Plant Villin, Lily P-135-ABP, Possesses G-Actin Binding Activity and Accelerates the Polymerization and Depolymerization of Actin in a Ca2+-Sensitive Manner

Abstract

From germinating pollen of lily, two types of villins, P-115-ABP and P-135-ABP, have been identified biochemically. Ca(2+)-CaM-dependent actin-filament binding and bundling activities have been demonstrated for both villins previously. Here, we examined the effects of lily villins on the polymerization and depolymerization of actin. P-115-ABP and P-135-ABP present in a crude protein extract prepared from germinating pollen bound to a DNase I affinity column in a Ca(2+)-dependent manner. Purified P-135-ABP reduced the lag period that precedes actin filament polymerization from monomers in the presence of either Ca(2+) or Ca(2+)-CaM. These results indicated that P-135-ABP can form a complex with G-actin in the presence of Ca(2+) and this complex acts as a nucleus for polymerization of actin filaments. However, the nucleation activity of P-135-ABP is probably not relevant in vivo because the assembly of G-actin saturated with profilin, a situation that mimics conditions found in pollen, was not accelerated in the presence of P-135-ABP. P-135-ABP also enhanced the depolymerization of actin filaments during dilution-mediated disassembly. Growth from filament barbed ends in the presence of Ca(2+)-CaM was also prevented, consistent with filament capping activity. These results suggested that lily villin is involved not only in the arrangement of actin filaments into bundles in the basal and shank region of the pollen tube, but also in regulating and modulating actin dynamics through its capping and depolymerization (or fragmentation) activities in the apical region of the pollen tube, where there is a relatively high concentration of Ca(2+).

Root Hairs

“…Most plant villins have, besides an actin bundling activity, Ca 2+ /calmodulin dependent actin severing, F-actin capping and monomeric actin binding activities (Tominaga et al, 2000;Yokota et al, 2005;Khurana et al, 2010;Zhang et al, 2010;Zhang et al, 2011b). Thus, while villins are actin bundling proteins at low Ca 2+ concentrations, they increase actin turnover at higher Ca 2+ concentrations.…”

Imaging of the Yellow Cameleon 3.6 Indicator Reveals That Elevations in Cytosolic Ca2+ Follow Oscillating Increases in Growth in Root Hairs of Arabidopsis

“…Thus, there is strong evidence supporting a regulatory role for the Ca 21 gradient through imposing spatial control on the site of cell expansion in these tip-growing systems. Indeed, the structures of the apical actin cytoskeleton and its regulatory proteins (villins, gelsolins, and actin-depolymerizing factors; Smertenko et al, 1998;Tominaga et al, 2000;Allwood et al, 2001;Ketelaar et al, 2003;Fan et al, 2004) are thought to be regulated by the tip-high Ca 21 (Yokota et al, 2005). Similarly, there is evidence that annexins (Blackbourn et al, 1992;Clark et al, 1992;Carroll et al, 1998), phosphoinositide metabolism (Preuss et al, 2006), and calmodulin and protein kinases (Moutinho et al, 1998;Yoon et al, 2006) also play roles in sustaining tip growth and are regulated via the cytosolic Ca 21 gradient.…”

Pollen Tube Growth Oscillations and Intracellular Calcium Levels Are Reversibly Modulated by Actin Polymerization

“…For example, the villin/gelsolin family includes several members that modify actin in a Ca 21 -dependent manner. Two villins, P-135-ABP (Yokota et al, 2000; and P-115-ABP (Yokota et al, 2005), which contain six gelsolin repeats plus the characteristic villin headpiece, cross-link and stabilize actin bundles in low Ca 21 , but stimulate depolymerization in regions of elevated Ca 21 -calmodulin (Yokota et al, 2005). Three different gelsolins, including PrABP80 from Papaver rhoeas (Huang et al, 2004), LdABP41 from Lilium davidii , and ABP29 from Lilium longiflorum (Xiang et al, 2007), which are presumably splice variants of villin, actively fragment and depolymerize F-actin, and cap the plus ends, in regions of elevated Ca…”