An integral membrane protein
An integral membrane protein (IMP) is a protein molecule (or protein aggregate) that is permanently attached to a biological membrane. Such proteins can only be separated from membranes by detergents, non-polar solvents, or in some cases denaturing agents.
Integral membrane proteins constitute a very significant fraction of proteins encoded by the genome of an organism.
The three-dimensional structures of only ~160 different integral membrane proteins have so far been determined at atomic resolution by X-ray crystallography or nuclear magnetic resonance due to difficulties associated with extraction and crystallization. In addition, the structures of many water-soluble domains are available in the Protein Data Bank. Their membrane-binding α-helices were removed to allow extraction and crystallization.
Peripheral membrane proteins
The human membrane proteome
Membrane proteins with known 3D structures
Database of protein orientations in membranes