Describe different ways of association of proteins with membrane.
Answers
Membrane proteins represent about a third of the proteins in living organisms. Based on their structure, there are main three types of membrane proteins: the first one is integral membrane protein that is permanently anchored or part of the membrane, the second type is peripheral membrane protein that is only temporarily attached to the lipid bilayer or to other integral proteins, and the third one is lipid-anchored proteins (Fig. 1). Here we only describe the first two types of membrane protein.
Structural classification of membrane proteins
Fig. 1 Structural classification of membrane proteins
According to their their relationship with the bilayer, integral membrane protein can be classified two primary types: integral polytopic proteins and Integral monotopic proteins. Integral polytopic proteins are also known as “transmembrane proteins” which can span across the membrane at least once (Fig. 2). These integral membrane proteins may have different transmembrane topology which refers to orientations (locations of N- and C-termini) of membrane-spanning segments with respect to the inner or outer sides of the biological membrane occupied by the protein. The first three types in the Fig. 2 are common forms in integral membrane proteins, such as, transmembrane α-helix protein, transmembrane α-helical protein and transmembrane β-sheet protein.
Integral monotopic proteins are one type of integral membrane proteins that are attached to only one side of the membrane and do not span the whole way across. There are 4 types of interaction between Integral monotopic membrane protein and cell membranes: by an amphipathicα-helix paralle, by a hydrophobic loop, by a covalently bound membrane lipid and electrostatic or ionic interaction with membrane lipids (No. 4, 5, 6,7 of Fig. 2). Integral membrane proteins can be separated from the biological membranes only using detergents, nonpolar solvents, or sometimes denaturing agents. Peripheral proteins dissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations.
7 types of Integral Membrane protein Structure
Fig. 2 Seven types of Integral Membrane protein Structure
The membrane is represented in yellow. 1. A single transmembrane α-helix (bitopic membrane protein). 2. A polytopic transmembrane α-helical protein. 3. A polytopic transmembrane β-sheet protein. 4. Interaction by an amphipathic α-helix parallel to the membrane plane (in-plane membrane helix). 5. Interaction by a hydrophobic loop. 6. Interaction by a covalently bound membrane lipid. 7. Ionic or electrostatic interactions with membrane lipids