Profilin in Phaseolus vulgaris is encoded by two genes (only one expressedin root nodules) but multiple isoforms are generated in vivo by phosphorylation on tyrosine residues

Actin-binding proteins such as profilins participate in the restructuration of the actin cytoskeleton in plant cells. Profilins are ubliquitous actin- , polyproline-, and inositol phospholipid-binding proteins, which in plants are encoded by multigene families. By PD-PAGE and immunoblotting, we detec ted as much as five profilin isoforms in crude extracts from nodules of Pha seolus vulgaris. However, by immunoprecipitation and gel electrophoresis of in vitro translation products from nodule RNA, only the most basic isoform of those found in nodule extracts, was detected. Furthermore, a bean profi lin cDNA probe hybridised to genomic DNA digested with different restrictio n enzymes, showed either a single or two bands. These data indicate that pr ofilin in P, vulgaris is encoded by only two genes. In root nodules only on e gene is expressed, and a single profilin transcript gives rise to multipl e profilin isoforms by post-translational modifications of the protein. By in vivo P-32-labelling and immunoprecipitation with both, antiprofilin and antiphosphotyrosine-specific antibodies, we found that profilin is phosphor ylated on tyrosine residues. Since chemical (TLC) and immunological analyse s, as well as plant tyrosine phosphatase (AtPTP1) treatments of profilin in dicated that tyrosine residues were phosphorylated, we concluded that tyros ine kinases must exist in plants. This finding will focus research on tyros ine kinases/tyrosine phosphatases that could participate in novel regulator y functions/pathways, involving not only this multifunctional cytoskeletal protein, but other plant proteins.