Proteins associated with or embedded in a biological cell membrane are called membrane proteins. Membrane proteins are of two types.
Proteins that are embedded deeply in the plasma membrane are integral proteins. They are held inside the membrane by hydrophobic interaction. Integral proteins may have their own transmembrane domain or may be linked with some special type of lipid-embedded deep inside the membrane. They can only be removed by using agents that interfere with hydrophobic interactions such as organic solvents, detergents, or denaturants.
Proteins that are attached to the surface of the membrane through electrostatic interactions as well as hydrogen bonding with the hydrophilic domains of integral proteins or polar head groups of membrane lipids. They can be released by treating with substances that can interfere with electrostatic interaction or hydrogen bonds such as salts and high pH.
Types of Integral Proteins
Depending upon the nature of the hydrophobic part of the protein and how they are embedded in the membrane, integral membranes are classified into six types. Type I and type II have only one transmembrane helix with the amino-terminal domain outside the cell in type I, while inside of the cell in type II integral membrane.
Type III integral proteins have multiple transmembrane helices within a single polypeptide and in type IV integral proteins, the transmembrane domains of several different polypeptides assemble to form a channel through the membrane.
Type V proteins are held to the lipid bilayers primarily through covalently linked lipids while the type VI integral proteins have both transmembrane helices and lipid anchors.
Interaction of proteins with membrane
Some membrane proteins contain one or more covalently linked lipids like long-chain fatty acids, isoprenoids, sterols, or glycosylated derivatives of phosphatidylinositol (GPI). The attached lipid provides a hydrophobic anchor that inserts into the lipid bilayer and holds the protein on the surface of the membrane.
The strength of such hydrophobic interaction between a bilayer and a protein-linked single hydrocarbon chain is enough to anchor the protein securely. However, many proteins have more than one attached lipid moiety.
Other interactions including ionic attractions between positively charged Lys residues in the protein and negatively charged lipid head groups, probably contribute to the stability of the attachment. However, the association of these lipid-linked proteins with the membrane is somewhat weaker than for the integral membrane proteins and probably in some cases reversible, even treatment with alkaline carbonate does not release GPI-linked proteins. Therefore, GIP-linked proteins are called integral proteins.