Tuesday, January 18, 2011

Membrane Proteins

Protein molecules are bound , dissolved,lodged in the lipid bilayer. Membrane proteins carryout almost all the functions of membranes.

  • Transport 
  • Sensors(Receptors)
  • Reactions(Scaffold Proteins)

Different proteins in the bio membrane carry out different functions.Membrane proteins also help to maintain the structural integrity.

According to how proteins associate with the lipid bilayer it is divided to two sections.
  1. Integral membrane proteins
  2. Peripheral membrane proteins

             1. Integral membrane proteins

Intrinsic membrane proteins
Integral membrane proteins are of two types

  • Protein molecules that are entirely or partly embedded or buried within the membrane.
The integral proteins that span across the lipid bilayer are called transmembrane proteins.
Transmembrane proteins are amphipathic
Contain at least one hydrophobic region that goes through the lipid bilayer.
This part is called the membrane spanning domain
Membrane spanning domain consists of alpha helix and beta barrels.
  • Protein molecules that are tightly bound to lipid molecules of the lipid bilayer

These are covalently linked with the hydrophobic tails of the lipid bilayer.
These do not enter or go through the lipid bilayer. They are non-transmembral.
Are attached only to one side of the membrane. Located entirely on the cytosolic side or entirely on the exoplasmic side.

Some integral proteins may have both hydrophobic interactions and covalent bonds with lipids.

             2. Peripheral membrane proteins

Extrinsic membrane proteins
Do not interact or form strong bonds with hydrophobic core of the bilayer.
Form Hydrogen/Ionic/other weak interactions with integral membrane proteins/heads of the lipid molecules of the bilayer.
Localized on cytosolic side or exoplasmic side of the plasma membrane.
These can be easily removed for analysis using;
high ionic streangth( high salt)
chemicals like EDTA that bind with Mg2+

Another way of classifying membrane proteins is;
  • Transmembrane proteins
  • Lipid Anchored proteins
  • Peripheral Proteins
Other Proteins that are closely associated with membranes

These are neither integral nor peripheral proteins.
From cytosolic side there are cytoskeleton proteins which are associated with peripheral proteins.
eg : Spectrin is a skytoskeleton protein in Red Blood Cells that give strength and flexibility to the cell.
From extracellular side these are proteins that are associated with peripheral proteins and form extracellular matrix in cell walls. These increase the elasticity. 


  1. Thanks!
    Think I've scored around 20 outta 30
    The other 10 not sure
    Didn't even had time to read the whole of cell signalling note! :-/