Actin filaments elongate from their membrane-associated ends

Area of Science:

• Cell Biology
• Cytoskeletal Dynamics
• Sperm Development

Background:

• Limulus sperm possess a unique, long actin filament bundle.
• This bundle extends from the acrosomal vacuole membrane through the nucleus and coils at its base.
• The bundle exhibits a distinct tapering, from 15 filaments at the acrosomal end to 85 filaments at the basal end.

Purpose of the Study:

• To elucidate the assembly mechanism and polarity of the actin filament bundle in limulus sperm.
• To determine the site of actin monomer addition during polymerization.
• To understand how the bundle's structure and tapering are achieved.

Main Methods:

• Observation of sperm during spermatid maturation using thin section electron microscopy.
• In vitro polymerization assays using isolated filament bundles and muscle G-actin.
• Decoration of polymerized actin filaments with myosin subfragment 1.

Main Results:

• Actin filament bundle assembly initiates on dense material at the acrosomal vacuole membrane.
• In vitro assays show a strong preference for monomer addition at the membrane-associated end.
• Myosin subfragment 1 decoration indicates filament polarity, with arrowheads pointing away from the acrosomal vacuole membrane.

Conclusions:

• The actin filament bundle is nucleated at the acrosomal vacuole membrane.
• Actin monomers are added preferentially to the membrane-associated end, driving assembly.
• The bundle's structure, including tapering and coiling, results from directed polymerization and filament capping.