ACTIN-BUNDLING PROTEIN ISOLATED FROM POLLEN TUBES OF LILY - BIOCHEMICAL AND IMMUNOCYTOCHEMICAL CHARACTERIZATION

A 135-kD actin-bundling protein was purified from pollen tubes of lily (Lilium longiflorum) using its affinity to F-actin. From a crude extr act of the pollen tubes, this protein was coprecipitated with exogenou sly added F-actin and then dissociated from F-actin by treating it wit h high-ionic-strength solution. The protein was further purified seque ntially by chromatography on a hydroxylapatite column, a gel-filtratio n column, and a diethylaminoethyl-cellulose ion-exchange column. In th e present study, this protein is tentatively referred to as P-135-ABP (Plant 135-kD Actin-Bundling Protein). By the elution position from a gel-filtration column, we estimated the native molecular mass of purif ied P-135-ABP to be 260 kD, indicating that it existed in a dimeric fo rm under physiological conditions. This protein bound to and bundled F -actin prepared from chicken breast muscle in a Ca2+-independent manne r. The binding of 135-P-ABP to actin was saturated at an approximate s toichiometry of 26 actin monomers to 1 dimer of P-135-ABP. By transmis sion electron microscopy of thin sections, we observed cross-bridges b etween F-actins with a longitudinal periodicity of 31 nm. Immunofluore scence microscopy using rhodamine-phalloidin and antibodies against th e 135-kD polypeptide showed that P-135-ABP was colocalized with bundle s of actin filaments in lily pollen tubes, leading us to conclude that it is the factor responsible for bundling the filaments.