References

6TargetsofSI 83
Fig.6.2. PrABP80 has potent calcium-dependent F-actin severing activity. a,b Actin filaments
(4 µM) labelled with rhodamine–phalloidin were incubated with PrABP80 in the
presence of 15 nM free Ca 2+ (a) or 160 µM Ca 2+ (b) and images were collected after 30-min
incubation. PrABP80 significantly reduced the length of actin filaments (mean 0. 4±0.4 µm)
after incubation in the presence of 160 µM free Ca 2+ (b). Reactions performed in the presence
of 15 nM free Ca 2+ had a mean filament length of 4. 7±3.2 µm (a). The bar represents 10 µm.
c The severing activity of PrABP80 was also shown by enhancement of profilin-mediated
depolymerizationinthepresenceof1µMfreeCa 2+ as monitored by fluorescence of pyrene
actin. Zea mays profilin alone (ZmPRO5; 2.5 µM) or ZmPRO5 together with PrABP80 were
added to an F-actin solution prepared from 2.5 µM actin (40–50% pyrene-labelled). Depolymerization
was recorded as a reduction in pyrene fluorescence (arbitrary units) beginning
from the addition of protein at time zero. The extent and rate of depolymerization were
increased by substoichiometric amounts of PrABP80 + profilin (bottom curve), compared
with profilin alone or PrABP80 alone (middle curves). Moreover, a similar level of depolymerization
was not observed when free calcium was reduced to 15 nM, a condition where
PrABP80 should function like a barbed-end cap (top curve)
signalling events continue after inhibition of growth. We therefore believe
that some of these events may act to make growth arrest irreversible. In
yeast and some animal cells, sustained actin depolymerization is observed
during apoptosis and the actin cytoskeleton has been shown to play a functional
role in the initiation of PCD by modulating PCD signalling cascades.
(Kayalar et al. 1996; Levee et al. 1996; Janmey 1998; Korichneva and Hammerling
1999; Rao et al. 1999; Morley et al. 2003; Gourlay et al. 2004; Gourlay
and Ayscough 2005). We noted that the sustained actin depolymerization
observed in incompatible pollen was highly reminiscent of apoptosis. This
suggested that a PCD signalling cascade might be triggered by SI in incompatible
pollen.