β-Strand twisting/bending in soluble and transmembrane β-barrel structures

The majority of β-strands in globular proteins have a right-handed twist and bend. The dominant driving force for β-strand twisting is thought to be inter-strand hydrogen bonds. We previously demonstrated that for water-soluble proteins, both the twisting and bending of β-strand are suppressed by the polar side chains of serine, threonine, and asparagine residues. To determine whether this also holds for transmembrane β-strands, we calculated and statistically analyzed the twist and bend angles of four-residue frames of β-strands in both transmembrane and water-soluble β-barrel proteins with known three-dimensional structures. In the case of transmembrane β-strands, we found that twisting was suppressed even for frames not containing serine, threonine, or asparagine residues. The suppression of twisting in transmembrane β-strands could be attributed to the propagation of the suppressive effect of serine, threonine, and asparagine residues within a frame to the neighboring, hydrogen-bonded strands under the restriction that all strands in the closed barrel structure must have similar twist angles. A similar tendency was also observed for water-soluble β-barrel proteins. We previously showed that the dominant driving force for β-strand bending is hydrophobic interactions involving aromatic residues within and outside the strand. Transmembrane β-barrels have no hydrophobic core; however, rather hydrophilic residues predominate inside the barrel or the β-strands of transmembrane β-barrels have larger bend angles than those of water-soluble β-barrels. Our results reveal that, in transmembrane β-barrel proteins, the glycine-aromatic ring motif is important for generating the β-strand bending necessary for barrel formation.