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Vol. 14, Issue 2, 396-406, February 2003
and
*Department of Molecular Microbiology, Washington
University School of Medicine, St. Louis, Missouri 63110;
Host cell entry by Toxoplasma gondii depends
critically on actin filaments in the parasite, yet paradoxically, its
actin is almost exclusively monomeric. In contrast to the absence of
stable filaments in conventional samples, rapid-freeze electron
microscopy revealed that actin filaments were formed beneath the plasma
membrane of gliding parasites. To investigate the role of actin
filaments in motility, we treated parasites with the
filament-stabilizing drug jasplakinolide (JAS) and monitored the
distribution of actin in live and fixed cells using yellow fluorescent
protein (YFP)-actin. JAS treatment caused YFP-actin to
redistribute to the apical and posterior ends, where filaments formed a
spiral pattern subtending the plasma membrane. Although previous
studies have suggested that JAS induces rigor, videomicroscopy
demonstrated that JAS treatment increased the rate of parasite gliding
by approximately threefold, indicating that filaments are rate limiting
for motility. However, JAS also frequently reversed the normal
direction of motility, disrupting forward migration and cell entry.
Consistent with this alteration, subcortical filaments in JAS-treated
parasites occurred in tangled plaques as opposed to the straight,
roughly parallel orientation observed in control cells. These studies reveal that precisely controlled polymerization of actin filaments imparts the correct timing, duration, and directionality of gliding motility in the Apicomplexa.
Department of Microbiology, University of
Pennsylvania, Philadelphia, Pennsylvania 19104; and
Department of Molecular Biology, Umeå
University, Umeå, Sweden 5-90187
Online version of this article contains video material. Online
version is available at www.molbiolcell.org .
§
Corresponding author. E-mail address:
sibley{at}borcim.wustl.edu.
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