Assembly Kinetics of Vimentin Tetramers to Unit-Length Filaments: A Stopped-Flow Study

Area of Science:

• Cell Biology
• Biophysics
• Structural Biology

Background:

• Intermediate filaments (IFs) are crucial cytoskeletal components providing mechanical stability and elasticity to cells and tissues.
• IF assembly involves tetrameric complexes forming unit-length filaments (ULFs) through lateral association.
• Previous studies used microscopy to analyze IF assembly kinetics over extended periods.

Purpose of the Study:

• To quantitatively determine the rate constants for lateral assembly of tetramers into ULFs.
• To elucidate the hierarchical steps and productive reactions driving ULF formation.
• To gain detailed insight into the early kinetics of IF assembly.

Main Methods:

• Utilized a stopped-flow apparatus combined with static light-scattering measurements.
• Applied quantitative time-lapse atomic force and electron microscopy for kinetic analysis.
• Employed a global fitting procedure to process recorded data and model assembly steps.

Main Results:

• Determined basic rate constants for tetramer lateral assembly into ULFs.
• Proposed that tetramers are rapidly converted to octamers, which are obligatory intermediates for ULF formation.
• Demonstrated that tetramer interaction is diffusion-controlled but geometrically driven for efficient subunit targeting.

Conclusions:

• The study reveals a hierarchical model for IF formation, starting with rapid tetramer-to-octamer conversion.
• The findings explain the occurrence of polymorphic ULFs and mature filaments based on tetramer number per cross-section.
• This provides a quantitative understanding of the early, rapid stages of intermediate filament assembly.