What Is an Integral Membrane Protein?
An integral membrane protein, also known as an IMP, is one which spans the entire biological membrane of a cell. These proteins are attached permanently to the cell membrane, and their function typically relies on being present in the membrane. Both structurally and functionally, they are integral parts of the membranes of cells.
Each integral membrane protein molecule has an intricate relationship with the membrane within which it is situated. Structurally, the IMP is usually placed such that protein strands are woven throughout the structure of the cell membrane. Sections of protein protrude through the cell wall inside or outside the cell, or in both directions. The protein molecule cannot function if it is not embedded within the membrane.
Another feature of the protein is that these proteins can be removed from the membrane only with very specific chemical treatment. This is because hydrophobic regions of the protein are protected within the phospholipid bilayer of the cell membrane. For this reason, detergents, denaturing solvents, and nonpolar solvents must be used to disrupt the phospholipid bilayer and extract the integral membrane protein.
Within the integral membrane protein class are several different categories of protein, many of which are receptors and other types of cell signaling molecules. They are categorized into two groups, based on their structure. These are integral transmembrane proteins, and integral monotopic proteins.
Integral transmembrane proteins are those which span the entire cell membrane. These proteins may span the membrane once, or may span it several times, weaving through the phospholipid bilayer such that there are several pieces of the protein protruding through the cell wall. Overall this is the most common type of IMP.
Examples of integral transmembrane proteins include voltage-gated ion channels such as those which transport potassium ions in and out of cells. Certain types of T cell receptors, the insulin receptor, and many other receptors and neurotransmitters, are all integral transmembrane proteins. In general, receptors, transmitters, and transporters tend to belong to this class of IMP because proteins that span the entire membrane are typically able to sense conditions both inside and outside of the cell simultaneously.
Integral monotopic proteins do not span the entire biological membrane. Instead they are attached to the membrane from only one side, with one end of the protein protruding either inside or outside the cell. This class of proteins includes enzymes such as monoamine oxidase and fatty acid amide hydrolase. Integral monotopic proteins are unable to sense conditions both inside and outside the cell, and are less likely to be involved in intercellular signaling.
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