The 135-kDa actin-bundling protein from lily pollen tubes arranges F-actininto bundles with uniform polarity

A plant 135-kDa actin-bundling protein (P-135-ABP) isolated from pollen tub es of Lilium longiflorum (Thunb.) binds stoichiometrically to F-actin filam ents and bundles them in vitro (E. Yokota et al., 1998, Plant Physiol. 116. 1421-1429). To further understand the mechanism of actin-filament bundle f ormation by P-135-ABP, the polarity of each F-actin filament in bundles was examined using myosin subfragment 1 (S-1). Dissociation of F-actin filamen ts from bundles organized by P-135-ABP was induced by S-1. However, F-actin filaments that remained in a bundle and decorated by S-1 showed uniform po larity. These results indicate that P-135-ABP arranges F-actin filaments in to bundles with uniform polarity and consequently plays a key role in the o rientation of cytoplasmic streaming in pollen tubes.