What are Membrane Proteins
Membrane proteins, as the name implies, are specialized proteins found exclusively embedded in the phospholipid bilayers of cell membranes. They may be either permanently or temporarily attached to the membrane.
About one-third of the proteins found in living organisms are membrane proteins. These proteins regulate the majority of the functions of the membrane that help in cell survival.
Glycophorin, bacteriorhodopsin, and porin are some examples of membrane proteins.
Types with Structure
Based on how the protein is associated with the membrane, they can be broadly classified into two types: (1) integral membrane proteins and (2) peripheral membrane proteins.
1) Integral Membrane Proteins
Integral membrane proteins, also called intrinsic proteins or transmembrane proteins, are permanently embedded within the plasma membrane. These proteins contain residues with hydrophobic side chains that penetrate the phospholipid bilayer, thus anchoring the protein to the membrane.
Integral proteins cannot be easily isolated from the cell membrane. The only way to remove them is using synthetic detergents, nonpolar solvents, and denaturing agents that destruct the hydrophobic interactions of the bilayer.
Depending on their relationship with the lipid bilayer, integral membrane proteins can be classified into two basic types: integral polytopic proteins and integral monotopic proteins.
1. Integral polytopic proteins: These transmembrane proteins span the membrane at least once. These proteins may have different transmembrane topologies, which means the orientations of amino-terminal or N-terminal and carboxyl-terminal or C-terminal on the inner or outer sides of the membrane may differ. Integral polytopic proteins regulate the transport of specific molecules across the membrane.
There are three basic types of transmembrane proteins:
- α-helix: Single-spanning transmembrane proteins
- α-helical: Span the membrane more than once.
- β-sheet or β-barrel: Span the membrane more than once.
2. Integral monotopic proteins:
These are integral membrane proteins attached to only one side of the membrane. They do not span the whole way across.
Here are some examples of integral membrane proteins along with their function:
- Glycoprotein: Helps in cell-to-cell interactions.
- Glucose permease: Acts as the reversible transporter protein of glucose.
- Na+/K+ ATPase: Maintains the electrochemical gradients of Sodium and Potassium ions across the plasma membrane.
- Gap junction proteins: Forms a direct connection between the cytoplasm of two adjacent cells, allowing various molecules and ions to pass freely.
- Ion channels and gates: Control the flow of ions across the cell membrane.
- Bacterial rhodopsins: Found exclusively in archaeans that use light energy to pump protons across the membrane.
2) Peripheral Membrane Proteins
Peripheral membrane proteins, also known as extrinsic proteins, are only temporarily associated with the membrane. As most of them are hydrophilic, they are either attached to integral membrane proteins or can be directly bound to the bilayer’s polar head. So, they can be easily removed using any polar solvent. On the cytoplasmic side, these proteins are held in position by the cytoskeleton.
Peripheral membrane proteins are primarily associated with ion channels and transmembrane receptors, allowing cell signaling.
Some examples of peripheral membrane proteins along with their functions are as follows:
- Spectrin: Links the plasma membrane to the actin cytoskeleton. Thus, it helps to determine the cell shape, arrange transmembrane proteins, and organize cellular organelles.
- Phospholipases: Hydrolyzes various bonds in the polar head of phospholipids which plays a vital role in the degradation of damaged or aged cell membranes.
- Kinase C: helps mediate signal transduction cascades by hydrolyzing lipids.
- Hormone receptors: Bind a hormone to the extracellular surface and activate a protein kinase inside the cell.
What are the Functions of Membrane Proteins
The six major functions of membrane proteins are listed below:
Enzymatic Functions: Some membrane proteins may be enzymes with their active sites exposed to substrate molecules in the adjacent solution. Sometimes, several enzymes in a membrane organize as a team, carrying out sequential steps of a metabolic pathway.
A protein that spans the cell membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Other transport proteins shuttle a substance from one side to another by a conformational change.
Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane.
Signal Transduction: A membrane protein receptor may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone.
The external messenger or signaling molecule may cause the protein to change shape, allowing it to relay the message to the inside of the cell, usually by binding to a cytoplasmic protein.
Thus, membrane proteins also function as receptors for peptide hormones
Cell Recognition: Some glycoproteins serve as identification tags that are recognized explicitly by membrane proteins of other cells. This type of cell-cell binding is usually short-lived.
Intercellular Joining: Membrane proteins of adjacent cells may connect through various kinds of junctions, such as gap junctions or tight junctions.
Attachment: Microfilaments or other cytoskeleton elements may be non-covalently bound to membrane proteins. It helps maintain cell shape and stabilizes the location of specific membrane proteins.
Integral vs Peripheral Membrane Proteins
From the above context, it is clear that there are some evident differences between integral and peripheral proteins. Let us summarize them:
|Integral Proteins||Peripheral Membrane Proteins|
|Permanently attached to the cell membrane.||Temporarily attached to the cell membrane.|
|Embedded in the whole membrane.||Located on the inner or outer surface of the cell membrane.|
|Interaction with the hydrophobic core of the lipid bilayer is high.||Interaction with the hydrophobic core of the lipid bilayer is low.|
|Constitute 70% of the total membrane proteins.||Constitute 30% of the total membrane proteins.|
|Contain both hydrophilic and hydrophobic parts.||Contain only hydrophilic part.|
|Act as carrier, channel, cell recognition, receptor, and enzymatic proteins.||Help in cell-cell signaling and interactions.|
|To isolate these proteins, detergents or non-polar solvents are required.||To isolate these proteins, polar solvent is required.|
|Examples: membrane-bound enzymes, hormone receptor, antigen, rhodopsin.||Examples: cytochrome C, acetylcholineesterase, mitochondrial ATPase.|
Ans. Integral membrane proteins are most likely to be found on the cell membrane of cells that respond to extracellular signals.
Ans. Yes, integral membrane proteins are amphipathic.
Ans. Yes, membrane proteins bind to ligands.
Ans. Yes, membrane proteins move.
Ans. Yes, membrane proteins regulate the passage of ions.
Article was last reviewed on Thursday, December 16, 2021