Dynamic Actin Filament Traps Mediate Active Diffusion of Vesicular Stomatitis Virus Ribonucleoproteins

Area of Science:

• Cellular and Molecular Biology
• Biophysics
• Virology

Background:

• Viral ribonucleoprotein (RNP) particles are too large for free diffusion in the cytoplasm.
• Their observed motion resembles diffusion, termed 'active diffusion', but lacks a clear molecular explanation.
• Previous models relied on indirect analyses like continuum microrheology.

Purpose of the Study:

• To provide a quantitative molecular explanation for active diffusion of viral RNP particles.
• To investigate the role of cytoskeletal elements in RNP particle motion.
• To elucidate the mechanisms underlying active diffusion in living cells.

Main Methods:

• Variational Bayesian analysis combined with pattern recognition and machine learning.
• Tracking of single viral ribonucleoprotein (RNP) particle movements in living cell cytoplasm.
• Perturbation of cellular structures (actin filaments, nonmuscle myosin II ATPase) to observe effects on RNP mobility.

Main Results:

• Machine learning identified dynamic traps formed by cytoskeletal elements confining RNP particles.
• Active diffusion occurs via particles escaping one trap and entering another.
• Disruption of actin filaments increased RNP mobility by enlarging traps.
• Inhibition of nonmuscle myosin II ATPase decreased mobility by reducing trap size.
• Trap size was observed to fluctuate dynamically, dependent on nonmuscle myosin II activity.

Conclusions:

• A model explaining active diffusion through hopping between dynamic, cytoskeletal traps is proposed.
• Nonmuscle myosin II activity regulates the size of these traps.
• This mechanism likely governs the motion of other viral and cellular components within the cytoplasm.