Calcium-Calmodulin Suppresses the Filamentous Actin-Binding Activity of a 135-Kilodalton Actin-Bundling Protein Isolated from Lily Pollen Tubes

We have isolated a 135-kD actin-bundling protein (P-135-ABP) from lily (Lilium longiflorum) pollen tubes and have shown that this protein is responsible for bundling actin filaments in lily pollen tubes (E. Yokota, K. Takahara, T. Shimmen [1998] Plant Physiol 116: 1421–1429). However, only a few thin actin-filament bundles are present in random orientation in the tip region of pollen tubes, where high concentrations of Ca2+ have also been found. To elucidate the molecular mechanism for the temporal and spatial regulation of actin-filament organization in the tip region of pollen tubes, we explored the possible presence of factors modulating the filamentous actin (F-actin)-binding activity of P-135-ABP. The F-actin-binding activity of P-135-ABP in vitro was appreciably reduced by Ca2+ and calmodulin (CaM), although neither Ca2+ alone nor CaM in the presence of low concentrations of Ca2+ affects the activity of P-135-ABP. A micromolar order of Ca2+ and CaM were needed to induce the inhibition of the binding activity of P-135-ABP to F-actin. An antagonist for CaM, W-7, cancelled this inhibition. W-5 also alleviated the inhibition effect of Ca2+-CaM, however, more weakly than W-7. These results suggest the specific interaction of P-135-ABP with Ca2+-CaM. In the presence of both Ca2+ and CaM, P-135-ABP organized F-actin into thin bundles, instead of the thick bundles observed in the absence of CaM. These results suggest that the inhibition of the P-135-ABP activity by Ca2+-CaM is an important regulatory mechanism for organizing actin filaments in the tip region of lily pollen tubes.