plant surface microbiology.pdf

6 Actin Binding Proteins
18 Mycorrhizal Development and Cytoskeleton 307
In eukaryotic cells, a high number of actin binding proteins (ABPs) regulate
polymerization and depolymerization, bundling and cross-linking of MFs,
and movement of cargo along the MFs. In animal cells, many of the ABP
encoding genes have been isolated and the function of the corresponding
proteins has been characterized. In plants and filamentous fungi, the
ABP research is just beginning. In Arabidopsis, the members of the gene
families encoding ADF proteins (actin depolymerizing factor/cofilin; Dong
et al. 2001), profilins (Ramachandran et al. 2000), Arp2 (Klahre and Chua
1999), villins (Klahre et al. 2000), and myosins (Reddy and Day 2001) have
been cloned and their expression patterns in different plant organs characterized.
ADF protein family members interact with actin monomers and filaments
in a pH-sensitive manner. When ADF/cofilin binds to filamentous (F) actin it
accelerates the dissociation of subunits from the pointed ends of actin filaments
(Bamburg 1999; Cooper and Schafer 2000). The properties of maize
cofilins have been analysed by using recombinant ZmADF1 and ZmADF3
proteins (Hussey et al. 1998). ZmADF3 has the ability to bind monomeric (G)
actin and F-actin and to decrease the viscosity of polymerized actin solutions,
indicating an ability to depolymerize actin filaments (Lopez et al. 1996).
ZmADF3 is phosphorylated on Ser6 by a calcium-stimulated protein kinase in
plant extracts (Smertenko et al. 1998), which suggests that phosphorylation
regulates cofilin’s actin binding activity and affects the stability of the actin
cytoskeleton in a manner shown in animal cells (Daniels and Bokoch 1999).
Profilin is a G-actin binding protein known to interact in animal and yeast
cells with proline-rich motifs of other proteins and with polyphosphoinositides.
The interaction of profilin with G-actin provides a mechanism to
sequester actin monomers and promote actin depolymerization. It appears
that profilin may also be involved in promoting actin polymerization. This
might take place by binding to proline-rich motifs in proteins that convey
intra- or extracellular signals to reorganization of actin cytoskeleton (Mullins
2000). In Arabidopsis,thePFN-1 gene, from profilin gene family with eight to
ten members, is expressed in root and root hairs, and in a ring of cells in the
elongating zone of the root (Ramachandran et al. 2000), in which the profilin
levels could be involved in the regulation of cell elongation as a rate-limiting
factor.
Plants have also several genes with high homology to animal villin (Klahre
et al. 2000). The first plant villin was isolated from pollen tubes as a 135-kDa
actin-bundling protein (Yokota et al. 1998; Yokota and Shimmen 1999). Its
identity as a villin-gelsolin family member (Vidali et al. 1999) was confirmed
by partial amino acid sequencing and by isolating the corresponding cDNA
from a pollen grain cDNA expression library. Immunodetection of villin
revealed its co-localization with actin bundles in pollen tubes. Due to the gel-