Integral membrane proteinAn integral membrane protein 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 reside in the oily lipid bilayer core of the membrane, while hydrophilic domains protrude into the watery environment inside and outside the cell or compartment. The structure of only a few 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 crystallize for analysis by X-ray diffraction. 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.