The three-dimensional structure of the Limulus acrosomal process: a dynamic actin bundle.

Area of Science:

• Cell biology
• Biophysics
• Structural biology

Background:

• Limulus sperm possesses a dynamic macromolecular structure enabling rapid extension.
• This structure, the true discharge, is crucial for sperm function.
• Its core is an ordered filament bundle of actin, scruin, and calmodulin.

Purpose of the Study:

• To determine the three-dimensional structure of the Limulus sperm true discharge filament bundle.
• To elucidate the molecular interactions governing bundle stability and dynamics.

Main Methods:

• Electron crystallographic reconstruction was employed to generate a high-resolution 3D map.
• Analysis focused on the arrangement and interactions of actin and scruin filaments.

Main Results:

• A detailed 3D map revealed an actin-scruin helix.
• Azimuthal modulation of the helix is influenced by inter-filament interactions.
• Scruin-scruin contacts, often extensive, were identified as key to bundle stability.

Conclusions:

• Promiscuous scruin-scruin contacts are the primary determinants of true discharge bundle stability.
• The identified structure suggests potential for rearrangements facilitating the coiled-to-extended transition.