Orientational Ambiguity in Septin Coiled Coils and its Structural Basis

Area of Science:

• Molecular biology
• Structural biology
• Biochemistry

Background:

• Septins are crucial proteins for cell division and other cellular processes.
• Their C-terminal coiled-coil domains are thought to mediate filament formation and bundling.
• Understanding septin assembly is key to deciphering their diverse cellular roles.

Purpose of the Study:

• To investigate the structural diversity of human septin coiled-coil regions.
• To provide direct structural information on these domains.
• To elucidate the molecular mechanisms underlying septin filament formation.

Main Methods:

• Integrated structural approach combining experimental and computational methods.
• X-ray crystallography or cryo-EM for structural determination.
• Molecular modeling and simulations to assess energetic accessibility of different arrangements.

Main Results:

• Direct structural data for the coiled-coil regions of five human septins were obtained.
• Unexpected dimeric structures with both parallel and antiparallel arrangements were identified.
• Molecular modeling confirmed the energetic accessibility of both parallel and antiparallel states.
• Antiparallel structures revealed a unique mixed coiled-coil interface with a hydrophilic core.

Conclusions:

• Human septin coiled-coil domains can adopt at least two metastable states: parallel and antiparallel.
• The parallel orientation likely facilitates intra-filament interactions, while antiparallel may mediate inter-filament cross-bridges.
• The unusual antiparallel coiled-coil structure presents novel opportunities for protein engineering and the design of new assemblies.