Integral membrane protein
In cell biology, an integral membrane protein (IMP) is a protein that is situated in the cell membrane and provides some sort of function for the cell.
Most commonly this function is to act as a transporter for various molecules that would otherwise not be able to move across the cell's membrane. When used as a transporter, its most common configuration is to have an extra-cellular domain and a cytoplasmic domain separated by a non-polar region that holds it tightly in the cell membrane.
An IMP is a protein molecule (or assembly of proteins) that in most cases spans the biological membrane with which it is associated (especially the plasma membrane) or which, in any case, is sufficiently embedded in the membrane to remain with it during the initial steps of biochemical purification (compare peripheral membrane protein). A hydrophobic domain of the protein resides in the oily core of the membrane, while hydrophilic domains protrude into the watery environment inside and outside the cell or compartment. The integral membrane proteins perform many fundamental cellular functions (for example, they can be receptors, channels or enzymes) and they comprise a very significant fraction of the proteins encoded in the genome. However, although thousand of structures of proteins have been obtained by X-ray diffraction and Nuclear magnetic resonance, the structure of only a few dozens of integral membrane proteins is known at atomic resolution, because they tend to denature on removal from the membrane, under which condition they are impossible to analyze. Cells assemble integral membrane proteins in the endoplasmic reticulum. A short signal sequence at the N-terminus typically marks a protein as destined for installation in the membrane.
Integral membrane proteins rarely diffuse freely within the membrane but rather most are anchored to the cytoskeleton.
Examples of the functions that different integral membrane proteins serve include the identification of the cell for recognition by other cells, the anchoring of one cell to another or to surrounding media, the initiation of intracellular responses to external molecules and the transportation of molecules across the membrane .
Types
Following is a list of types of integral membrane proteins:
- See also: membrane protein, membrane topology, Lipid anchored protein
Categories: Cell biology | Proteins | Integral membrane proteins | Membrane biology