Antiparallel four-stranded coiled coil specified by a 3-3-1 hydrophobic heptad repeat

Area of Science:

• Protein structure and folding
• Biochemistry and molecular biology
• Structural biology

Background:

• Coiled-coil protein structures are common in biology.
• These structures typically feature a seven-amino acid repeat (heptad repeat) with nonpolar amino acids at the 'a' and 'd' positions.

Purpose of the Study:

• To investigate the role of a 3-3-1 hydrophobic repeat (including 'a', 'd', and 'g' positions) in coiled-coil structure determination.
• To explore how altering hydrophobic-polar patterns affects coiled-coil stability and formation.

Main Methods:

• Utilized mutants of the GCN4 leucine zipper dimerization domain.
• Replaced charged residues at 'g' positions with nonpolar amino acids (alanine or valine).
• Determined structures of resulting tetramers using X-ray crystallography.

Main Results:

• Mutants with nonpolar amino acids at 'g' positions formed stable, four-stranded antiparallel coiled coils (tetramers).
• X-ray crystal structures revealed interlocking of 'a', 'd', and 'g' side chains via knobs-into-knobs and knobs-into-holes packing.
• Demonstrated that hydrophobic-polar patterns beyond the canonical heptad repeat can dictate interfacial interactions.

Conclusions:

• The study expands the understanding of coiled-coil structural determinants beyond the traditional heptad repeat.
• Findings suggest that conserved charged residues at 'g' positions in GCN4 leucine zipper may act as a 'negative design' element, preventing the formation of more stable tetramers.