3 June 2005
Hao Yuan Kueh
Mitchison Lab
Department of Systems Biology
Harvard Medical School
Actin forms cytoskeletal structures that play important roles in cell motility, division and integrity. These structures turnover rapidly in vivo, allowing the cell to maintain a large actin monomer pool and make rapid rearrangements to their actin cytoskeleton in response to environmental signals. To study mechanisms of actin depolymerization in vivo, we imaged actin comet-tails of the intracellular pathogen listeria monocytogenes. Actin disassembles rapidly from listeria comet tails, and the decay profile is well-fit by a single decaying exponential with a half life ~30 seconds.
To gain insight into the kinetic processes driving in vivo actin turnover, we developed models of actin dynamics to determine disassembly mechanisms consistent with an exponential decay. We studied three main classes of kinetic mechanisms: 1) depolymerization from filament-ends, 2) filament-severing, and 3) biochemical or structural transitions that destabilize filaments. Simple end-depolymerization models yield exponential decay curves only if the initial filament length distribution is exponential. Models in which a slow first-order transition precedes depolymerization also yield exponential decay curves independent of the initial filament length-distribution. Severing-based models yield inflected curves and are inconsistent with experimental data. Modeling results suggest that in vivo actin disassembly occurs via end-dependent depolymerization, which may be preceded by a possible slow first-order transition.