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Transmembrane segments (TM) are important structural elements that often define the functional domain of membrane proteins.’. 2 Structural studies of TM segments are thus becoming essential for understanding the structure/function of membrane proteins, although their physical properties have often complicated characterization at high resolution. Compositional analyses of putative TM segments of membrane proteins have shown that they are dominated by hydrophobic amino acids such as Ile and Val, which are known to promote 8-sheet formation in globular protein^.^ However, in the membrane environment, this propensity for (3-structure is reflected only in a few special situations such as in (3-barrel protein^.^ Instead, TM segments are found to adopt predominantly a-helical conformations (Refs. 5, 6, and references therein).

The characteristic stretch of hydrophobic residues in TM segments can be prone to …