Tryptophans in membrane proteins. X-ray crystallographic analyses.

While tryptophans are generally found in low abundance in soluble proteins, in many integral membrane proteins they comprise a significantly higher proportion of the amino acid composition. Now that crystal structures are available for a number of membrane proteins, it has been possible to examine the distribution and disposition of the tryptophans within these structures. The tryptophan locations with respect to the lipid bilayer (along the direction normal to the membrane surface) are strikingly non-uniform in nearly all of the membrane proteins examined. They tend to cluster at the interface between the polar head group region and the hydrophobic interior, in a relatively uniform layer just below the surface. In many cases, their distributions with respect to the extra- and intra-cellular surfaces tend to be asymmetric. These observations provide evidence for possible structural roles for tryptophans in transmembrane sheets and helices, where they may play a part in the stabilization of the transmembrane segments and perhaps in the orientation and bilayer insertion processes.