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Transmembrane segment usually denotes a single transmembrane alpha helix of a transmembrane protein, also known as an integral protein. More broadly, a transmembrane segment is any three-dimensional protein structure which is thermodynamically stable in a membrane. This may be a single alpha helix, a transmembrane beta barrel, a beta-helix of gramicidin A, or any other structure. Transmembrane helices are usually about 20 amino acids in length.
Identification of transmembrane helices
Transmembrane helices are visible in structures of membrane proteins determined by X-ray diffraction. They may also be predicted on the basis of hydrophobicity scales. Because the interior of the bilayer and the interiors of most proteins of known structure are hydrophobic, it is presumed to be a requirement of the amino acids that span a membrane that they be hydrophobic as well. However, membrane pumps and ion channels also contain numerous charged and polar residues within the generally non-polar transmembrane segments.
Using hydrophobicity analysis to predict transmembrane helices enables a prediction in turn of the "transmembrane topology" of a protein; i.e. prediction of what parts of it protrude into the cell, what parts protrude out, and how many times the protein chain crosses the membrane. Such prediction methods are commonly applied with a limited success.