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

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