Macromolecular crowding gives rise to microviscosity, anomalous diffusion and accelerated actin polymerization

Area of Science:

• Biochemistry
• Cell Biology
• Biophysics

Background:

• Macromolecular crowding (MMC) mimics cellular environments and influences stem cell behavior.
• MMC is known to enhance extracellular collagen deposition and adipogenic differentiation.
• The precise mechanisms by which MMC affects biochemical reaction rates are not fully understood.

Purpose of the Study:

• To investigate the effect of macromolecular crowding on reaction rates, specifically actin polymerization.
• To differentiate the roles of excluded volume and viscosity in MMC-induced rate enhancements.
• To elucidate the impact of microviscosity versus bulk viscosity in crowded biological systems.

Main Methods:

• Fluorimetry was used to measure the rate of actin polymerization.
• Fluorescence correlation spectroscopy (FCS) was employed to measure the diffusion of various probes.
• Actin polymerization assays were conducted in solutions with Ficoll (crowder) and glycerol (viscosity modifier).

Main Results:

• Ficoll enhanced actin polymerization rate despite increasing bulk viscosity.
• FCS measurements indicated a minor component of anomalous diffusion.
• Glycerol experiments ruled out bulk viscosity as the cause of rate enhancement, suggesting microviscosity is key.

Conclusions:

• MMC enhances reaction rates primarily through excluded volume effects, increasing effective reactant concentrations.
• Microviscosity, rather than bulk viscosity, becomes significant at the molecular scale in crowded environments.
• This study provides a detailed mechanistic understanding of how macromolecular crowding influences biochemical reaction kinetics.