Seven-helix bundles: molecular modeling via restrained molecular dynamics

Area of Science:

• Computational biology
• Structural biology
• Biophysics

Background:

• Seven-helix bundles are crucial structural motifs in membrane proteins like bacteriorhodopsin and rhodopsin.
• These bundles are also found in bacterial toxins, such as Bacillus thuringiensis delta-endotoxin.
• Understanding the assembly and stability of these bundles is key to elucidating their function.

Purpose of the Study:

• To model compact bundles of seven approximately antiparallel alpha-helices using computational methods.
• To investigate the influence of different starting configurations and polypeptide chain arrangements on bundle formation.
• To identify stable helix-packing motifs and compare them with known biological structures.

Main Methods:

• Simulated annealing via restrained molecular dynamics (SA/MD) was employed to generate ensembles of seven-helix bundle models.
• Two types of models were considered: separate peptide chains and single polypeptide chains with interhelix loops.
• Three distinct C-alpha templates (circular, bacteriorhodopsin-like, zig-zag) were used as starting points for simulations.

Main Results:

• SA/MD successfully generated stable seven-helix bundle models.
• The simulations produced structures resembling both bacteriorhodopsin and delta-endotoxin.
• A novel helix-packing mode, characterized by a "4+1" core, was identified and occurred independently of the starting template and loop presence.

Conclusions:

• Computational modeling can effectively reproduce known seven-helix bundle structures.
• A new, stable helix-packing motif ("4+1" core) was discovered, offering insights into protein folding and stability.
• The findings provide a structural basis for understanding the diverse functions of seven-helix bundles in biological systems.