Arp2/3 complex and cofilin modulate binding of tropomyosin to branched actin networks

Area of Science:

• Cell Biology
• Cytoskeletal Dynamics
• Protein-Actin Interactions

Background:

• Tropomyosins are coiled-coil proteins crucial for cytoskeletal functions, regulating actin dynamics.
• Tropomyosins inhibit actin nucleation by the Arp2/3 complex and prevent filament disassembly by cofilin.
• The interplay between tropomyosins, Arp2/3 complex, and cofilin in actin network regulation is not fully understood.

Purpose of the Study:

• To investigate how the Arp2/3 complex and cofilin regulate tropomyosin binding to actin filaments.
• To elucidate the molecular mechanism underlying tropomyosin's localization and function in actin networks.

Main Methods:

• Fluorescence microscopy was used to visualize tropomyosin polymerization on actin filaments.
• In vitro assays were performed using purified actin, tropomyosin (Drosophila Tm1A), Arp2/3 complex, and cofilin.

Main Results:

• Tropomyosin polymerizes along actin filaments, initiating from nuclei on ADP-bound regions near the pointed end.
• Tropomyosin fails to bind Arp2/3 complex-generated dendritic networks because the Arp2/3 complex blocks pointed ends.
• Cofilin promotes phosphate dissociation and filament severing, creating new pointed ends that enable tropomyosin binding to Arp2/3 networks.

Conclusions:

• Tropomyosin exhibits a strong preference for pointed ends of actin filaments.
• Arp2/3 complex and cofilin dynamically regulate tropomyosin binding, controlling its access to actin networks.
• This interaction provides a mechanism for insulating lamellipodial actin networks from tropomyosin's effects.