MICROINJECTED PROFILIN AFFECTS CYTOPLASMIC STREAMING IN PLANT-CELLS BY RAPIDLY DEPOLYMERIZING ACTIN MICROFILAMENTS

Background: Cytoplasmic streaming is a conspicuous feature of plant ce ll behaviour, in which organelles and vesicles shuttle along cytoplasm ic strands that contain actin filaments. The mechanisms that regulate streaming and the formation of actin filament networks are largely unk nown, but in all likelihood involve actin-binding proteins. The monome ric actin-binding protein, profilin, is a key regulator of actin-filam ent dynamics in animal cells and it has recently been identified in pl ants as a pollen allergen. We set out to determine whether plant profi lin can act as a monomeric actin-binding protein and influence actin d ynamics in plant cells in vivo. Results: Recombinant birch-pollen prof ilin was purified by polyproline affinity chromatography and microinje cted into Tradescantia blossfeldiana stamen hair cells. After profilin injection, a rapid and irreversible change in cellular organization a nd streaming was observed: within 1-3 minutes the transvacuolar cytopl asmic strands became thinner and snapped, and cytoplasmic streaming ce ased. Fluorescein-labelled-phalloidin staining confirmed that this was due to depolymerization of actin filaments. To confirm that the effec ts observed were due to sequestration of monomeric actin, another mono meric actin-binding protein, DNase 1, was injected and found to produc e comparable results. Conclusions: Profilin can act as a potent regula tor of actin organization in living plant cells. Its rapid effect on t he integrity of cytoplasmic strands and cytoplasmic streaming supports a model in which, organelle movements depend upon microfilaments that exist in dynamic equilibrium with the pool of monomeric actin.