Time-resolved X-ray scattering study of actin polymerization from profilactin

Area of Science:

• Biochemistry
• Biophysics
• Cell Biology

Background:

• Actin polymerization is crucial for cell motility and structure.
• Profilactin is a complex of actin and profilin, a key regulator of actin dynamics.
• Previous studies using viscometry suggested a lag phase in actin polymerization from profilactin.

Purpose of the Study:

• To investigate the polymerization kinetics of actin from profilactin using synchrotron radiation X-ray solution scattering.
• To clarify the structural organization of polymerized actin derived from profilactin.
• To reconcile discrepancies between viscometry and other methods regarding actin polymerization from profilactin.

Main Methods:

• Synchrotron radiation X-ray solution scattering (SAXS) was employed to monitor actin polymerization.
• Electron microscopy was used for direct visualization of filament structures.
• Model calculations were performed to interpret scattering data and structural arrangements.

Main Results:

• Actin polymerization from profilactin occurs without a lag phase at the studied concentrations.
• Filaments formed from profilactin aggregate into bundles or networks.
• This aggregation phenomenon was observed regardless of the polymerization induction method (temperature change, salt addition).

Conclusions:

• The formation of actin filaments from profilactin is immediate, without a lag phase.
• Filament aggregation into bundles/networks is a key characteristic of profilactin polymerization.
• The observed aggregation explains the lower final viscosity in profilactin polymerization compared to pure actin solutions.