6.2. Capping protein
A gelsolin-like protein has been isolated from Papaver
pollen [100]. This protein tightly binds to the barbed ends of
actin filaments in vitro, in this way preventing polymerisation
and depolymerisation at the barbed ends in a calcium
dependent way. The gelsolin-like protein also has actin
filament nucleation and severing properties. The Arabidopsis
genome, however, does not contain sequences for gelsolinlike
proteins [100]. The closest sequence homologues in
Arabidopsis to gelsolin are villin-like proteins. Plant villins
have been shown to bundle actin filaments [14,101–103]. The
actin bundling activity of some villins is calcium-dependent
[101] and of others not [102]. Villin-like proteins from lily
can inhibit growth of barbed ends at high calcium concentrations,
which could be caused by actin capping activity [103].
Huang et al. [104] demonstrated that in plants, a capping
protein is present that binds to the barbed ends of actin
filaments, in this way preventing polymerisation and
depolymerisation. The capping protein forms heterodimers
and binding of this protein to the barbed end is regulated by
phosphatidic acid (PA): in the presence of PA, the actinbinding
activity of the capping protein is inhibited, which
leads to extensive actin filament growth [105]. It is not
known whether the presence of capping proteins is required
for Arp2/3 complex-dependent growth processes in plants [1]