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

Area of Science:

• Structural Biology
• Protein Folding
• Biophysics

Background:

• Beta-strands in globular proteins typically exhibit right-handed twist and bend.
• Inter-strand hydrogen bonds are considered the primary drivers of beta-strand twisting.
• Polar residues (serine, threonine, asparagine) were previously shown to suppress twist and bend in water-soluble proteins.

Purpose of the Study:

• To investigate whether polar side chains also suppress twisting and bending in transmembrane beta-strands.
• To analyze the structural differences in beta-strands between transmembrane and water-soluble beta-barrel proteins.
• To identify key motifs responsible for beta-strand bending in transmembrane beta-barrels.

Main Methods:

• Statistical analysis of twist and bend angles in four-residue frames of beta-strands.
• Comparison of beta-strands from known 3D structures of transmembrane and water-soluble beta-barrel proteins.
• Examination of residue composition within beta-strand frames.

Main Results:

• Twisting of transmembrane beta-strands is suppressed, even in the absence of serine, threonine, or asparagine residues.
• Suppression of twisting in transmembrane beta-strands is attributed to the propagation of polar residue effects to adjacent strands, maintaining uniform twist in the barrel.
• Transmembrane beta-barrels exhibit larger bend angles compared to water-soluble ones, with the glycine-aromatic ring motif being crucial for this bending.

Conclusions:

• Polar residue effects on beta-strand twisting extend to transmembrane proteins, influencing the overall structure of beta-barrels.
• The unique structural constraints of transmembrane beta-barrels necessitate specific residue motifs, like glycine-aromatic interactions, to facilitate necessary beta-strand bending for barrel formation.
• Understanding these structural principles is vital for deciphering the function and assembly of transmembrane protein structures.