<P> An integral membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane . All transmembrane proteins are IMPs, but not all IMPs are transmembrane proteins . IMPs comprise a significant fraction of the proteins encoded in an organism's genome . Proteins that cross the membrane are surrounded by "annular" lipids (see annular lipid shell), which are defined as lipids that are in direct contact with a membrane protein . Such proteins can be separated from the biological membranes only using detergents, nonpolar solvents, or sometimes denaturing agents . </P> <P> Three - dimensional structures of ~ 160 different integral membrane proteins have been determined at atomic resolution by X-ray crystallography or nuclear magnetic resonance spectroscopy . They are challenging subjects for study owing to the difficulties associated with extraction and crystallization . In addition, structures of many water - soluble protein domains of IMPs are available in the Protein Data Bank . Their membrane - anchoring α - helices have been removed to facilitate the extraction and crystallization . Search integral membrane proteins in the PDB (based on gene ontology classification) </P>

A membrane embedded segment of an integral membrane protein