The unusual dynamics of parasite actin result from isodesmic polymerization

Kristen M. Skillman, Christopher I. Ma, Daved H. Fremont, Karthikeyan Diraviyam, John A. Cooper, David Sept () and L. David Sibley ()
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Kristen M. Skillman: Washington University School of Medicine
Christopher I. Ma: Washington University School of Medicine
Daved H. Fremont: Washington University School of Medicine
Karthikeyan Diraviyam: University of Michigan
John A. Cooper: Washington University School of Medicine
David Sept: University of Michigan
L. David Sibley: Washington University School of Medicine

Nature Communications, 2013, vol. 4, issue 1, 1-8

Abstract: Abstract Previous reports have indicated that parasite actins are short and inherently unstable, despite being required for motility. Here we re-examine the polymerization properties of actin in Toxoplasma gondii, unexpectedly finding that it exhibits isodesmic polymerization in contrast to the conventional nucleation–elongation process of all previously studied actins from both eukaryotes and bacteria. Polymerization kinetics of actin in T. gondii lacks both a lag phase and critical concentration, normally characteristic of actins. Unique among actins, the kinetics of assembly can be fit with a single set of rate constants for all subunit interactions, without need for separate nucleation and elongation rates. This isodesmic model accurately predicts the assembly, disassembly and the size distribution of actin filaments in T. gondii in vitro, providing a mechanistic explanation for actin dynamics in vivo. Our findings expand the repertoire of mechanisms by which actin polymerization is governed and offer clues about the evolution of self-assembling, stabilized protein polymers.

Date: 2013
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DOI: 10.1038/ncomms3285

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