  • showed globular proteins 

• Evidence for fluid mosaic model

Classes of MP

• Integral - proteins embedded in the lipid bilayer

• Peripheral - hydrophilic and located on the surface

• Lipid Anchored - hydrophilic and attached to bilayer by covalent attachments to lipid molecules embedded in bilayer.

Integral Proteins

• Possess one or more hydrophobic regions with an affinity for the interior of the lipid bilayer. 

• Can only be extracted by dissolving membrane to get them out

• Integral monotopic proteins - embedded in just one side of bilayer

• Most are however Transmembrane proteins that span the membrane and protrude on both sides

  • Can cross once (singlepass) or several times (multipass)

Peripheral Membrane Proteins

• Lack discrete hydrophobic regions and do not penetrate lipid bilayer

• bound to membrane surfaces through weak electrostatic forced and hydrogen bonds (potentially also hydrophobic residues)

• Can be removed by changing pH or ionic strength

Lipid Anchored Membrane Proteins

• Covalently bound to lipid molecules embedded in the bilayer

• Those bound to the inner surface are linked by fatty acids (prenyl groups) 

Hydropathy Plots

• used to determine the # and location of transmembrane segments in a membrane protein which can be inferred if the protein sequence is known. 

• Calculate a hydropathy index for successive “windows” along the protein

Membrane Protein Function (Part 2)

• Enzymes

• Electron Transport Proteins - involved in energy production

• Transport proteins - nutrients

• Channel proteins - hydrophilic passageways

• Transport ATPases - uses ATP to transport ions

• Receptors - recognize outside stimuli

Membrane Protein Asymmetry

• Asymmetric orientation with respect to lipid bilayer

  • Once in place they cannot move across the membrane from one surface to another

• All of the molecules of a particular protein are oriented the same way in the membrane 

Glycosylation

Glycoproteins - membrane proteins with carbohydrate chains covalently linked to amino acid side chains

The addition of carbohydrate side chain to a protein is called glycosylation 

• Can be to the nitrogen atom of an amino group

• Can be to the oxygen atom of a hydroxyl group

• Can be either straight or branched

Play a role in cell-cell recognition and often forms protective barrier (glycocalyx)

Membrane Mobility

• More variable than lipids in their ability to move freely within the membrane

• Some constrained some can move freely

• Cell fusion experiments proved mobility (David Frye and Michael Edidin).

Membrane Domains

• Differ in protein composition and function

  • Lipid Rafts

• Structures that become barriers to diffusion

Lipid Rafts

• Lipids of certain types self associate and MPs of the right size and electrostatics can partition to the lipid domains. 

  • Sphingolipids and cholesterol are particularly prone to domain formation

• Form distinct membrane identities 

Erythrocyte

• Meshwork of peripheral and integral membrane proteins

• Membrane System

• Supports the membrane, helps maintain cell shape, and helps cell withstand stress.