The 135 kDa actin-bundling protein from Lilium longiflorum pollen is the plant homologue of villin - Rapid communication

Actin microfilaments, which are essential for cell growth and cytoplasmic s treaming in pollen tubes, are closely dependent on actin-binding proteins f or their organization and regulation. We have purified the plant 135 kDa ac tin-bundling protein (P-135-ABP) from Lilium longiflorum pollen and determi ned that its amino acid composition is highly similar to members of the vil lin-gelsolin family of proteins. We used antibodies against P-135-ABP to pr obe an expression cDNA library of L. longiflorum pollen and isolated a full -length clone (ABP135) that corresponds to a 106 kDa polypeptide. The deduc ed amino acid sequence of ABP135 shows homology with members of the villin- gelsolin family of proteins and contains the characteristic six repeats of this family, as well as an extended carboxy-terminal domain that includes t he villin headpiece preceded by a highly variable region. Using two-dimensi onal polyacrylamide gel electrophoresis we detected at least 5 isoforms of P-135-ABP, with isoelectric points (pI) ranging between 5.6 to 5.9. The mos t abundant P-135-ABP isoform has a pi of 5.8, closely approximating the pi predicted from the deduced ABP135 amino acid sequence. These data, together with the partial amino acid sequence from a proteolytic peptide of the pro tein, indicate that P-135-ABP is a plant villin. Immune-detection of Lilium villin in rapidly frozen pollen tubes localized it to actin bundles. Liliu m villin is also ubiquitously expressed in all tissues tested. Since villin s, like gelsolins, are also Ca2+-dependent severing, capping, and nucleatin g proteins, Lilium villin may participate in F-actin fragmentation and nucl eation in the apex of the pollen tube where there is steep Ca2+ gradient.