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Department of Anatomy and Structural Biology, Albert Einstein College of Medicine of Yeshiva University, Bronx,
New York 10461
The first step in the directed movement of
cells toward a chemotactic source involves the extension of pseudopods initiated by the focal nucleation and
polymerization of actin at the leading edge of the cell.
We have previously isolated a chemoattractant-regulated barbed-end capping activity from Dictyostelium
that is uniquely associated with capping protein, also
known as cap32/34. Although uncapping of barbed
ends by capping protein has been proposed as a mechanism for the generation of free barbed ends after stimulation, in vitro and in situ analysis of the association of
capping protein with the actin cytoskeleton after stimulation reveals that capping protein enters, but does not
exit, the cytoskeleton during the initiation of actin polymerization. Increased association of capping protein
with regions of the cell containing free barbed ends as visualized by exogenous rhodamine-labeled G-actin is
also observed after stimulation. An approximate threefold increase in the number of filaments with free
barbed ends is accompanied by increases in absolute filament number, whereas the average filament length remains constant. Therefore, a mechanism in which preexisting filaments are uncapped by capping protein, in
response to stimulation leading to the generation of
free barbed ends and filament elongation, is not supported. A model for actin assembly after stimulation, whereby free barbed ends are generated by either filament severing or de novo nucleation is proposed. In
this model, exposure of free barbed ends results in actin
assembly, followed by entry of free capping protein into
the actin cytoskeleton, which acts to terminate, not initiate, the actin polymerization transient.
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